Investigating interferences of a whole-blood point-of-care creatinine analyzer: Comparison to plasma enzymatic and definitive creatinine methods in an acute-care setting

BACKGROUND: Although measurement of whole-blood creatinine at the point of care offers rapid assessment of renal function, agreement of point-of-care (POC) results with central laboratory methods continues to be a concern. We assessed the influence of several potential interferents on POC whole-blood creatinine measurements. METHODS: We compared POC creatinine (Nova Stat- Sensor) measurements with plasma enzymatic (Roche Modular) and isotope dilution mass spectrometry (IDMS) assays in 119 hospital inpatients. We assessed assay interference by hematocrit, pH, pO ₂, total and direct bilirubin, creatine, prescribed drugs, diagnosis, red blood cell water fraction, and plasma water fraction. RESULTS: CVs for POC creatinine were 1.5- to 6-fold greater than those for plasma methods, in part due to meter-to-meter variation. Regression comparison of POC creatinine to IDMS results gave a standard error (S _y|x) of 0.61 mg/dL (54 μmol/L), whereas regression of plasma enzymatic creatinine to IDMS was S _y|x 0.16 mg/dL (14 mol/L). By univariate analysis, bilirubin, creatine, drugs, pO ₂, pH, plasma water fraction, and hematocrit were not found to contribute to method differences. However, multivariate analysis revealed that IDMS creatinine, red blood cell and plasma water fractions, and hematocrit explained 91.8% of variance in POC creatinine results. CONCLUSIONS: These data suggest that whole-blood POC creatinine measurements should be used with caution. Negative interferences observed with these measurements could erroneously suggest adequate renal function near the decision threshold, particularly if estimated glomerular filtration rate is determined. Disparity between whole-blood and plasma matrices partially explains the discordance between whole-blood and plasma creatinine methods.