My Kidney Is Fine, Can't You Cystatin C?

Area of Science:

• Nephrology
• Clinical Chemistry

Background:

• Serum creatinine (sCr) levels are commonly used to estimate kidney function, but can be elevated in individuals of African American race and those with high muscle mass.
• These elevated sCr levels may lead to an overestimation of chronic kidney disease (CKD) and subsequent misdiagnosis.

Observation:

• A case presentation of a 35-year-old African American man with elevated sCr, initially diagnosed with CKD stage 3A based on estimated glomerular filtration rate (eGFR).
• Further testing revealed isolated sCr elevation without other indicators of kidney disease, and cystatin C (CysC) testing confirmed the absence of CKD.

Findings:

• The 2009 CKD Epidemiology Collaboration equation for eGFR, which includes race, and even the 2021 update excluding race, show limitations in accuracy for certain populations.
• Factors like race, creatine supplements, and high protein intake can affect sCr levels, leading to inaccurate eGFR_Cr estimations.
• Serum cystatin C (CysC) based eGFR (eGFR_cys) and combined creatinine-cystatin C eGFR (eGFR_Cr-Cys) offer improved accuracy and concordance with measured GFR across diverse populations.

Implications:

• Confirmation screening with CysC can significantly reduce the misdiagnosis of CKD in at-risk populations, including African Americans and individuals with high muscle mass.
• Accurate kidney function assessment prevents inappropriate treatment, incorrect drug dosing, and misrepresentation of fitness for duty.