Kidney function: creatinine and eGFR

Creatinine is muscle waste

Creatinine is a by‑product of normal muscle metabolism, cleared almost entirely by the kidneys. If the kidneys filter less, it accumulates. That is the whole principle, and it is why the test is cheap and universal.

It also explains the test's central weakness: how much creatinine you produce depends on how much muscle you have. A heavily muscled man and a small, frail older woman with identical kidney function will have very different creatinine levels. The number is not comparable between people in the way a blood sugar is.

eGFR: a calculation, not a measurement

eGFR — estimated glomerular filtration rate — is an attempt to correct for that. It is calculated from your creatinine together with your age and sex, using a published equation. The current standard is the CKD‑EPI equation; your laboratory may say which it used.

Two things follow from it being an estimate:

• It is least reliable where people most want it — in the near‑normal range, and in anyone whose muscle mass is unusual. Bodybuilders, amputees, people with wasting illness, and the very frail are all poorly served by it.
• Creatine supplements raise creatinine without kidney damage, and so lower the estimate. Say if you take it.

The race coefficient, and its removal

Older versions of these equations included a coefficient that raised the estimated GFR for Black patients, based on population differences in average creatinine. In 2021 a joint task force of the American Society of Nephrology and the National Kidney Foundation recommended removing it, and a race‑free CKD‑EPI equation was adopted widely.

Worth knowing because results from before and after that change are not directly comparable for some patients. If you are tracking eGFR over several years, the equation may have changed underneath you.

Cystatin C, the second opinion

Cystatin C is a different filtered protein, and its level depends much less on muscle. Where the creatinine‑based estimate is doubtful — unusual body composition, or a result that does not fit the person — it gives an independent estimate, and combining both is more accurate than either.

It is an immunoassay, so it sits in the expensive half of the price list, and not every laboratory here offers it. It is a second‑line test, ordered for a reason rather than routinely.

What creatinine alone does not catch

Filtration is only one kidney job. The other common early sign of damage is protein leaking into the urine, and that is invisible to any blood test.

Urine albumin‑to‑creatinine ratio is the test for it. In diabetes and hypertension it often turns abnormal before eGFR falls, which makes it the earlier warning of the two. It needs a urine sample rather than blood, which is the only reason it is not on more panels.

Why it is in our baseline

Two reasons, and the second is the one that matters operationally.

First, kidney disease is common in exactly this population — it travels with diabetes, high blood pressure and central obesity — and it is silent until it is advanced.

Second, dosing. Several medicines used in metabolic care are cleared by the kidneys or are not recommended below a certain filtration rate. Metformin is the obvious one. Prescribing without a baseline creatinine is prescribing without knowing whether the dose is safe, and at ₡8,000 it is among the cheapest tests on the list.

The boundary

A single eGFR is a snapshot of an estimate. Chronic kidney disease is defined by abnormality persisting over at least three months, which means the repeat is part of the diagnosis rather than an optional extra. What your number means — and whether it changes what you can safely take — is a question for a physician with the whole panel in front of them.