Large difference but high correlation between creatinine and cystatin C estimated glomerular filtration rate in Mesoamerican sugarcane cutters

OBJECTIVES: To explore the relationship between creatinine and cystatin C based estimated glomerular filtration rate (eGFR) in actively working sugarcane cutters. METHODS: This cohort study included 458 sugarcane cutters from Nicaragua and El Salvador. Serum samples were taken before and at end of harvest seasons and analysed for creatinine and cystatin C. Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) formulas were used to calculate eGFRs based on creatinine (eGFR(cr)), cystatin C (eGFR(cys)) and both creatinine and cystatin C (eGFR(crcys)) at each time point. Bland-Altman plots and paired t-tests were used to compare the difference between eGFR(cr) and eGFR(cys), and the difference in eGFRs between before and at end of the harvest seasons. RESULTS: The mean eGFR(cr) was higher than eGFR(cys) in both cohorts; absolute difference 22 mL/min/1.73 m(2) (95% CI 21 to 23) in Nicaragua and 13 mL/min/1.73 m(2) (95% CI 11 to 15) in El Salvador. Correlations between eGFR(cr) and eGFR(cys) were high, with r=0.69, 0.77 and 0.67 in Nicaragua at pre-harvest, end-harvest and cross-harvest, and r=0.89, 0.89 and 0.49 in El Salvador. CONCLUSIONS: Creatinine increases among heat-stressed workers reflect reduced glomerular filtration as estimated using eGFR(cys), a marker independent of muscle mass and metabolism. The discrepancy between eGFR(cr) and eGFR(cys) may indicate reduced glomerular filtration of larger molecules and/or systemic bias in CKD-EPI performance in this population.