Two models may offer accurate assessment of GFR in older adults

Two newly developed estimates of glomerular filtration rate (GFR) may provide more accurate assessment of kidney function in older adults, a study found.

To derive the Berlin Initiative Study (BIS) equation, researchers designed a cross-sectional, random community-based population of participants age 70 years or older (mean age, 78.5 years) from a large insurance company. Data were split for analysis into two sets for equation development and internal validation.

Two estimates of GFR were developed and validated, one based on creatinine only (BIS1) and one based on both creatinine and cystatin C measurements (BIS2). Results appear in the Oct. 2 Annals of Internal Medicine.

The new BIS2 equation yielded the smallest bias, followed by BIS1 and Cockcroft-Gault equations. All other equations considerably overestimated GFR.

The BIS equations confirmed a high prevalence of persons older than 70 years with a GFR less than 60 mL/min per 1.73 m² (BIS1, 50.4%; BIS2, 47.4%; measured GFR, 47.9%). The total misclassification rate for this criterion was smallest for the BIS2 equation (11.6%), followed by the cystatin C equation 2 (15.1%), a GFR estimation equation proposed by the Chronic Kidney Disease Epidemiology Collaboration. Among the creatinine-based equations, BIS1 had the smallest misclassification rate (17.2%), followed by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation (20.4%).

Researchers noted that the BIS2 equation should be used to estimate GFR in persons age 70 years or older with normal or mild to moderately reduced kidney function. If cystatin C is not available, the BIS1 equation is an acceptable alternative.

“Compared with current creatinine-based or creatinine- and cystatin C-based equations, the new BIS1 and BIS2 equations showed better precision and excellent agreement with [measured]GFR, especially in a population with an [estimated]GFR greater than 30 mL/min per 1.73 m² (CKD [chronic kidney disease] stages 1 to 3),” researchers wrote. “This is important because a validated equation to estimate GFR in older adults, especially in cases of normal or only moderately reduced kidney function, has been lacking.”

Also, researchers noted that older adults may be a unique population in which traditional assumptions about GFR are not necessarily true and the existing, commonly used methods for estimating GFR may not be accurate. In elderly participants, the Modification of Diet in Renal Disease (MDRD) study equation yielded higher estimated GFRs across CKD stages than did the CKD-EPI and especially the Cockcroft-Gault equation. This contrasts with the situation in younger adults in whom implementation of the CKD-EPI equation has reduced CKD prevalence but agrees with current results seen in older adults.

“The most striking result was that incorporation of cystatin C in the equation decreased the effect of age and sex,” researchers wrote. “This confirms the independence of cystatin C from age- and sex-associated conditions and may thus make it the preferred laboratory variable to be included in a GFR-estimating equation in an elderly population where reduction in muscle mass is common.”