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

Cystatin C measurement leads to lower metformin dosage in elderly type 2 diabetic patients

The aim of this study was to provide evidence for the hypothesis that estimated glomerular filtration rate from serum Cystatin C (eGFRcys) is better to be determined for all elderly type 2 diabetes mellitus (T2DM) patients based on eGFRcys upward and downward reclassification rate for hypothetical metformin dose reduction by eGFRcys at the GFR decision point of 45 mL/min/1.73 m². A total of 265 consecutive T2DM elderly patients (age range 65‐91 years) from outpatient diabetic clinic were included in the study. The Kidney Disease Improving Global Outcomes (KDIGO) guidelines for metformin dosing were strictly followed. Estimated glomerular filtration rate from serum creatinine (eGFRcrea) led to results of metformin eligibility. Each of the results of eGFRcrea‐based eligibility was further compared to eGFRcys‐based eligibility. Creatinine was measured by enzymatic method standardized against international reference material SRM 967. Cystatin C was determined by method traceable to DA ERM 471 international standard. eGFRcrea and eGFRcys were calculated according to Chronic Kidney Disease Epidemiology Collaboration (CKD‐EPI) equations. A downward reclassification rate was higher than upward reclassification rate (31 vs 3, respectively; P < 0.0001). The median (IQR) eGFRcrea was higher than eGFRcys (73 (58‐85) vs 63 (50‐75) mL/min/1.73 m², respectively; P < 0.0001). eGFRcys reclassified significant proportion of patients with T2DM from metformin eligible CKD stages to less or non‐eligible stages. The downward reclassification was more frequent in patients older than 80 years (P < 0.01). Cystatin C‐based eGFR selects more complicated patients, where lower doses of metformin are possibly advisable. We recommend calculating both eGFRcrea and eGFRcys for metformin dosing in elderly patients with T2DM.

Creatinine Versus Cystatin C: Differing Estimates of Renal Function in Hospitalized Veterans Receiving Anticoagulants

BACKGROUND

Current practice in anticoagulation dosing relies on kidney function estimated by serum creatinine using the Cockcroft-Gault equation. However, creatinine can be unreliable in patients with low or high muscle mass. Cystatin C provides an alternative estimation of glomerular filtration rate (eGFR) that is independent of muscle.

OBJECTIVE

We compared cystatin C-based eGFR (eGFR_cys) with multiple creatinine-based estimates of kidney function in hospitalized patients receiving anticoagulants, to assess for discordant results that could impact medication dosing.

DESIGN

Retrospective chart review of hospitalized patients over 1 year who received non-vitamin K antagonist anticoagulation, and who had same-day measurements of cystatin C and creatinine.

PARTICIPANTS

Seventy-five inpatient veterans (median age 68) at the San Francisco VA Medical Center (SFVAMC).

MAIN MEASURES

We compared the median difference between eGFR by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) study equation using cystatin C (eGFR_cys) and eGFRs using three creatinine-based equations: CKD-EPI (eGFR_EPI), Modified Diet in Renal Disease (eGFR_MDRD), and Cockcroft-Gault (eGFR_CG). We categorized patients into standard KDIGO kidney stages and into drug-dosing categories based on each creatinine equation and calculated proportions of patients reclassified across these categories based on cystatin C.

KEY RESULTS

Cystatin C predicted overall lower eGFR compared to creatinine-based equations, with a median difference of - 7.1 (IQR - 17.2, 2.6) mL/min/1.73 m² versus eGFR_EPI, - 21.2 (IQR - 43.7, - 8.1) mL/min/1.73 m² versus eGFR_MDRD, and - 25.9 (IQR - 46.8, - 8.7) mL/min/1.73 m² versus eGFR_CG. Thirty-one to 52% of patients were reclassified into lower drug-dosing categories using cystatin C compared to creatinine-based estimates.

CONCLUSIONS

We found substantial discordance in eGFR comparing cystatin C with creatinine in this group of anticoagulated inpatients. Our sample size was limited and included few women. Further investigation is needed to confirm these findings and evaluate implications for bleeding and other clinical outcomes.

NIH TRIAL REGISTRY NUMBER

Not applicable.