People with chronic kidney disease faced a higher risk of death and serious health problems when two tests of kidney function did not agree with each other.
The findings suggest that using both of these blood tests to track kidney health could help identify patients at increased risk for health problems.
Two common blood tests can be used to estimate kidney filtration rates.
The kidneys keep us healthy by filtering toxins, electrolytes, and excess water out of our blood. Unfortunately, more than 35 million people nationwide have chronic kidney disease. Affected people are far more likely to experience kidney failure, develop cardiovascular disease, or suffer from other life-threatening health problems.
One way doctors estimate those risks is by tracking a measure called estimated glomerular filtration rate (eGFR). People with a higher eGFR have kidneys that are better at filtering their blood.
A person’s eGFR can be determined with a blood test that measures either a byproduct of metabolism called creatinine or a small protein called cystatin C. Various factors besides kidney function can influence each test. As a result, the two measures sometimes produce different eGFR values for the same person. Previous studies suggest a 30% difference is clinically concerning.
A research team led by Drs. Morgan Grams and Josef Coresh at the New York University Grossman School of Medicine recently explored how often creatinine and cystatin C produce conflicting eGFR results. They used data from an NIH-funded initiative that has been collecting data on kidney function and damage from people around the world since 2009. The results were published on November 7, 2025, in the Journal of the American Medical Association.
The team analyzed data from more than 800,000 patients who had their eGFR determined by using both creatinine and cystatin C tests in an outpatient setting. They also included nearly 40,000 hospitalized patients who received both tests during their hospital stay. The team also tracked the outpatient participants’ health for an average of 11 years after their eGFR tests.
The team found that about 11% of the outpatient participants and 35% of the hospitalized participants had cystatin C-based eGFR values at least 30% lower than their creatinine-based eGFR. Among outpatients, those with this large difference were more likely to die from any cause during the study period. They were at increased risk of developing cardiovascular disease and dying from it. They also had an elevated risk of heart failure and kidney failure. The larger the difference between the two eGFR values, the greater the risk.
On the other hand, outpatient participants whose cystatin C-based eGFR was at least 30% higher than their creatinine-based eGFR had a lower risk for most of these health outcomes compared to those with smaller eGFR differences.
The study highlights the importance of determining eGFR by measuring both cystatin C and creatinine levels. Doing so could help identify patients with chronic kidney disease who have increased health risks. They may need more intensive monitoring and treatment. More study will be needed to understand what underlies the differing eGFR test results.
“Our findings highlight the importance of measuring both creatinine and cystatin C to gain a true understanding of how well the kidneys are working, particularly among older and sicker adults,” Grams says. “Since neither test is perfect, evaluating both biomarkers may identify far more people with poor kidney function, and earlier in the disease process, allowing for preventative intervention."
Source: U.S. National Institutes of Health
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