Glomerular filtration rate measurement: Iohexol(TM) method

The purpose of this guideline is to provide guidance about the glomerular filtration rate measurement: IohexolTM method at Great Ormond Street Hospital.

NOTE: We review our guidelines regularly and this guideline is now past its review date. The content of the guideline below may not reflect the most recent evidence based practice. Please use with caution.

Flowchart showing glomerular filtration rate measurement: Iohexol(TM) method
The glomerular filtration rate (GFR) refers to the rate of renal clearance of a substance from the plasma compartment. Clearance is assumed to be due to the glomerular filtration of the solute. See Appendix 1 (Word document, 34 KB).

A formula relating creatinine and height has been advocated as a rough estimate of GFR:

eg GFR = (height in cms x 33)/plasma creatinine concentration (Rationale 1)

In routine clinical practice the GFR is inferred from the plasma creatinine concentration using this formula. It is assumed that a normal creatinine concentration indicates normal glomerular function. This may not apply in some children, hence the need for a more accurate GFR test.

The measurement of GFR is based on the determination of the clearance of a marker, either endogenous (eg urea, creatinine) or exogenous (eg iohexol, inulin).

This assumption may be invalid in children:

  • who are malnourished
  • whose dietary protein is severely restricted
  • whose growth is poor
  • who have reduced muscle mass

In these children and inpatients that need an accurate assessment of renal function, eg renal impairment prior to nephrotoxic therapy such as chemotherapy, GFR is formally determined by the iohexol method.

Various methods have been used to assess GFR. In the UK the single injection slope clearance method has been adopted as the most practical. The test assumes that after intravenous injection, the marker is instantaneously mixed throughout the plasma compartment and only excreted by glomerular filtration. This method has been established for many years and is widely used (Chantler 1969; Counahan 1976). 

51 Chromium EDTA (a radioactive the isotope), administered by intravenous injection, has now been replaced with iohexol (replaced in 2007) at Great Ormond Street Hospital (GOSH), a non-radioactive substance. See Appendix 2 (Word document, 27 KB) (Rationale 2).

Three 1mL blood samples are usually taken: one before the iohexol injection, and at three and four hours after a further sample at six hours if expected GFR<40mL/min (Nilsson-Ehle 2001)(Rationale 3).

The advantages of this method are:

  • It is established and safe and cost effective.
  • It is reliable and iohexol is not radioactive.
  • It involves a small dose of marker, which is safe to use in a ward environment provided this guideline is followed.
  • It uses much smaller blood samples than the chromium EDTA method (1mL).
  • Samples can be stored without loss of accuracy.
  • There is a sensitive, reproducible assay for iohexol in clinical biochemistry.

A wider range of staff can perform the test than with the chromium EDTA method. Basic IV training plus iohexol-specific training will be required.

The disadvantages of this method are:

  • As with other GFR test methods, it is most useful for detecting mild to moderate renal impairment and is less accurate at ‘normal’, ‘hyperfiltration’ and ‘severe’ GFR values.
  • The test has to be booked in advance and preferably performed in working hours.

An aseptic non-touch technique (ANTT) should be employed throughout the procedure, whether it is performed peripherally or utilising a central venous access device (CVAD) (Rationale 4).


Clinical indications

  • Suspicion of reduced renal function, eg a raised plasma creatinine and children with a progressive renal disease (Rationale 5).
  • Follow up of children with urological abnormalities, eg solitary functioning kidney, posterior urethral valves (PUV) or other abnormalities.
  • Follow up of children following extensive renal surgery, eg renal transplant.
  • Children taking nephrotoxic drugs, eg ciclosporin.
  • Oncology patients who are due to receive certain types of cytotoxic chemotherapy.
  • A GFR test may also be requested as part of a research study where there is a clear need to accurately define renal function (Rationale 6).


  • Ascites or generalised oedema, eg relapse of nephrotic syndrome (Rationale 7).
  • Manifest thyrotoxicosis (Rationale 8).
  • Previous severe reaction to iohexol or iodine products (severe reaction is one in which steroid and/or other medications such as anti-histamines were administered) (Rationale 9).
  • Severely compromised venous access (Rationale 10).
  • Presence of external drain, eg an external ventricular drain or chest drain (Rationale 11).
  • Fasted/dehydrated individuals: The child should have had their usual fluid intake during the previous twenty-four hours and for the duration of the test (Rationale 12).
  • Hyper-hydrated individuals (eg those undergoing hyper-hydration for certain types of chemotherapy)(Rationale 13).

