Glomerular filtration rate, GFR, is a measure of how fast the kidney is filtering through its 1 million glomeruli. GFR is the most useful test of overall kidney function and we have a separate section on measuring renal function. However because it is not straightforward to measure GFR directly, most of the time we use estimated GFR, eGFR, to give an approximate idea of how good kidney function is.
Most labs currently report eGFR estimated using the MDRD equation. This uses age, sex, and creatinine level in a calculation. It is complex so it is usually calculated in the lab where creatinine is measured and the result returned along with the creatinine result. It can also be calculated (a little less accurately) using an online calculator, e.g. the RA MDRD eGFR calculator – but read the important info on that page, and linked from it. Here are some of the things to be aware of:
- It is an estimate not a precise measure. It is most accurate in people who are quite well and in reasonable health. People at extremes – amputees, body builders, people who have lost a lot of weight during an illness – are likely to get the least accurate results.
- It was worked out from results in white and black patients in the US. It may not be so accurate for other races.
- It’s least accurate at good levels of kidney function, and gets better the worse kidney function is.
- It shouldn’t be used in under-18s, in pregnancy, or relied on in acute illnesses.
- More information about eGFR from the UK CKD eGuide.
The CKD-EPI equation is an alternative that is more accurate at higher GFRs, and which is gaining currency internationally. It is also too complex to calculate easily: the Kidney Australia CKD-EPI calculator is good. All the caveats described above for the MDRD equation apply equally to CKD-EPI.
CKD stages: eGFR is used to categorise reduced kidney function into 5 CKD stages.
Prescribing: eGFR is very useful for identifying patients who might be at risk from drug overdoses because their kidneys don’t get rid of the drug as fast as usual. However most drug datasheets only have instructions for adjusting dose according to creatinine clearance – so in some circumstances you may need to work that out before calculating the dose. More on measuring renal function
Improvements to come: research to identify more accurate equations, and to test MDRD and other methods with patients of different countries and races, is under way.
- Measuring Renal Function (edren textbook) – another page of this book