Discordance among creatinine- and cystatin C-based eGFR equations and its impact on chronic kidney disease staging in older adults: a cross-sectional study
In a cross-sectional study of older Malaysian adults, cystatin C-based eGFR equations were found to systematically yield lower estimates and reclassify a majority of patients into more advanced chronic kidney disease stages compared to creatinine-based equations, while combined creatinine-cystatin C equations provided more consistent intermediate estimates.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
The kidneys act as the body's internal filtration system, constantly cleaning the blood and removing waste. To know how well these organs are working, doctors often look at a substance called creatinine, a natural byproduct of muscle activity that the kidneys usually flush out. If the kidneys slow down, creatinine builds up in the blood. However, this test has a blind spot in older adults. As people age, they often lose muscle mass and eat less protein, which means their bodies produce less creatinine to begin with. This can trick the test into showing a healthy number even when the kidneys are struggling, leading to a false sense of security. To get a clearer picture, scientists have developed a second test using a different marker called cystatin C, a protein produced by all cells that is less affected by muscle loss. The question facing doctors is whether relying on this second marker, or a combination of both, changes how they understand a patient's health, particularly in the elderly.
A team of researchers at the University Malaya Medical Centre in Kuala Lumpur set out to investigate this exact issue among older adults in Malaysia. They gathered data from 721 patients, all aged 65 or older, who had both creatinine and cystatin C levels measured during routine medical care. The team did not perform a new, invasive test to measure the actual flow of fluid through the kidneys; instead, they compared the different mathematical formulas doctors use to estimate kidney function. They wanted to see if switching from the standard creatinine formula to one based on cystatin C, or using a mix of both, would result in a different diagnosis for these patients.
The results revealed a significant disconnect between the two methods. When the researchers calculated kidney function using the standard creatinine formula, many patients appeared to have mild or moderate issues. However, when they applied the cystatin C formula, the picture changed dramatically. Nearly two-thirds of the patients were reclassified into a more severe category of kidney disease. On average, the cystatin C test suggested kidney function was about 16.7 units lower than the creatinine test. This gap was not just a small statistical blip; it was large enough to move a patient from a stage requiring routine check-ups to a stage demanding closer monitoring, stricter medication adjustments, and earlier specialist referrals.
The study found that this disagreement was not random. It happened most often in specific groups of people. The likelihood of a large discrepancy increased with age, with every additional year of life making a mismatch more probable. It was also more common in individuals with a higher body mass index and those living with diabetes. In these groups, the creatinine test seemed to be underestimating the severity of the problem, likely because their reduced muscle mass was masking the true level of kidney decline. The cystatin C test, being less dependent on muscle, appeared to reveal the hidden damage.
To address this uncertainty, the researchers also looked at formulas that combine both markers. These mixed equations acted as a middle ground. They did not show the extreme drop in estimated function seen with cystatin C alone, nor did they stay as high as the creatinine-only results. Instead, they provided a more stable and consistent estimate that fell between the two extremes. The study suggests that for older adults, especially those who are diabetic or have a higher body weight, relying on a single test can be misleading. The combination of both markers offers a more reliable view, helping to ensure that patients receive the correct level of care without the risk of missing a serious decline in kidney health.
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