Note: this does not apply to patients receiving maintenance fluids (appropriate for body weight), total parenteral nutrition (TPN) or electrolyte supplements. GFR testing may take place in these patients.

The child’s temperature must be recorded prior to commencement of the test. The test should be rescheduled in the event of pyrexia (>38°C) (Rationale 14). Confirm with the appropriate team prior to cancelling the test.

Patients due to receive radioiodine therapy in the next week (Rationale 15).

Heavy meal including large protein load or large caffeine intake (>1 cup of tea/coffee) on the morning of, or during the test (Rationale 16).


The following groups of patients may be at increased risk of allergic-type reaction (Morcos 2005) (Rationale 17):

  • Children with asthma who require regular inhaled corticosteroids, or have had a hospital visit due to asthma in the past two years.

  • Children with allergies to other foods/drugs including patients with severe hay fever requiring regular anti-histamines.

  • These patients should be closely monitored for 15 minutes after injection of iohexol, and if the dose was given through a cannula, it should be left in until at least the end of the observation period.

Note: There is a small chance of allergic-type reactions in all patients; no patient should be allowed to leave the ward for at least 15 minutes after iohexol administration.

  • Infants (under 1 year) require special consideration (Rationales 18 and 19).
  • Patients with phaeochromocytoma were considered at risk, but this is no longer a contraindication (Mukherjee et al 1997) (Rationale 20).
  • Children taking metformin (eg Type 2 diabetes, polycystic ovary syndrome) should be assessed individually by the consultant requesting the test. If the renal function is predicted to be near normal, the patient should be advised to avoid taking any further doses from the start of the test until 48 hours after the test. If the child has renal impairment, it is advisable to stop metformin 48 hours before the test and not to re-start until at least 48 hours after (Thomson Healthcare Inc. 2006) (Rationale 21).
  • Children taking amiodarone (eg cardiac problems such as arrhythmias) should be individually assessed. If a child is currently taking amiodarone, refer to the consultant who requested the GFR for them to make a risk/benefit assessment (Rationale 22).

Note: the iohexol assay has been modified and paracetamol no longer affects the measurement of iohexol. 

Pregnancy and breastfeeding

Adolescent/menstruating girls, generally ten years or older, should be asked if there is a possibility of their being pregnant (Rationale 23). If they suspect, or know they are pregnant, the benefit of obtaining an accurate GFR must be weighed against the risk to the developing foetus. Adolescent mothers who are breastfeeding could be advised to avoid doing so for 24 hours after the test (Rationale 24). Staff must exercise caution in dealing with this sensitive situation (Rationale 25).

Pregnant or breastfeeding staff must also be aware of any hazards associated with exposure to iohexol (Rationale 26).


Only a healthcare professional (HCP) who has been trained and is assessed as competent in venepuncture/cannulation or central line handling/management should undertake the GFR procedure and be qualified to give intravenous drugs (Rationale 27).

The trainee should read appropriate literature and research around renal investigations. HCPs who take on this role should be responsible for ensuring that they work within the guidelines contained in their codes of professional conduct and that they identify their own moral and legal accountability (Nursing and Midwifery Council (NMC) 2002; General Medical Council 2006; NMC 2008)(Rationale 29).

The HCP should undertake and complete the GFR training tool (Rationale 30).

The newly trained HCP should continue to make him/herself aware of developments in practice, research and available products (Rationale 31).

Laboratory results

Receive batched samples along with the PiMS Request form and GFR Record Form via the chute or from the porters, where possible before 4:30pm and three additional patient name labels should be provided (Rationale 32).

Separate serum and store whilst awaiting assay (Rationale 33).

Assay iohexol concentration; usually run as a weekly or bi-weekly batch of samples.

Calculate GFR using formula programmed in to Excel spreadsheet.

Report results via pathology computer system.

Glomerular filtration rate measurement

Preparation of child and family

Explain the entire procedure to the child and family, including the reason for the GFR, avoiding medical jargon and language. Information must be given according to the child’s age and developmental understanding (Rationales 34 and 35).

Refer also to our Glomerular Filtration Rate (GFR) test factsheet for families.

Explain the following elements:

  • What is a GFR test?
  • How long will it take?
  • Why does the child need the test?
  • Are there any side effects?
  • Are there any alternatives?
  • What happens before the test, what does the test involve and what happens between blood samples?
  • What happens afterwards and when the child returns home?

