Problems/concerns with respect to using eGFR for the indentification of patients with CKD

Concerns regarding the use of eGFR in monitoring/assessing renal function (1):

• a test to monitor change in renal function over time in individual patients will need to be reproducible
  • serial serum creatinine measurements are useful in this context because they are analytically precise and vary little over time in patients with stable disease, although meat rich meals and other non-renal factors can interfere
• eGFR is a mathematical transformation of serum creatinine that takes into account three factors that have no measurement error and that do not change (sex and ethnic group) or change slowly (age)
  • the authors of a review (1) argue that, in comparison to serum creatinine, eGFR is NOT a more reliable measure of renal function - except in the longer term, as changes in estimated GFR mirror changes in creatinine
  • eGFR is arguably accurate enough to stage patients with known chronic kidney disease with a GFR less than 60 ml/min/1.73 m2 (1)
    • however different patients with the same serum concentration of creatinine can have widely divergent degrees of renal impairment
    • as eGFR improves, the scatter of estimated GFR plotted against reference GFR becomes progressively wider
      • an eGFR at the threshold value of 60 ml/min/1.73 m^2 could correspond to a reference method GFR between about 40 ml/min/1.73 m^2 and more than 100 ml/min/1.73 m^2
      • the review authors state that eGFR reflects the true filtration rate in patients with chronic kidney disease - however several studies have shown that formula estimated GFR underestimates renal function in people without known kidney disease

NICE outline other concerns about the use of eGFR (2):

• interpret reported values of eGFR 60 ml/min/1.73 m2 or more with caution, bearing in mind that estimates of GFR become less accurate as the true GFR increases
• where eGFR is simply reported as 60 ml/min/1.73 m2 or more, use a rise in serum creatinine concentration of more than 20% to infer significant reduction in renal function
• where a highly accurate measure of GFR is required - for example, during monitoring of chemotherapy and in the evaluation of renal function in potential living donors - consider a gold standard measure (inulin, 51Cr-EDTA, 125I-iothalamate or iohexol)
• in cases where there are extremes of muscle mass - for example, in bodybuilders, amputees or people with muscle wasting disorders - interpret the eGFR with caution. (Reduced muscle mass will lead to overestimation and increased muscle mass to underestimation of the GFR)
• advise people not to eat any meat in the 12 hours before having a blood test for GFR estimation. Avoid delaying the despatch of blood samples to ensure that they are received and processed by the laboratory within 12 hours of venepuncture
• an eGFR result less than 60 ml/min/1.73 m2 in a person not previously tested should be confirmed by repeating the test within 2 weeks. Make an allowance for biological and analytical variability of serum creatinine (+/-5%) when interpreting changes in eGFR

Reference:

• (1) Giles PD, Fitzmaurice DA. Formula estimation of glomerular filtration rate: have we gone wrong?BMJ 2007;334:1198-1200
• (2) NICE (September 2008). Chronic Kidney Disease - Early identification and management of chronic kidney disease in adults in primary and secondary care