Estimated GFR

eGFR (Estimated Glomerular Filtration Rate) is a calculated measure of how well your kidneys are filtering waste products out of your blood each minute. The key word is *estimated*, eGFR is not measured directly. It is derived from your creatinine level using a formula that adjusts for age and sex, based on the assumption that creatinine production is broadly predictable. For most people that assumption holds and eGFR is an excellent screen for kidney function. For some people it does not, and the result can be misleading in both directions.

Because eGFR is derived from creatinine, anything that raises your creatinine level independently of kidney function will make your eGFR look lower than your actual filtration capacity. In other words, your kidneys may be working fine, but the formula reports a deceptively low number. The most common causes of this falsely low pattern are: high lean muscle mass (bodybuilders, rugby players, heavy lifters), a high-protein diet, creatine supplementation, very strenuous exercise in the days before the test, and dehydration. A healthy 30-year-old on creatine who lifts weights five days a week can easily show an eGFR in the 60s or 70s without any kidney problem at all, the creatinine is simply elevated from muscle turnover. Conversely, people with unusually low muscle mass (frail elderly, some chronic illnesses) can show a deceptively normal eGFR while their actual kidney function is reduced.

When an eGFR result seems at odds with clinical context (young, healthy, active individuals with low numbers; frail individuals with high numbers), Cystatin C is the right next step. Cystatin C is a protein produced at a roughly constant rate by all nucleated cells in the body and is cleared almost entirely by the kidneys. Unlike creatinine, it is not meaningfully affected by muscle mass, diet, or exercise. The eGFR calculated from Cystatin C (eGFR-cystatin) is a more accurate reflection of true kidney filtration in anyone whose creatinine-based eGFR might be confounded, and it is increasingly used in primary care as a confirmatory test before diagnosing chronic kidney disease.

Even with those caveats, eGFR is still the primary screening marker for kidney function because it is cheap, automatic from any standard chemistry panel, and works well for the typical patient. A persistently low eGFR (confirmed on repeat testing, ideally with Cystatin C in unclear cases) indicates reduced kidney function and warrants investigation for causes including diabetes, hypertension, and glomerular disease. A high eGFR is usually not a concern, it can reflect hyperfiltration in early diabetes or pregnancy but is often simply an efficient kidney in a young adult. If your eGFR is below the expected range or has changed significantly since a previous test, discuss it with your GP, who can consider your clinical context, ask about muscle mass and supplements, and arrange Cystatin C if appropriate.