Reliability of Three Urine Specific Gravity Meters Measuring Brix and Urine Solutions at Different Temperatures.

CONTEXT The measurement of urine specific gravity should be performed at room temperature (20 °C) but sample temperature is not always taken in consideration. OBJECTIVE Evaluate the effect of sample temperature on the measurement accuracy of a digital (DIG) and optical (MAN) refractometer and a hydrometer (HYD). DESIGN Quantitative comparison between measurement outcomes for a reference solution (sucrose, degrees Brix) and fresh collected urine samples. SAMPLES Experiment 1 used a 24 Brix (°Bx) samples and experiment 2 used 33 fresh urine samples. MAIN OUTCOME MEASURE Urine specific gravity (USG). RESULTS Experiment 1 showed DIG and MAN did not differ from reference, but HYD reported lower or inconsistent values compared to Bx, while highly correlating with Bx solutions (r: > 0.89). The overall diagnostic ability of elevated USG (≥ 1.020; ≥ 1.025; ≥ 1.030) was high for all tools (AUC > 0.92). Misclassification of samples increased from 0 to 2 at 1.020 to 1 to 3 samples at cutoff 1.025 and 1.030 USG. Bland-Altman analysis showed DIG 5 °C underreports slightly without reporting bias (r: -0.344, P = 0.13); all other plots for DIG, MAN, and HYD showed considerably larger underreporting at higher concentrations (r ranging from -0.21 to -0.97 with P > .02) at all temperatures. The outcomes of experiment 2 using DIG 20°C as standard, showed only negligible differences between DIG and MAN at all temperatures, but larger differences using HYD. CONCLUSIONS All tools showed reporting bias when compared to °Bx solutions which can impact classification of low and high urine concentration at higher USG cutoff values, especially at a sample temperature of 37 °C.