Combination of creatinine and cystatin C more accurate than either alone for GFR

Combining creatinine and cystatin C measurements provided a more accurate estimation of glomerular filtration rate (GFR) than using either measurement alone, a new study found.

Researchers used data from more than 5,000 participants in 13 studies to develop estimating equations for GFR using cystatin C alone and in combination with creatinine. They then used more than 1,000 participants in five studies in which GFR had been measured to validate the equations. Results were published in the July 5 New England Journal of Medicine.

In the validation cohort, the combined cystatin C-creatinine equation showed similar bias to the equations using only one marker, and it was more precise and accurate than either of them. The median difference between measured and estimated GFR was 3.9 mL/min/1.73 m² with the combined equation compared to 3.7 mL/min/1.73 m² with creatinine alone and 3.4 mL/min/1.73 m² with cystatin C alone. Using the combined equation, only 8.5% of estimates were more than 30% off the measured value, compared to 12.8% and 14.5% with creatinine and cystatin C, respectively.

The combined equation performed better than either of the single-marker ones, study authors concluded. One advantage of cystatin C as a measure is that it is less subject to variation due to age, sex and race, and in fact doesn't require race for its calculation, the authors noted. The combined equation was also more accurate in patients with a body mass index less than 20 kg/m², a difficult-to-measure subgroup.

However, the results do not suggest that cystatin C should replace creatinine as a marker in clinical practice, according to the authors. Routine use of it could increase laboratory costs, so it may be most useful as a confirmatory test for diagnosis of chronic kidney disease, they said. An accompanying editorial suggested that clinicians also keep in mind the value of presence or absence of albuminuria for diagnosis and staging of kidney disease. The editorialist suggested use of the cystatin C measurement to diagnose patients who have no albumin in the urine and an estimated GFR close to 60 mL/min/1.73 m² according to creatinine measurement.