Previous experience needs to be considered together with preferred methods of coping, timing of the preparation and readiness of parent/carer(s) to take an active role (Rationale 36).

There is evidence that tolerance to pain increases with age and maturity when the child no longer perceives medical interventions as punitive (Haslam 1969)(Rationale 37).

For communication to be effective, the non-verbal aspects of the practitioner-parent-child relationship must be understood (Rationale 38).

If a CVAD is not being used, a topical anaesthetic should be offered and/or applied to two potential vein sites on separate limbs, prior to commencing the test (Rationale 39).

Alternatively the child may choose a local anaesthetic spray that should be applied according to product instructions.

Provide play preparation if appropriate, involving the play specialist if possible (Rationale 40).

Consider involvement of a clinical psychologist if appropriate, particularly if previous procedures have been very stressful for the child or if he/she is known to have, or exhibits signs of anticipatory anxiety and distress, eg procedural phobia (Claar, Walker & Smith 2002; Duff 2003).

Prepare appropriate methods of distraction for the child to use during the procedure itself. Attempt to discover from the child and family what techniques are most likely to consume his attention. For example, pop-up or musical books, blowing bubbles and guided imagery where the child is encouraged to imagine something pleasant, eg a favourite holiday (Rationale 41).

Check that the child has had their normal fluid intake over the previous 24 hours (Rationale 42).

Measure and record the child’s weight and height prior to commencing the procedure. For the test to be interpreted, it is essential that an accurate height and weight is recorded (see height and weight CPC guidelines)(Rationale 43).

Take a full drug history and list all medicines take in the last week in the healthcare record, including over-the-counter medicines (Rationale 44).

Check that the child has not had any iohexol administered within last 72 hours, ie radiological test with contrast (Rationale 45).

Any radiological test or scan involving the injection of contrast is likely to have used iohexol. If the child has had a radiological test or scan involving an injection within the previous 72 hours, check with the radiology department who performed the imaging if iohexol was used.

If the patient is currently taking metformin, take advice from the consultant who ordered the test or if unavailable, a member of the renal team responsible for GFR measurement. See cautions section for further guidance (Rationale 46).

If the patient is currently taking amiodarone, take advice from the consultant who ordered the test or if unavailable, a member of the renal team responsible for GFR measurement. See cautions section for further guidance (Rationale 47).

Ascertain whether the child suffers from asthma. See cautions section for further guidance (Rationale 48).

Ascertain whether the child may have any allergies. See cautions section for further guidance (Rationale 49).

If the child/family report an allergy to latex, ward staff should initiate the relevant policy (iodine/products) alert medical staff (Rationale 50).

The child undergoing the procedure must be wearing an identity bracelet or have photographic ID (Rationale 51).

Tea, coffee, fizzy drinks, ice cream, chocolate (bananas to be avoided pre-test but can be eaten during) or a heavy meal are best avoided for the duration of the test (see appendix 3). If the child has had a small intake of these proceed with the test and note on the GFR Record Form (printed at the same time as the PiMS request form)(Rationale 52).

Preparation of equipment, iohexol and environment

Print out the request forms for GFR test using PiMS (Rationale 53). Two forms are required:

  1. PiMS Request - A5 size with stickers for blood bottles.

  2. GFR Record Form - A4 size with space to record all the required information for calculating the GFR (accessed via patient – documents – advice letter in PiMS) See Appendix 4 (Word document, 144 KB)

Check and record the following:

  • The child’s name, GOSH hospital number, date of birth, gender, ward name.
  • Body weight and height (Rationale 54).

Check that there is sufficient iohexol 300 to perform the required number of GFRs (Rationale 55). Iohexol should be stored with other intravenous medicines in the ward stock cupboard. If there is insufficient stock, order some from pharmacy via the blue stock order book or via the ward pharmacist.

Note: The vials are meant for single use only. Multiple doses may be drawn from the vial on a single occasion. Opened vials containing iohexol should be disposed of as with other medicines according to the waste policy (Rationale 56).

The iohexol should, wherever possible, be administered before midday, so that samples can be delivered to chemical pathology by 4:30pm (Rationale 57).

Prepare personal protective clothing, ie clean gloves and apron (Rationale 58).

Gloves should be of a comfortable fit but tight to the skin, particularly at the fingertips (Rationale 59).

