eGFR - a simple guide
Estimated GFR – what is it?
Serum creatinine is one of the most common requested tests, and is commonly felt to be an indicator of renal function. That, to a certain extent, is true, but the relationship between serum creatinine and renal function is not linear, and is not identical between individuals. There is a clinical need to pick up renal disease as early as possible, and the Renal NSF has indicated that eGFR should be adopted on a national basis.
eGFR is an estimation of Glomerular Filtration Rate based on serum Creatinine level, age, sex, and race. The most widely used method for this estimation is the abbreviated MDRD equation, as it has proved the most robust and accurate. It was developed in the Modification of Diet in Renal Disease study. All but the race are generally available on the request form, and consequently an eGFR can be generated for virtually all patients in whom a serum creatinine is requested. The only ethnic correction that is required is to multiple by about 1.20 for black ethnic groups. As the normal GFR is about 100 mls/min/1.73 m2, one can think of the eGFR as percentage kidney function. In addition, unlike creatinine clearance results, the GFR is corrected for body surface area. It is not perfect, but it is probably better than using serum creatinine alone, and is more reliable and easier to carry out than a 24 hour urine collection for creatinine clearance.
The laboratory will use the 4 variable (abbreviated) MDRD. The only correction that is required from the reported value is to adjust for ethnic background (multiply by 1.21 if Afro-Caribbean).
The interpretation of eGFR has a number of caveats, especially in patients with normal or near normal function. When the serum creatinine is well in the normal range it does tend to underestimate the true GFR, for example. It is not robust at the extremes of body habitus, particularly if they have very little muscle mass, or have large muscle bulk, as seen in atheletes. A few drugs, such as trimethoprim, inhibit the excretion of creatinine and can elevate the serum creatinine concentration, reducing the eGFR. It is not appropriate to quote an eGFR based on a serum creatinine in an unstable situation, such as acute renal failure - eGFR is based on a steady state situation.
Importantly, the staging is only applicable if the abnormality has been present for more than 3 months. However, it is a useful serial measurement and is helpfully in guiding investigation and treatment.
What does this mean in Southern Derbyshire?
For GPs or any clinician requesting basic biochemistry, no additional work will be required. The clinical chemistry department will make a calculation of the estimated GFR and report the value (if below 60 mls/min) and the stage of chronic kidney disease (CKD). The full guidelines are given on this website, plus access to renal advice (telephone, fax, email or letter) will be available when this project is launched, since we realise that it is a relatively new concept. However we hope to provide a simple and robust system to meet the clinical need and at the same time improve the care of patients with CKD.
Thanks to Dr Nigel Lawson (Department of Clinical Pathology, Derby Hospitals NHS Foundation Trust) for the slides.