By Susan George Former Head of Department (Biochemistry)
The GFR is a vital measure of kidney function. It is used for staging and monitoring progression of Chronic Kidney Disease (CKD), as well as drug dosing decisions. Serum Creatinine is the most widely performed test for the assessment of GFR, however interpretation is complicated by the reciprocal relationship between serum Creatinine and GFR, as well as the effects of age, sex, body size and diet.
Recently a formula for estimating the GFR has been developed for which the only inputs are the age, sex and serum creatinine. This formula is called 'abbreviated MDRD' named after the Modification of Diet in Renal Disease study where it was developed.
Routine reporting of eGFR will be done for every serum creatinine request in adults. The aim is to enhance recognition of moderate to severe kidney disease at a stage when it may still be clinically silent and allow valuable intervention to delay the disease progression. For this reason, serum creatinine results should be reported in μmol/L as any calculations should be performed on the most precise available data.
The equation/formula takes into consideration a result corrected for body surface area (BSA) for an average person. Results will be reported in mL/min/1.73 m2.
The eGFR should be reported for all requests for serum creatinine with the following exceptions
- Patients below 18 years of age (no eGFR will be reported)
- Patients on dialysis treatment (no eGFR will be reported)
All requests should therefore have the gender of patient and whether patient is on dialysis clearly stated on request form.
Comments that follows result:
|If result 30-59 mL/min/1.73m2:
||eGFR 30-59 mL/min/1.73m2 suggests moderate chronic kidney disease and indicates the need for further investigation including assessment of proteinuria and cardiovascular risk factors.
|If result is <30mL/min/1.73m2:
||eGFR <30 mL/min/1.73m2 usually indicates a need for referral for assessment and management of Chronic Kidney Failure disease
It is important to note that a result of > 60 does not imply a normal GFR. When the GFR is reported > 60, then assessment should be made on usual criteria e.g. comparison of serum creatinine with previous result from the same patient if available or against population-based reference intervals if previous results not available.
Limitations of the MDRD eGFR
The MDRD formula generally provides an excellent estimate of GFR. In majority of cases it is more accurate and convenient than Cockcroft and Gault estimates or measurement of 24hr creatinine clearance urine sample. There are however limitations of the MDRD estimate of GFR that the doctors need to be aware of:
- The formula is not appropriate for persons under 18 years of age. Use of other formula or direct measurement is recommended.
- The formula will give erroneous results in patients on dialysis.
- The formula may misrepresent GFR in cases of very rapidly changing renal function due to delays in accumulation or removal of creatinine from serum in such cases. Note this limitation applies to any assessment of renal function based on serum creatinine.
- Exceptional dietary intake such as vegetarian, high protein or creatine intake may influence tests results.
- Severe liver disease may markedly influence results with over-estimation renal function in the moderate or severe renal impairment range.
- Extremes of body composition such as emaciation, limb amputation or paraplegia. Note that the formula has been validated in obesity.
- The formula is not validated in pregnancy.
- The MDRD formula has not been validated for drug dosing.
- The formula is dependent on the creatinine measurement; so any drug interference in the serum creatinine will affect the results.
- The reliance of serum creatinine measurements also affects precision and accuracy. When monitoring a patient over time it is preferable to use results from the same laboratory. A change in GFR of > 15% in results from the same laboratory indicates that the change is unlikely to be due to random variance. Repeat testing on a new sample should be performed to confirm significant changes in eGFR.
- Serum creatinine remains an important test. Changes of serum creatinine in a patient are the most sensitive test of changes in renal function, especially with results in or near the reference interval. A serum creatinine above the population reference interval, even if the eGFR is normal, may require further investigation.
eGFR is a crude index of kidney function, susceptible to a variety of confounding factors. However, despite major limitation, the use of eGFR will improve the recognition and probably the subsequent management of patients with CKD, compared with the use of serum creatinine alone.
It is very likely that the MDRD formula will gain widespread acceptance since patient weight is not required to enable calculation of eGFR and hence implementation is likely to be facilitated.
Further validation of MDRD formula in a variety of clinical settings and at varying levels of kidney function is still required. In the longer term, we anticipate that improved formulae based on reference creatinine methodology or alternative approaches to GFR measurement will supplant the use of the MDRD formula.
The Australasian Creatinine Consensus Working Group; The Clinical Biochemist Reviews Vol 26(3) Aug 2005