Reference intervals for 24-hour and random urine porphyrins.

Urine porphyrin analysis is an important part in evaluation of photosensitivity. Since porphyrin excretion is variable throughout the day, analysis is traditionally based on 24-hour collections. To facilitate the use of random specimens, as well as poorly collected 24-hour specimens, reference limits based on the porphyrin to creatinine ratio have been developed. Based on 1,171 adult specimens, it is estimated that the 95 percent reference limit (90 percent confidence interval) is < or = 3.9 (3.5-5.7) mumol/mol of creatinine for uroporphyrin and < or = 22 (19-34) mumol/mol for coproporphyrin. These values apply to both 24-hour and random specimens, although random specimens show a higher degree of variability. Modest differences exist between males and females, but they are not significant given the degree of uncertainty in the confidence intervals. In terms of more traditional 24-hour units, reference limits correspond to < or = 37 (32-63) nmol/day for uroporphyrin and < or = 221 (195-320) nmol/day for coproporphyrin.