JAMA Diagnostic Test Interpretation 

August 8, 2022

Serum Cystatin C for Estimation of GFR

Michael G. Shlipak, Lesley A. Inker, Josef Coresh

JAMA. Published online August 8, 2022. doi:10.1001/jama.2022.12407

Case

An 80-year-old man with type 2 diabetes and hypertension was referred to the nephrology clinic for assessment of persistent hyperkalemia despite discontinuation of his angiotensin-converting enzyme inhibitor. The patient was frail, sedentary, and had a body mass index of 37. Select blood testing results are shown in Table 1. His estimated glomerular filtration rate (eGFR) was calculated using 3 equations that incorporated creatinine (eGFRcr), cystatin C (eGFRcys), and both creatinine and cystatin C (eGFRcr-cys), as shown in Table 2.

How Do You Interpret These Test Results?

1. The estimated glomerular filtration rate calculated using creatinine (eGFRcr) may be inaccurate because the patient is frail and sedentary.
2. The eGFRcr of 64 mL/min/1.73 m² indicates normal kidney function.
3. The eGFR calculated using cystatin C (eGFRcys) may be low because the patient has decreased muscle mass.
4. The eGFRcys may be reduced because the patient has type 2 diabetes.