THE VALIDITY OF SPOT URINE SAMPLES FOR LOW-LEVEL OCCUPATIONAL MERCURYEXPOSURE ASSESSMENT AND RELATIONSHIP TO PORPHYRIN AND CREATININE EXCRETION RATES

Hg and porphyrin levels in single void urine specimens (spot samples) were compared with calculated 24-hr urine levels in 35 (25 male and 15 female) practicing dentists who had been occupationally exposed to lo w levels of elemental Hg. The study aimed to: 1) determine the individ ual variability for Hg and porphyrin concentrations in spot samples ov er a 24-hr period; 2) test for the presence of diurnal variation in ur inary Hg and porphyrin concentrations; and 3) determine the time of da y at which a spot sample would give a Hg concentration closest to the 24-hr average concentration. Results confirmed previous reports of a f irst-order diurnal pattern with a mid-morning Peak for Hg concentratio n (P < .0001). A second-order model best described creatinine excretio n (P = .0089), with peaks at about 5:00 and 19:00. The use of creatini ne adjustment for Hg concentration Significantly reduced the intraindi vidual variation around the diurnal curve. No diurnal patterns were fo und for any of the porphyrins examined. We recommend that, for small c linical studies using urinary Hg concentration, 24-hr sampling would b e ideal, but that for mass screenings and cross-sectional studies, spo t samples may be useful because they correlate fairly well with 24-hr averages (creatinine adjusted, r = 0.61; unadjusted, r = 0.74). Becaus e of the existence of diurnal variation, for all cases using serial sa mpling attention should be paid to time of day.