Use of cystatin C to inform metformin eligibility among adult veterans with diabetes

• Creatinine-based GFR estimates (eGFRcr) may contribute to misuse of metformin.
• We examined the impact of cystatin C GFR estimates (eGFRcys) on metformin eligibility.
• eGFRcys reclassified 20% of Veterans into different eGFR categories.
• Old age and albuminuria were associated with more severe eGFRcys vs. eGFRcr.

AimsRecommendations for metformin use are dependent on eGFR category: eGFR >45 ml/min/1.73 m2 – “first-line agent”; eGFR 30–44 – “use with caution”; eGFR<30 – “do not use”. Misclassification of metformin eligibility by creatinine-based MDRD GFR estimates (eGFRcr) may contribute to its misuse. We investigated the impact of cystatin c estimates of GFR (eGFRcys) on metformin eligibility.MethodsIn a consecutive cohort of 550 Veterans with diabetes, metformin use and eligibility were assessed by eGFR category, using eGFRcr and eGFRcys. Discrepancy in eligibility was defined as cases where eGFRcr and eGFRcys categories (<30, 30–44, 45–60, and >60 ml/min/1.73 m2) differed with an absolute difference in eGFR of >5 ml/min/1.73 m2. We modeled predictors of metformin use and eGFR category discrepancy with multivariable relative risk regression and multinomial logistic regression.ResultsSubjects were 95% male, median age 68, and racially diverse (45% White, 22% Black, 11% Asian, 22% unknown). Metformin use decreased with severity of eGFRcr category, from 63% in eGFRcr >60 to 3% in eGFRcr <30. eGFRcys reclassified 20% of Veterans into different eGFR categories. Factors associated with a more severe eGFRcys category compared to eGFRcr were older age (aOR = 2.21 per decade, 1.44–1.82), higher BMI (aOR = 1.04 per kg/m2, 1.01–1.08) and albuminuria >30 mg/g (aOR = 1.81, 1.20–2.73).ConclusionsMetformin use is low among Veterans with CKD. eGFRcys may serve as a confirmatory estimate of kidney function to allow safe use of metformin among patients with CKD, particularly among older individuals and those with albuminuria.