Prepare the required equipment (Rationales 60 and 61):

  • appropriate tray (usually a clean plastic tray washed thoroughly with soap and water)
  • apron
  • non-sterile gloves (latex free if required)
  • 2 x 5mL syringes or 1 x 5mL and 1 x 2mL depending on the dose to be administered
  • blue or green butterfly needle or appropriate cannula for venepuncture
  • blue needle for drawing up the iohexol
  • green needle for drawing up the 0.9% sodium chloride
  • 5mls 0.9% sodium chloride
  • latex-free filter (if required)
  • 2% chlorhexidine gluconate/70% isopropyl alcohol solution applicator (see antiseptic skin preparation - first line use of 2% chlorhexidine/70% isopropyl alcohol for vascular line insertion for further information)
  • sterile gauze
  • plaster spot dressing
  • sharps disposal box

Perform a thorough hand wash and put on personal protective clothing (Rationale 58).

Draw up 5mls of 0.9% sodium chloride; label the syringe. 

Take a vial of Iohexol 300 and calculate dose according to the following ranges. This should be checked by two qualified staff and dose to be given written on the GFR Record Form. The dose should also be prescribed separately (see PGD)(Rationales 62, 63 and 64).

 Weight Iohexol 300 dose (ml) 
 <40kg 2ml 
 >=40kg 5ml

Draw up the iohexol in an appropriate sized syringe (for a 2mL dose use a 2mL syringe and for a 5mL dose use a 5mL syringe) using a blue needle. Have this checked by a trained member of staff (Rationale 65). Discard the blue needle once the iohexol has been drawn up, label the syringe (Rationale 66).

Detach the blue needle and connect the syringe to the spare port of the three-way tap (Rationale 66).

Performing the test using peripheral venous access

An aseptic non-touch technique (ANTT) and standard precautions must be used throughout the procedure (Rationale 67).

Position the child on a chair, on a treatment couch or on his/her parent’s lap as appropriate (Rationale 68).

Whenever possible allow the child the freedom to select their chosen position (RCN 2010)(Rationale 69).

Ensure that the chosen position will be comfortable for the child, parents and HCPs for the duration of the procedure. Consider moving and handling risks.

Perform a thorough hand wash (Rationale 70).

Put on personal protective clothing and ensure that all HCPs assisting are similarly attired (Rationale 71).

Place yourself in a comfortable position, sitting facing the child and family (Rationale 72).

Check the child’s name, date of birth, hospital number and allergies against the prescription datasheet (Rationale 73).

Remove the local anaesthetic cream if used and wipe dry with a tissue or gauze.

If an occlusive transparent dressing was used, remove by stretching parallel with the skin (Rationale 74).

Confirm with the child, if appropriate, that the cream has caused numbness of the skin effectively (Rationale 75).

Apply an appropriate size tourniquet 5-8 centimetres above the chosen vein but not so tight to occlude arterial supply. Indications of occluded arterial supply include: loss of colour, compromised pulse and pain (Rationale 76).

Lightly tapping the vein or instructing the child to clench or pump the fist can encourage further venous filling.

Palpate the chosen vein (Rationale 77 and 78).

Cleanse the skin at the site at which it is intended to insert the needle, working outwards, with a 2% chlorhexidine gluconate/70% isopropyl alcohol solution for at least 30 seconds (Rationale 79). GOSH currently use Chloraprep® devices for this procedure (see antiseptic skin preparation)

Do not fan or blow-dry or otherwise attempt to accelerate the drying process.

Do not re-palpate the vein once the skin has been sterilised (Rationale 80).

Apply local anaesthetic spray if this is the child’s preferred pain management strategy, in accordance with product instructions eg should not be used on children aged under five years.

Advise the child that venepuncture will now occur (Rationale 81).

Using the butterfly needle or cannula, access one of the prepared venous sites.

If any routine blood tests are required, this is a good time to take them.

Collect a ‘pre-dose’ sample (0.5-1mL) in a white serum bottle. Label this as: pre-dose sample (Rationale 82).

Flush with 1-2mLs of sodium chloride (Rationale 83).

Remove the syringe with 0.9% sodium chloride and attach the iohexol syringe.

Administer the iohexol over 20-30 seconds. Note down the time to the nearest minute that the iohexol is administered on the GFR Record Form (Rationale 84).

Detach the iohexol syringe once administered and reattach the 0.9% sodium chloride syringe. Flush the remaining 0.9% sodium chloride through the butterfly or cannula (Rationale 85).

