A new study shows that when two common tests for kidney function give very different results, it may be a sign of serious health risks.
This mismatch could point to a higher chance of developing kidney failure, heart problems, or even dying earlier.
For many years, doctors have used a blood test called creatinine to check how well a person’s kidneys are working. Creatinine is a waste product made by muscles, and healthy kidneys filter it out of the blood.
But in recent years, another test has become more common. This test looks at a protein called cystatin C, which is made by all the cells in the body. Like creatinine, high levels of cystatin C in the blood can mean the kidneys aren’t working well.
The problem is that these two tests often don’t match. In fact, the new study found that many people—especially those who are sick—had very different results from the two tests. In more than one-third of hospitalized patients in the study, the cystatin C test showed kidney function was at least 30% worse than what the creatinine test showed.
This mismatch could be a warning sign. The study, led by researchers at NYU Langone Health, found that people with these large differences between the two tests were more likely to have serious health problems later. They were more likely to die, have heart disease, or develop severe kidney failure requiring dialysis or a kidney transplant.
These findings suggest that using both tests together gives a clearer picture of kidney health. One test alone might miss early signs of trouble. This is especially important for older people or those already dealing with other health issues.
The research team looked at health data from nearly 861,000 people from several countries. All of them had both creatinine and cystatin C levels measured on the same day.
The participants were followed for an average of 11 years to see what happened to their health over time. The researchers also considered other things that might affect the test results, like smoking, being overweight, or having cancer.
This study was part of a larger international effort called the Chronic Kidney Disease Prognosis Consortium. The group works to improve how doctors understand and treat kidney disease. According to the researchers, this is the biggest study so far to explore the differences between these two kidney tests.
The results were clear. People whose cystatin C results showed much worse kidney function than their creatinine results faced higher risks. This was true not just for patients in hospitals, but also for some people in outpatient clinics and even for those who seemed healthy.
The study also pointed out a problem in the U.S. healthcare system: cystatin C testing is still not widely used. Even though experts recommended this test over a decade ago, only a small number of hospitals and labs in the U.S. regularly perform it. Among hospitalized Americans in the study, fewer than 1% had been tested for cystatin C.
Experts say this needs to change. Having access to both tests could help doctors catch kidney problems earlier and make better decisions about treatment. It can also help doctors safely adjust doses of medications, like those used in cancer treatment or antibiotics, that are filtered through the kidneys.
Reviewing the findings, this study reveals that when there is a big gap between the two test results, it should not be ignored. It might be a sign that the person’s kidneys are in worse shape than the creatinine test alone suggests. If doctors only rely on one test, they may miss early signs of serious illness.
By encouraging more widespread use of the cystatin C test, especially now that large labs like Quest Diagnostics and Labcorp offer it, the medical community has an opportunity to improve patient care. Early action could mean better outcomes, fewer complications, and lives saved.
If you care about kidney health, please read studies about how to protect your kidneys from diabetes, and drinking coffee could help reduce risk of kidney injury.
For more information about kidney health, please see recent studies about foods that may prevent recurrence of kidney stones, and eating nuts linked to lower risk of chronic kidney disease and death.
This important research was published in JAMA and presented at the annual Kidney Week conference hosted by the American Society of Nephrology.
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