eGFR: normal values and what a low eGFR means

Your eGFR estimates how many millilitres of blood your kidneys filter each minute. The e stands for estimated. The value is calculated rather than measured directly. The laboratory enters your creatinine, age and sex into an equation.

The standard equation is CKD-EPI. Its 2021 version deliberately works without a race coefficient. Older equations did apply one. They overestimated kidney function in Black patients and delayed referral. Some European laboratories also use the EKFC equation. CKD-EPI and EKFC can differ slightly in the same person. Check which equation your report names.

The figure is scaled to a standard body surface area of 1.73 square metres. It answers what your filtration would be at an average body size. For a very large or very small body the true filtration differs.

Creatinine comes from muscle tissue. Muscle mass therefore affects the result. Men on average carry more muscle than women. That is why sex appears in the equation. In a woman with little muscle the eGFR can look better than her kidney function truly is.

An eGFR of 90 or above counts as normal. Between 60 and 89 filtration is mildly reduced. On its own that is often not disease, particularly at older ages. Between 45 and 59 the stage is called G3a. Between 30 and 44 it is stage G3b. Between 15 and 29 the reduction is severe. Below 15 the term used is kidney failure.

A single value under 60 does not mean kidney disease. That diagnosis needs the value to stay low for at least three months. Protein in the urine also counts in that assessment. One result of 58 is mainly a reason to repeat the test.

A high eGFR is not a good report card. Many laboratories simply report greater than 90. The equations are imprecise at high filtration rates. Sustained high filtration is called hyperfiltration. It is seen in early diabetes and in obesity. It is a risk signal rather than a sign of health.

eGFR also falls with age without any disease. After roughly age forty the decline is about 1 ml/min per year. A value of 65 at age 80 is therefore often simply normal.

eGFR is usually calculated automatically whenever creatinine is measured. That happens in standard blood work. The value carries more weight in diabetes and in high blood pressure. Cardiovascular disease and kidney disease in the family also count.

The equation is only valid in a steady state. During acute illness the result does not hold. Creatinine then lags a day or more behind true filtration. The equations are also not validated during pregnancy. Filtration rises physiologically by 40 to 50 percent then.

Several everyday things lower a calculated eGFR without touching the kidney. Creatine supplements raise blood creatinine. A large meat meal does the same. Hard training and dehydration push the same way. The antibiotic trimethoprim blocks creatinine excretion. In all these cases kidney function itself is unchanged.

When the result does not fit the person, cystatin C helps. That marker does not come from muscle tissue. In case of doubt the combined equation gives the most reliable picture. Always discuss an abnormal value with your doctor.