There is no need to flush the iohexol syringe. A small amount of iohexol will remain in the tip of the syringe, but this is accounted for during syringe calibration.

Continuously observe the butterfly/cannula site for blanching and any other adverse local reaction (Rationale 86).

In the event of iohexol infiltration into the surrounding tissue stop immediately.

If the infiltration occurs whilst administering the initial 0.9% sodium chloride flush, repeat venepuncture and attempt to administer the iohexol.

If infiltration occurs whilst administering the iohexol the test should be abandoned (Rationale 87).

Iohexol is non-ionic and the maximum volume used is 5mL for GFR: there is very little risk of tissue damage in the event of infiltration. However, if infiltration does occur, this must be documented in the health record and the test will be abandoned (Rationale 88).

Note the exact time of administration to the nearest minute using the same watch/clock throughout (Rationale 89).

Hold a piece of sterile gauze over the exit site but do not apply pressure (Rationale 90).

Slowly withdraw the butterfly needle/cannula, maintaining a neutral angle with the child’s skin.

Immediately apply gentle digital pressure to the exit site for three to four minutes (Rationale 91).

Place the used equipment carefully into the tray (Rationale 92).

Lift the soiled gauze and inspect the exit site to check that bleeding has ceased.

DO NOT wipe the exit site with the gauze (Rationale 93).

Once the bleeding has ceased apply the spot plaster (Rationale 94).

Record accurately (using the same watch/clock throughout) on the GFR Record form, to the nearest minute, the time the iohexol injection was administered. Note which clock/watch was used and use the same one to record blood sampling times. Also note which limb was used as blood sampling should ideally be taken from an alternate limb (Rationale 95 and 96).

Dispose of used equipment and sharps according to the Waste Policy, remove protective clothing and perform a thorough hand wash (Rationale 97 and 98).

If the procedure is abandoned for any reason dispose of any iohexol as for other IV drugs: empty vials may be disposed in a sharps bin, partially full vials should be disposed of as with other medicines (Rationale 99).

Performing the test using a central venous access device (CVAD)

If the child has a double lumen CVAD, the iohexol may be given through one lumen and the blood samples taken from the other lumen (Rationale 100).

Accurate recording of which lumen was used for which function is vital. There is space for this on the GFR Record Form (Rationale 101).

If the child has a single lumen CVAD, PICC or implantable port, the iohexol may be given through a butterfly needle and the blood samples collected through the CVAD, or the iohexol given through the CVAD and the blood samples collected peripherally (Rationale 101).

In either of these events the procedure for performing the test remains as described above.

Positive reinforcement and reward

Time should be taken to give positive feedback to the child for tolerating the invasive procedure (Rationale 102).

The parents should also be given positive feedback for their valuable contribution (Rationale 103).

If a story or game has been used for distraction purposes, allow the child to complete the activity (Rationale 104).

Any reward, however small, will help the child to feel a sense of achievement and control.

Subsequent blood sampling

When collecting the subsequent blood samples, all HCPs involved must:

Dispose of all waste and sharps in accordance with the hospital Waste Policy.

It is important that there is a delay of three hours between administration of iohexol and the second blood sample and an hour delay after the second blood sample (T=0hrs, T=3hrs and T=4hrs) (Rationale 106).

Take 1mL of blood at 180 and 240 minutes post injection from a either new butterfly or cannula (preferably inserted into an alternative limb from which the iohexol was administered), a finger prick, the indwelling central line (if the patient has one and provided iohexol was not administered via this) or an alternate lumen of a multiple lumen central line to the lumen used to administer iohexol. If the first sample is delayed, eg at 200 minutes, take the second sample 60 minutes later (Rationale 107).

If the expected GFR is less than 40mL/min, a further sample at 360 minutes (six hours) post injection is required. The requesting doctor should state whether this is the case (Rationale 108).

In some circumstances it may be appropriate to take blood samples using a finger prick (see capillary blood sampling guideline). An absolute minimum of 0.5mL of blood is required (Rationale 109).

It is very important to accurately record on the GFR Record Form the exact times at which samples are taken, to the nearest minute, from the same clock/watch (Rationale 110).

Place the blood sample in a white serum bottle and label accurately with the child’s details, date and time of sampling to the nearest minute. Place in a clear specimen bag.

