eGFR explained: what does this kidney value say about your kidneys?

The eGFR is arguably the key value for kidney function, and at the same time one of the most confusing. It is not a measured substance but a calculated number. I find that estimate makes people uneasy, while the calculation is exactly what makes it more useful than creatinine alone (Levey 2009).

This article expands on the overview about your kidney function, focusing on the eGFR.

What is the eGFR?

eGFR stands for estimated glomerular filtration rate. Put simply, it is an estimate of how much blood your kidneys filter per minute. A higher number means better filtering. It is expressed in millilitres per minute per 1.73 square metres of body surface, which makes results comparable between people. The e stands for estimated: it is a calculation, not a direct measurement.

How is the eGFR calculated?

The eGFR is derived from your creatinine, plus your age and sex. Because creatinine depends heavily on muscle mass, the formula partly corrects for that. Different formulas exist, so your eGFR can vary slightly between labs without anything changing in your kidneys.

What do the numbers mean?

A mildly low eGFR is fairly common in older people, since filtering naturally declines with age (Levey 2012). See also high creatinine.

Why one eGFR is a snapshot

A single eGFR says less than you might think. It can fluctuate through low fluid intake, fever or heavy exercise before the test. Imagine testing on a hot day after a hard run with too little water: your eGFR can read lower than usual. A repeat measurement on a calm day often gives a fairer picture.

What can you do?

To map your kidney function, you can have a kidney function test at Vitalcheck without a referral. If your eGFR is low, discuss it with your GP, who combines it with your creatinine and symptoms. See also eGFR and kidney function blood test.

References

• Levey AS, et al. A new equation to estimate glomerular filtration rate. Ann Intern Med. 2009. PMID: 19414839.
• Levey AS, et al. Chronic kidney disease. Lancet. 2012. PMID: 21840587.
• Thuisarts.nl. About your kidneys.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.