What eGFR actually estimates, and what it does not

Estimated glomerular filtration rate appears on most routine chemistry panels and is one of the more widely misread numbers on a laboratory report.

The reason is that it looks like a measurement and is not one. It is the output of an equation whose main input is creatinine, and almost everything surprising about it follows from that.

What is being estimated

Glomerular filtration rate is the volume of fluid the kidneys filter per minute. Measuring it directly requires infusing a marker substance and collecting timed samples, which is accurate, slow and impractical for routine use.

The estimate substitutes creatinine, a waste product of muscle turnover that the kidneys clear. If production is steady, the concentration in blood reflects how well it is being removed.

The equation then adjusts for age and sex, because those correlate with muscle mass, which drives creatinine production.

Why muscle mass drives everything

A muscular person produces more creatinine and will have a higher blood level at any given filtration rate, which the equation reads as worse kidney function. A person with low muscle mass produces less, and the equation reads better function than is really present.

This is why eGFR is least reliable at the extremes of body composition, and why a bodybuilder and a frail elderly person can both have estimates that misstate their actual kidney function in opposite directions.

A large meat meal shortly before the sample raises creatinine measurably, as does intense exercise in the preceding day.

Why it is reported with a ceiling

Many laboratories report anything above about 90 as greater than 90 rather than an exact figure. This is deliberate.

The equations were developed and validated mostly in people with reduced kidney function, and they are imprecise at the upper end. Reporting a precise-looking 112 would imply an accuracy the calculation does not have.

An estimate reported as above the ceiling is therefore not a maximum score. It means the calculation cannot distinguish usefully within that region.

When the estimate should not be trusted

The equations assume a steady state — creatinine production and clearance in balance. During rapidly changing kidney function, that assumption fails and the estimate lags behind reality, sometimes by a day or more.

Pregnancy, significant limb amputation, severe liver disease and unusual diets all shift the relationship between creatinine and filtration. Where accuracy matters in these situations, alternative markers such as cystatin C are sometimes used instead.

Different equations exist and laboratories have moved between them over the years. A change in reported eGFR that coincides with a laboratory switching equations is not a change in the person.

How it is actually used

The trend matters more than any single value. A stable estimate over years carries a very different meaning from the same number arriving after a run of higher ones.

It is also used practically, to adjust doses of medicines cleared by the kidneys. This is one reason it appears on routine panels even when kidney function is not the question being asked.

Because it is calculated rather than measured, it is worth reading alongside the creatinine it came from, which is the actual measurement.