All blood samples, PiMS Request Form and the GFR Record Form should be packaged together and sent to the biochemistry by chute or by porter collection on completion of the test. Additional patient name labels should also be included which should be on the PIMS forms. Where possible they should arrive in chemical pathology by 4:30pm (Rationale 111).

The HCP performing the procedure must sign and date the GFR Record Form upon completion of the test (Rationale 112).

Serum will be separated promptly and stored until the assay is performed (Rationale 113).

Summary of flowchart

  • Admit child, check allergies and contraindications.
  • Measure and record height and weight.
  • Peripheral dosing
    • Identify:
      • sites (x2) on different limbs
      • appropriate device
      • topical anaesthetic (cream/spray)
      • check prescribed dose of iohexol against body weight and draw up required dose in appropriate-sized syringe
      • access vein
  • Central line dosing
    • Single lumen central line - identify:
      • site for dosing
      • appropriate device
      • topical anaesthetic (cream/spray)
    • Multi-lumen central line: Choose one lumen for dosing and one lumen for post-dose sampling.
    • Check prescribed dose of iohexol against body weight and draw up required dose in appropriate sized syringe.
    • Clean needle-free access device on identified lumen as per guidelines.
  • Draw pre-dose blood sample.
  • Administer prescribed dose of iohexol, flush line/cannula/butterfly after dosing. Record time to nearest minute (NB use same watch/clock for subsequent sampling).
  • Dispose of equipment as per hospital policy.
  • 15 minutes after the dose has been administered the patient may leave the ward. Give patient/parents details of time to return for blood samples and dietary restrictions.
  • Post-dose blood sampling: Take samples at 180 and 240 minutes post-dose noting time to nearest minute. If GFR<40mL/min then third sample must be taken at 360 minutes post-dose.
  • Samples labelled with patient details and times, PiMS Request Form and GFR Record Form (signed and dated by practitioner performing procedure) should be sent to chemical pathology within working hours.
  • Document procedure in patient’s Health Care Record.


