Diagnostic role of urine specific gravity to detect kidney impairment on heat-exposed workers in a shoe factory in Indonesia: a cross-sectional study

OBJECTIVES: Globally, there are increasing cases of chronic kidney disease of unknown origin among heat-exposed workers. We aimed to see the kidney damages of indoor heat-exposed workers and whether urine specific gravity can predict any kidney damages. DESIGN: A cross-sectional study. SETTING: A shoe-making factory in West Java, Indonesia. PARTICIPANTS: 119 subjects were included. Minimum total sample size was 62. Subjects were indoor heat-exposed workers who were exposed to occupational wet-bulb globe temperature (WBGT) of 28°C–30°C for 8 hours daily with 1 hour break, 5 days a week. The inclusion criterion was healthy subjects according to the result from annual medical check-up in 2019. The exclusion criteria were subjects who were taking vitamins and/or supplements that might cause disturbance in urine specific gravity and/or hydration status, pregnant and fasting. PRIMARY AND SECONDARY OUTCOME MEASURES: Area under the curve (AUC), sensitivity and specificity of urine specific gravity for the detection of urinary nephrin and urinary kidney injury molecule-1 (KIM-1) were analysed. Estimated glomerular filtration rate (eGFR) and quantitative albuminuria were also measured. RESULTS: WBGT in the work area of the subject was 28°C–30°C. There were 15 (12.6%) subjects who had eGFR <90 mL/min, but ≥60 mL/min. High serum vasopressin levels were found in 79 subjects with a mean of 6.54 (95% CI 5.94 to 7.14) ng/mL. Most subjects had nephrinuria (87.4%) with preserved renal function (87.4%). Several subjects had elevated urinary KIM-1 (10.9%) and albuminuria (7.6%). AUC of urine specific gravity for increased urinary nephrin was 81.7% (95% CI 68.8% to 94.6%) and statistically significant (p<0.001). Cut-off value of ≥1.018 for urine specific gravity has sensitivity of 71.2% and specificity of 80% for detecting elevation of urinary nephrin levels. CONCLUSION: Urine specific gravity with a cut-off value of ≥1.018 could be used to detect nephrinuria among heat-exposed workers.