Objectives Sugarcane harvesters in Costa Rica are exposed to heat stress, a likely major risk factor in the Mesoamerican nephropathy epidemic. Routine urinalyses provide important information about kidney function, but have not been reported in detail for this population.

Method Sugarcane cutters (n = 48) provided six spot urine samples, pre- and post-workshift for three days during one week in mid-harvest. Chemical analyses with dipstick and microscopic examinations of sediment were performed. Changes over the workday in markers for hydration status and kidney responses to heat stress were evaluated with McNemar test on paired proportions.

Results Preliminary results indicate percentages of workers with specific parameters in at least one morning versus in at least one afternoon sample were with dipstick: pH ≤5 30% vs 82%, specific gravity ≥1.025 30% vs 51%, proteinuria (1+ and up) 86% vs 8%, and blood (traces and up) 28% vs 12%; and in sediment: >5 leucocytes 76% vs 57%, >3 erythrocytes 50% vs 37%, and casts (granular, leukocyte and some erythrocyte) 75% vs 39%. Except for erythrocytes in sediment, these differences were statistically significant.

Conclusions Concentration and acidification of urine over the workday indicate insufficient hydration. Positive blood on dipstick may be a sign of low-grade hematuria or mild rhabdomyolysis. Predominance of proteinuria, leucocytes, erythrocytes and casts in morning urine was unexpected but may reflect improvement in glomerular filtration with hydration overnight or, alternatively, effects from vasopressin at night. This study adds to an increasing body of evidence of kidney dysfunction among heat stress exposed sugarcane workers.