This morning we welcomed Dr. Dena Rifkin, nephrologist extraordinaire, to discuss the limitations of serum creatinine and the use of Cystatin C as an alternative for the estimation of renal function. We started by considering a 59-year-old man that presented to primary care clinic to discuss an elevated Cr to 1.43 (eGFR of 46). There was no clear sign of personal risk factors for CKD, although he occasionally used Ibuprofen and had a family history of Type II Diabetes and Hypertension. He had no microalbuminuria and his Cr was stable compared to three years prior. We obtained a Cystatin C level that indicated an eGFR of 82.
We then discussed why serum creatinine needs to be interpreted within the context of the patient’s physiology and how estimation calculators (including the preferred CKD-EPI equation) make adjustments for age, race, and sex. Cystatin C is an alternate biomarker produced by all nucleated cells that is freely filtered by glomeruli that avoids those problematic corrections. Cystatin C allows for increased detection of GFR reductions in the “creatinine blind range” where large changes in GFR are not reflected by substantial changes in GFR. Caution needs to be exercised when using Cystatin C in inflammatory states (infection, malignancy) and obesity as those will create higher levels of the protein. We then went through three patient cases that demonstrate the discrepancy in eGFR when using serum Cr versus CysC.
Cystatin C is available with rapid turnaround times at both the VA and UCSD – use it today!