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We studied the accuracy and precision of a HPLC method for determination of porphyrins in urine. A commercial standard solution appeared to contain less porphyrins than indicated by the manufacturer, since calibration resulted in lower concentrations of uroporphyrin and coproporphyrin: 16% and 8%, respectively. Coefficients of variation for the measurement of uro-, hepta-, copro I and copro III porphyrins in samples of patients with and without porphyria were often much less than 15%. Comparison of measurements with and without calibrated standards revealed differences for uroporphyrin and coproporphyrin of 27% and 5%, respectively. Recovery of added uroporphyrin and coproporphyrin was 99%. The main cause of the variability in test results was apparently the improperly calibrated standard solutions. The precision of porphyrin measurements was not influenced by the type of porphyria.

On Accuracy and Precision of a HPLC Method for Measurement of Urine Porphyrin Concentrations