Rationale 1: Plasma creatinine concentration increases with age due to growth.
Rationale 2: Iohexol is a contrast media used for imaging. It is completely eliminated by glomerular filtration and not radioactive.
Rationale 3: An accurate result is calculated by measuring the quantity of the marker remaining in the circulating blood at these times.
Rationale 4: To prevent cross infection.
Rationale 5: To monitor any deterioration in renal function.
Rationale 6: To prevent excessive dosing of cytotoxic chemotherapy.
Rationale 7: The test is likely to fail due to abnormal distribution of the marker.
Rationale 8: Some free iodine may be liberated which is taken up by the thyroid gland.
Rationale 9: Risk of anaphylactic reaction.
Rationale 10: There is an increased risk of extravasation and likelihood that adequate post injection samples will not be available.
Rationale 11: This is likely to lead to abnormal losses of the marker.
Rationale 12: Dehydration may lead to deranged electrolytes, which could cause an inaccurate result. Iohexol may rarely precipitate acute renal failure in dehydrated individuals.
Rationale 13: Hyperhydration may affect the distribution of iohexol, and therefore the result of the test.
Rationale 14: Pyrexia may cause a false result due to insensible loss.
Rationale 15: This is to prevent avid uptake of any free iodine from iohexol ‘blocking’ the uptake of the radioiodine by the thyroid gland. GFR should be postponed until after therapy.
Rationale 16: Protein and caffeine may affect the measured GFR.
Rationale 17: There is a theoretically increased risk of allergic-type reactions in these children. This must be weighed against the benefits of obtaining an accurate GFR.
Rationale 18: Renal function matures during the first year of life so the test result may be less useful in this age group.
Rationale 19: Venous access may be difficult in very young children.
Rationale 20: Iohexol does not cause significant changes in plasma catecholamine levels in patients with phaeochromocytomas.
Rationale 21: There is a theoretical risk of developing lactic acidosis if metformin is given after iohexol treatment (Thomson Healthcare Inc. 2006).
Rationale 22: Concurrent administration of iohexol and amiodarone can prolong QT-interval (Goernig et al 2004). The risk is probably small as the dose of iohexol used for GFR measurement is low. However, the benefit of obtaining an accurate GFR must be weighed against the risk of ventricular arrhythmia.
Rationale 23: Iohexol is classified as risk category B by the US FDA. This means there is no known teratogenicity in animals. Furthermore, no human teratogenicity has been seen but there is an absence of clinical trial data to support this (Thomson Healthcare Inc. 2006). There may be a risk to the developing foetus with iohexol exposure, but it is probably small compared with many other medicinal products.
Rationale 24: Iohexol is excreted in small amounts in to breast milk. Iohexol is poorly absorbed from the gastrointestinal tract so there is minimal risk to the infant; cautious mothers may wish to avoid breast-feeding for 24 hours after the test.
Rationale 25: To maintain the child’s confidentiality, privacy and dignity and offer appropriate advice, support and counselling (United Nations General Assembly 1989).
Rationale 26: Iohexol is not known to be teratogenic and it can be handled safely by pregnant staff.
Rationale 27: The HCP should feel confident before undertaking the procedure to maximise the potential for success.
Rationale 28: The GFR and the sequence of events is a complex procedure requiring numerous skills. The potential for distress to the child, family and practitioners involved should not be underestimated.
Rationale 29: To develop a sound theoretical base and to assist him/her with problem solving in practice.
Rationale 30: To assure his/her competency and theoretical knowledge.
Rationale 31: To ensure that his/her knowledge remains valid, up to date and his/her practice safe and effective (GMC 2006; NMC 2002; NMC 2008).
Rationale 32: Samples from each patient can be processed together to ensure all paperwork/samples are present.
Rationale 33: Iohexol assays can be run as a batch either once or twice a week.
Rationale 34: To ensure that the child and family understand the reason for the procedure and are psychologically prepared. Well-informed parents are more likely to stay calm and will be in a position to support their child (Frederick 1991).
Rationale 35: To ensure that informed consent is obtained and to allow the family to develop coping strategies.
Rationale 36: Children do not usually develop coping strategies by themselves. Parents can often predict whether their child will co-operate.
Rationale 37: The better informed the child, the better able he will be to develop coping strategies (Sclare & Waring 1995).
Rationale 38: Active listening appears to be demonstrated mainly through non-verbal communication (Magnusson 1996).
Rationale 39: To anaesthetise the venepuncture sites.
Rationale 40: To enable explanation in a non-threatening manner and give the child the opportunity to express fears in a familiar environment (Broome 1990; Lansdown 1993; Action for Sick Children 2003).
Rationale 41: To prevent the child’s whole attention being centred on the invasive procedure. These techniques can help to distract and relax the child (Heiney 1991; Langley 1999).
Rationale 42: Dehydration may lead to deranged electrolytes, which could cause an inaccurate result. Iohexol may rarely precipitate acute renal failure in dehydrated individuals.
Rationale 43: To facilitate accurate calculation of the dose and to allow correction of the result based on surface area.
Rationale 44: Some drug metabolites may interfere with the assay (Nilsson-Ehle 2001). These can be corrected for using the ‘pre-dose’ sample but it is important to alert biochemistry to any potential interactions.
Rationale 45: Iohexol given in other tests will affect parameters used to calculate the GFR.
Rationale 46: There is a risk of developing lactic acidosis if metformin is given after iohexol treatment.
Rationale 47: There is a risk of iohexol causing a prolonged QT-interval in patients taking amiodarone.
Rationale 48: Children with bronchial asthma may be at increased risk of allergic-type reactions (Morcos 2005).
