Comparing the Accuracy of Standard Equations to Predict Glomerular Filtration Rate for Persons with Spinal Cord Injury: Which Is the "best Fit"

Patients with spinal cord injury (SCI) are at increased risk of renal insufficiency, so their renal function must regularly be monitored. Glomerular filtration rate (GFR) assessment is challenging as it requires measuring clearance of exogenous markers, which is impractical in most clinical settings. Thus, equations have been formulated to estimate GFR that utilize serum creatinine (Cr) or cystatin C (CysC). Objectives: Given loss of muscle mass after SCI, we hypothesized equations using CysC would be more accurate than those using Cr after SCI. Methods: Fifty-eight persons (51 male/7 female; age 22-87 years) with SCI level C2-L1/AIS A-D were enrolled. Serum CysC and Cr, 24-hour urine creatinine (24hrUCr) and 24- hour urine urea (24hrUurea) were collected. Average of the 24hrUCr clearance and 24hrUurea clearance was calculated (AvgCl_CrUr). Six GFR estimating equations were compared to AvgCl_CrUr. For each equation, mean bias (AvgCl_CrUr - eGFR) was calculated followed by the Pearson correlation calculation between AvgCl_CrUr and eGFR. In addition, the percentage of estimated values within 15%, 30%, and 50% of AvgCl_CrUr values are reported for each estimating equation. Results: 2012 CKD-EPI CysC equation (bias 9.32 mg/dL, 95% CI, -1.23 to -17.41) was the most accurate predictor of GFR. This model accurately predicted GFR of 81%, 57%, and 33% within ±50%, ±30%, and ±15% of the AvgCl_CrUr, respectively. Conclusion: In persons with SCI, the CKD-EPI 2012 CysC equation, which uses cystatin C rather than creatinine, is the most accurate of the six equations tested in estimating GFR in persons with SCI.