Reliability of 3 Urine Specific Gravity Meters for Measuring Brix and Urine Solutions at Different Temperatures

Urine specific gravity (USG) should be measured at room temperature (20°C), but the temperature of the sample is not always considered.

To evaluate the effect of sample temperature on the measurement accuracy of a digital refractometer (DIG), manual optical refractometer (MAN), and hydrometer (HYD).

Descriptive laboratory study.

Urine specific gravity.

Experiment 1 (24 Brix (°Bx) samples) showed that measurements via the DIG and MAN did not differ from the reference, but HYD provided lower or inconsistent values compared with °Bx and was highly correlated with °Bx solutions (r, > = 0.89). The overall diagnostic ability of elevated USG cut-off values (≥1.020, ≥1.025, ≥1.030) was high for all tools (area under the curve >0.92). Misclassification of samples increased from 0 to 2 at 1.020 to 1 to 3 samples at cutoffs of 1.025 and 1.030 USG. Bland-Altman analysis showed that the DIG 5°C underreported slightly without reporting bias (r = -0.344, P = .13); all other plots for the DIG, MAN, and HYD showed considerably larger underreporting at higher concentrations (r = -0.21 to -0.97 with P >.02) at all temperatures. The outcomes of experiment 2 (33 fresh urine samples) using DIG 20°C as the standard demonstrated only negligible differences between the DIG and MAN at all temperatures but larger differences using the HYD.

All tools showed reporting bias compared with the °Bx solutions, which can affect the classification of low and high urine concentration at higher USG cutoff values, especially with a sample temperature of 37°C.