The validity urine color scoring at different light conditions to assess low versus high urine concentration.

CONTEXT Urine color (Uc) is used to asses urine concentration when lab techniques are not feasible. OBJECTIVE To compare the accuracy of Uc scoring using four different light conditions and two different scoring techniques with a 7-color Uc chart. Additionally to assess the results' generalizability, a subsample was compared to scores obtained from fresh samples. DESIGN Descriptive laboratory study. SAMPLES 178 previously frozen urine samples were scored and n=78 samples were compared to its own fresh outcomes. MAIN OUTCOME MEASURE Urine color and accuracy for classifying urine samples were calculated using receiver operating characteristics (ROC) analysis, allowing to compare the diagnostic capacity against a 1.020 urine specific gravity (USG) cut-off and defining optimal Uc cut-off value. RESULTS Uc was significantly different between light conditions (P<0.01), with the highest accuracy (80.3%) of correctly classifications of low or high urine concentrations occurring at the brightest light condition. Lower light intensity scored 1.5-2 shades darker on a 7-color Uc scale than bright conditions (P<0.001), with urine color but no further practical differences for accuracy between scoring techniques. Frozen was 0.5-1 shade darker than freshly measured Uc (P<0.004), but they were moderately correlated (r=0.64). A Bland-Altman plot showed that reporting bias mainly affects darker Uc without impacting the diagnostic ability of the method. CONCLUSIONS Uc scoring, accuracy and Uc cut-off values are affected by lighting condition but not by scoring technique, with higher accuracy and a one-shade lower Uc cut-off value at the brightest light (i.e. LED flashlight).