Understanding Kidney Tests: Creatinine and Beyond
For decades, clinicians have relied on blood tests that measure creatinine to gauge kidney function. Creatinine is a waste product produced by muscles, and its level in the blood helps estimate how well the kidneys are filtering. The most common tool derived from this measurement is the estimated glomerular filtration rate (eGFR). A lower eGFR suggests reduced kidney function and can trigger closer monitoring or treatment to slow progression.
In recent years, researchers have explored additional markers that may paint a more complete picture of kidney health. One such marker is cystatin C, a protein produced by all nucleated cells. Because cystatin C is filtered by the kidneys in a way that is less influenced by muscle mass, it can sometimes reveal problems that creatinine misses. As a result, doctors increasingly compare eGFR estimates derived from creatinine with those derived from cystatin C to identify potential discrepancies.
What Do Discrepancies Between Tests Signify?
A mismatch between creatinine-based eGFR and cystatin C-based eGFR, or other kidney function assessments, may signal several important possibilities. In some cases, a discrepancy could indicate unusually high muscle mass or recent illness that temporarily affected creatinine levels. In other situations, the divergence between tests raises concern that the kidneys are not filtering in a consistent, reliable manner, which may be a harbinger of underlying kidney disease or future health risks.
Emerging evidence suggests that people whose kidney function estimates diverge between tests face higher risks independent of the measured eGFR. Studies have linked such discrepancies to an increased likelihood of kidney disease progression, cardiovascular events, and even mortality. The exact reasons are still being investigated, but several factors could contribute: variability in muscle mass, inflammation, comorbid conditions like diabetes or high blood pressure, or early-stage kidney damage that alters filtration in ways not captured by a single test.
What This Means for Patients and Providers
If your doctor notes a discrepancy between kidney function tests, it does not mean an immediate crisis. Rather, it should prompt a closer look. Physicians may:
– Confirm results with repeat testing to rule out lab error.
– Consider additional tests such as cystatin C alongside creatinine to refine eGFR estimates.
– Check for other signs of kidney damage, including protein in the urine (albuminuria).
– Assess cardiovascular risk factors and address them aggressively where possible (blood pressure, cholesterol, diabetes management).
– Review medications and exposures that could affect kidney function.
For patients, awareness is key. If you’ve recently learned that your creatinine-based eGFR doesn’t align with a cystatin C-based estimate, discuss what this means in your specific context. Ask about the need for follow-up tests, lifestyle changes, or therapies that can protect kidney function and reduce cardiovascular risk.
Practical Steps to Protect Kidney Health
Beyond medical management, there are actionable steps people can take to support kidney health:
– Maintain blood pressure within target ranges and control diabetes if present.
– Follow a heart-healthy diet rich in fruits, vegetables, and whole grains while limiting processed foods high in sodium.
– Stay hydrated and avoid overuse of NSAIDs or other nephrotoxic substances without medical supervision.
– Engage in regular physical activity tailored to your abilities.
– Monitor kidney-related lab results with your healthcare provider and seek clarification whenever results are unclear or conflicting.
Looking Ahead: Why Research on Test Discrepancies Matters
As scientists continue to investigate why some tests disagree and what that means for long-term health, clinicians may gain better tools to predict who is at risk and tailor prevention strategies. The ultimate goal is clear: identify people at higher risk before serious kidney or cardiovascular problems develop and intervene early with personalized care.
Bottom Line
Discrepancies between common kidney function tests, such as creatinine-based eGFR and cystatin C-based estimates, can be a meaningful alert signal. They may indicate a higher risk of kidney failure, heart disease, and death, especially when paired with other risk factors. If your results show discordant readings, work with your healthcare provider to investigate further, refine your risk assessment, and implement proactive steps to protect your kidneys and heart.