Rationale 49: To ensure that the child has had no previous allergic reaction to iohexol/other contrast media containing iodine.
Rationale 50: To enable appropriate equipment to be used and prescription of relevant medications to be considered (see Latex Allergy Policy).
Rationale 51: To ensure correct identification of the patient and to prevent critical incidents (see Patient Identification Policy).
Rationale 52: Some foods, eg protein load and/or caffeine stimulate the kidneys causing a false result (see appendix 3).
Rationale 53: To ensure samples and test data are attributed to the correct patient.
Rationale 54: The dose will be calculated using a recent measurement of weight.
Rationale 55: To ensure there is sufficient iohexol available for the test to proceed.
Rationale 56: Whilst iohexol is a relatively stable compound, dosing accuracy is crucial so new vials should be used wherever possible. This also reduces the risk of microbial contamination.
Rationale 57: To ensure that blood sampling can be completed with the same staff members, giving continuity of care and arriving at chemical pathology before staffing levels fall in the evening.
Rationale 58: To prevent cross infection.
Rationale 59: To allow easier palpation of the vein and manual dexterity in handling equipment.
Rationale 60: To prepare for the procedure.
Rationale 61: To ensure the smooth and efficient running of the procedure.
Rationale 62: To ensure an appropriate dose for the child’s size.
Rationale 63: To ensure that the correct dose is given to the correct child.
Rationale 64: To prevent medication errors.
Rationale 65: Using the correct sized syringe improves the accuracy of the dose.
Rationale 66: To minimise drug error
Rationale 67: To minimise the risk of cross infection.
Rationale 68: To minimise the trauma of the procedure.
Rationale 69: To maximise the chance of a successful procedure and to aid compliance by the child.
Rationale 70: To prevent cross-infection (see clinical procedure guideline: hand-washing).
Rationale 71: To prevent cross-infection.
Rationale 72: Good eye contact should be maintained to aid compliance with the procedure.
Rationale 73: To prevent medication error.
Rationale 74: This breaks the glue, causing less irritation and pain (Smith & Nephew Ltd 2003).
Rationale 75: To re-assure the child that the pain of the procedure will be minimised.
Rationale 76: To facilitate filling of the vein, enhance visualisation and to allow the assisting nurse to have his/her hands free for other tasks (Millam 1992).
Rationale 77: To ensure that it can still be found and is still considered suitable.
Rationale 78: To ascertain the calibre and direction of the vein.
Rationale 79: To clean the skin and prevent introduction of infection (Franklin 1998).
Rationale 80: To prevent re-contaminating the skin (Millam 1992).
Rationale 81: To allow the child to initiate planned coping strategies and to confirm implied consent.
Rationale 82: This is to ensure no iohexol is present before the test and to correct for any substances that may affect the assay.
Rationale 83: To check patency of the vein.
Rationale 84: To ensure the GFR can be accurately calculated.
Rationale 85: To ensure the whole dose is administered.
Rationale 86: To monitor for infiltration.
Rationale 87: Infiltration is a major cause of inaccurate results, as the whole dose will not enter the circulation.
Rationale 88: To ensure accurate record keeping and to conform to Trust policy.
Rationale 89: To facilitate accurate calculation of the GFR.
Rationale 90: Pressure should not be applied until the needle has been fully removed or it will cause the needle to be dragged out of the vein causing pain and venous damage.
Rationale 91: To absorb any blood that escapes and to prevent the formation of a haematoma.
Rationale 92: To prevent possible inoculation injury.
Rationale 93: This could re-activate bleeding by dislodging the thrombus.
Rationale 94: To prevent infection of the puncture site.
Rationale 95: To facilitate accurate calculation of the GFR and to determine the sites and times for the subsequent blood samples to be collected.
Rationale 96: To ensure accurate record keeping.
Rationale 97: To prevent inoculation injury.
Rationale 98: To prevent cross infection.
Rationale 99: To facilitate safe disposal.
Rationale 100: To ensure that the same lumen is not used for both functions.
Rationale 101: To ensure an accurate result.
Rationale 102: To pass control back to the child.
Rationale 103: To acknowledge the value of their involvement and teach parents mastery of the event, equipping them with coping strategies for future occasions.
Rationale 104: To conclude the procedure with a positive outcome and maintain/improve compliance for subsequent blood sampling.
Rationale 105: To minimise the risk of cross infection.
Rationale 106: To ensure the iohexol level will have dropped sufficiently to accurately measure the line slope.
Rationale 107: Sampling from the same limb as that used for administration of the iohexol may give a false result.
Rationale 108: Elimination of the marker is slower in these patients.
Rationale 109: Finger pricks may be preferred to cannulation where venous access is difficult.
Rationale 110: Accurate time keeping is imperative to calculate an accurate result.
Rationale 111: Keeping all blood samples and paperwork together for each patient enables.
Rationale 112: To ensure that laboratory staff know who to contact in the event of a query and to comply with the medication policy.
Rationale 113: To ensure that the samples are collected and processed in the right way at the right time.


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Document control information

Lead Author(s)

Zoe Wilks, Head of Nursing, Outpatients Department
Herdip Sidhu-Bevan, Lead Nurse, MDTS

Additional Author(s)

Joseph Standing, Pharmacist, Infection, Pharmacy
William van't Hoff, Consultant nephrologist, Nephro-Urology

Document owner(s)

Herdip Sidhu-Bevan, Lead Nurse, MDTS

Approved by

Guideline Approval Group

Reviewing and Versioning

First introduced: 
31 March 2004
Date approved: 
25 February 2014
Review schedule: 
Three years
Next review: 
25 February 2017
Document version: 
Previous version: