Summarized & reviewed by The Peptide Dispatch Editorial Team · Last reviewed July 20, 2026
Almost every blood panel run in the United States reports a number called eGFR, the estimated glomerular filtration rate. It is the standard measure of kidney function, it comes back as a tidy two-digit figure, and it is calculated from a single input: serum creatinine. What almost nobody is told is that there is a second way to estimate the same thing, using a protein called cystatin C, and that…
This dispatch covers Your Kidney Function Has Two Different Answers, and the Gap Between Them Predicts Mortality in the metabolic research category, authored by The Peptide Dispatch Editorial Team. Estimated reading time: 9 minutes. The Peptide Dispatch curates peer-reviewed peptide research for self-directed learners. All summaries are presented for Research Use Only and do not constitute medical advice.
Almost every blood panel run in the United States reports a number called eGFR, the estimated glomerular filtration rate. It is the standard measure of kidney function, it comes back as a tidy two-digit figure, and it is calculated from a single input: serum creatinine. What almost nobody is told is that there is a second way to estimate the same thing, using a protein called cystatin C, and that the two methods frequently disagree. Not by rounding-error amounts. By thirty percent or more, in roughly one in nine ordinary outpatients. The interesting part is what happens next. Over the past three years the research has converged on a finding that is genuinely counterintuitive: the disagreement between the two tests carries prognostic information that neither number carries on its own. The gap is the signal. ## Why there are two tests in the first place Creatinine is a waste product of muscle metabolism. Your muscles produce it at a fairly steady rate, your kidneys filter it out, and the concentration left in your blood is used as a proxy for how well the filtering is going. High creatinine implies poor filtration. The obvious weakness is that creatinine production depends on how much muscle you have and what you eat. More muscle mass means more creatinine generated at any given level of kidney function. Less muscle means less. Meat intake raises it. Some medications block its tubular secretion without touching filtration at all. The equations used to convert creatinine into eGFR try to correct for this with age and sex terms, but those are population averages applied to individuals, and individuals vary. Cystatin C is a small protein produced at a relatively constant rate by essentially all nucleated cells in the body. It is freely filtered by the glomerulus and then almost entirely reabsorbed and broken down in the tubules. Critically, its production is far less dependent on muscle mass and diet. It is not perfectly independent of everything, and we will get t…
Educational content — not medical advice. Effects described are drawn from cited research in study subjects.