Difference in EGFR by Creatinine vs Cystatin C May Predict Outcomes in Older Patients

By Marilynn Larkin

July 23, 2020

NEW YORK (Reuters Health) - The difference between cystatin C- and creatinine-based kidney function could be used to predict frailty and other adverse outcomes in older adults, researchers suggest.

Normal kidney function is typically an estimated glomerular filtration rate (eGFR) >90 mL/min, and chronic kidney disease is diagnosed when eGFR is consistently <60 mL/min. "The idea of the current project came from commonly seeing patients in clinic whose eGFR by creatinine and cystatin C were different by >15 mL/min," Dr. O. Alison Potok of the University of California, San Diego said in an email to Reuters Health.

"We examined the intra-individual difference in eGFR by cystatin C versus creatinine (eGFRDiff) and found that eGFRDiff holds prognostic information," she said. "A negative eGFRDiff (i.e., a cystatin C-based eGFR lower than a creatinine-based eGFR) at any level of kidney function is associated with higher risk of frailty, cardiovascular disease, and death."

"Rather than using a combined equation to determine the eGFR, which would mask this information, clinicians may acknowledge this difference and use it as a clinical tool," she said.

Dr. Potok and colleagues analyzed the difference in EGFR based on cystatin C and serum creatine in more than 9,000 hypertensive adults (mean age 68; about 64% men; about half white and one-third black).

A validated 35-item frailty index (FI) included questionnaire data on parameters such as general and physical health, activity limitations, pain, depression, sleep, energy level, self-care, smoking status, and past medical history. Frailty was defined as an FI score >0.21.

As reported in the American Journal of Kidney Diseases, the mean eGFRCys was 73; mean eGFRCr was 72; and the mean eGFRDiff was 0.5 mL/min/1.73 m2.

After adjustment, each standard deviation higher eGFRDiff was associated with 24% lower odds of frailty (OR=0.76), as well as with a lower incidence of injurious falls (HR=0.84), hospitalization (HR=0.91), cardiovascular events (HR=0.89), and all-cause mortality (HR=0.71).

Summing up, the authors state, "The difference between eGFRCys and eGFRCr is associated with frailty and health status. Positive eGFRDiff is strongly associated with lower risks of longitudinal adverse outcomes and mortality, even after adjusting for CKD stage and baseline frailty.

Dr. Potok said, "While this may not yet be practice-changing at this time, we believe that clinicians should be attentive to this difference. The next step is to understand what is driving this difference, whether it is true kidney function, or other determinants."

Dr. David Goldfarb, Clinical Chief, Nephrology at NYU Langone Medical Center in New York City, told Reuters Health, "We are not currently using cystatin C routinely. From a clinical point of view, estimates of GFR are close but not necessarily identical to the actual, or measured, GFR, whether estimated by cystatin C or by creatinine-based equations. So, the instances where we measure cystatin C to aid decision making about clinical care of chronic kidney disease are few."

"As the article suggests, these numbers may be important to assess patients regarding other variables, like cardiovascular risk," he noted. "In this case, the difference between the cystatin C and creatinine-based GFR results correlated with frailty."

"Clinicians recognize frailty, or should," he said. "How to deal with it and reverse it is not so easy. Whether this interesting finding would lead clinicians to take a different course in order to anticipate and manage frailty is not revealed in this study."

SOURCE: https://bit.ly/39kFzXO American Journal of Kidney Diseases, online July 16, 2020.