Cystatin C Versus Creatinine: Why Two eGFR Estimates Can Disagree

Two readings from one blood draw

When a laboratory returns two estimated glomerular filtration rates from the same sample, one anchored on creatinine and one on cystatin C, the two numbers can diverge by 15 mL/min/1.73 m2 or more, the same size gap that studies use to define large discordance. Neither molecule is a pure measure of filtration, which is the whole reason they disagree. Creatinine rises and falls with muscle mass, meat intake, and drugs that block its tubular secretion, while cystatin C tracks body fat, inflammation, thyroid status, and steroid exposure. The 2024 KDIGO guideline answers this by keeping creatinine as the first-line estimate and adding cystatin C when precision matters, and a 2025 meta-analysis shows that when the cystatin C estimate falls well below the creatinine one, the gap itself predicts higher mortality.

What each marker is actually reporting

Creatinine is a byproduct of muscle metabolism. It is freely filtered at the glomerulus and partly secreted by the tubules, so its blood level reflects far more than kidney function. A frail, low-muscle patient generates little creatinine, so a creatinine-based estimate (eGFRcr) can read reassuringly high while true filtration is poor. Diet, physical activity, and medicines such as trimethoprim or cimetidine shift the number further.

The pattern shows up predictably in specific bodies. A heavily muscled person or someone eating a high-meat diet can look worse on creatinine than their kidneys really are, while a person with sarcopenia, cirrhosis, or a spinal cord injury can look deceptively well because there is simply less muscle to generate the marker.

Cystatin C is a small protein produced at a fairly steady rate by every nucleated cell, freely filtered and not meaningfully secreted. Because it is largely independent of muscle, it often outperforms creatinine in people with unusually low or high muscle mass. It carries its own baggage, though. As the NIDDK summarizes, adiposity, corticosteroids, thyroid dysfunction, and inflammation can all raise cystatin C independently of filtration. Each marker corrects for some of the other's blind spots without being clean on its own.

Why KDIGO 2024 leans on cystatin C

The 2024 KDIGO guideline still starts with eGFRcr, because creatinine is cheap, fast, and universally available. Its notable step is a graded recommendation (strength 1C) to use the combined creatinine and cystatin C estimate, eGFRcr-cys, in clinical situations where eGFRcr is less accurate and where GFR actually drives a decision. The evidence for combining the two comes largely from the 2021 CKD-EPI work published in the New England Journal of Medicine, where Inker and colleagues built race-free equations and found the combined equation more accurate than either marker alone, with smaller differences between Black and non-Black participants. The guideline also asks laboratories to use enzymatic creatinine assays, and it points to three groups who gain most: frail or multimorbid patients, anyone needing a precise GFR for a specific decision such as chemotherapy dosing or transplant workup, and those at high risk of kidney-related complications.

The disagreement is itself a signal

A discordant pair is often read as noise to be averaged away. The 2025 systematic review and meta-analysis in Clinical Kidney Journal by Liu and colleagues argues the opposite. Across 18 studies ranging from 373 to more than 363,000 participants, the authors standardized the exposure so that a lower cystatin C estimate relative to the creatinine estimate was the reference direction. They treated a large discordance as the cystatin C estimate falling to 60 percent or less of the creatinine estimate, or a difference of at least 15 mL/min/1.73 m2 between the two. When eGFRcys ran substantially below eGFRcr, all-cause mortality was markedly higher (hazard ratio 1.58, 95% CI 1.42 to 1.76) and cardiovascular events rose as well (hazard ratio 1.32, 95% CI 1.25 to 1.39). The mirror image held: a higher cystatin C estimate tracked with lower risk. One proposed explanation is selective glomerular hypofiltration syndrome, in which filtration of mid-sized molecules is impaired out of proportion to creatinine handling, layered on top of the two markers' different production rates.

How to read the mortality signal

For anyone appraising this evidence, direction is everything. The prognostic weight sits on the eGFRcys-lower side, so the first question is always which marker is lower before any conclusion is drawn. The second point is that the association is not a clean causal arrow. The discordant pattern travels alongside muscle wasting, chronic inflammation, and multimorbidity, each of which independently raises mortality, so the gap may be partly a barometer of overall illness burden rather than a distinct renal lesion. It flags a sicker patient; it does not show that cystatin C is harmful. Heterogeneity and residual confounding across the pooled studies reinforce that caution, though the consistency of direction across 18 datasets is the reassuring part. The practical payoff is concrete: a single confirmatory cystatin C can reclassify a CKD stage, change dosing for narrow-margin drugs, and sharpen a risk estimate that a creatinine reading alone would have gotten wrong.

This article is educational and not medical advice; decisions about kidney testing and interpretation belong with a qualified clinician.