Creatinine in urine – a method comparison

Drug screening in urine is widely applied in forensic toxicology. Contrary to blood analysis, excessive or reduced fluid intake can substantially alter the concentration of substances in urine. As a standard to detect urinary dilution, creatinine concentrations are analyzed. A sample with a concentration below 20 mg/dL is generally defined as too diluted to provide a valid result in abstinence control samples. This work investigates the potential of three different methods for the determination of creatinine concentrations in urine samples: A ZIC®‐HILIC‐based liquid chromatography–tandem mass spectrometry (LC–MS/MS) method, a spectrophotometric method on an AU 480 chemistry system, and a portable, chemical‐reaction‐based, point‐of‐care testing device were compared by measuring 200 urine samples. When comparing the two laboratory methods, LC–MS/MS and spectrophotometry, a mean difference of 3.7 ± 14 mg/dL was found, indicating that the spectrophotometric method slightly overestimates the creatinine concentration. When comparing the LC–MS/MS method with the point‐of‐care testing device, a mean concentration difference within the calibration range for POCT (>20 mg/dL (excluding 16 samples) and <500 mg/dL (excluding 4 samples)) of 21 ± 37 mg/dL was found, indicating that the point‐of‐care testing device overestimates the measured creatinine concentration. A point‐of‐care testing device as used during this study can provide valuable information for on‐site analysis. However, reported concentrations above 500 mg/dL should be further evaluated, for example by dilution of the sample. Copyright © 2017 John Wiley & Sons, Ltd.