On accuracy and precision of a HPLC method for measurement of urine porphyrin concentrations.

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.

Comparison of the Bio-Rad Porphyrin Column Test with a simple spectrophotometric test for total urine porphyrin concentration.

We compared two screening methods for increased urine porphyrin concentration and compared the results with a high-performance liquid chromatography (HPLC) method. The screening methods were the Bio-Rad (Porphyrin) Column Test and a simple spectrophotometric method. Results were obtained for urines with three different porphyrin patterns. Both screening methods were easy to perform. The accuracy and precision of the spectrophotometric method were both slightly better than that of the Bio-Rad Column Test. Recovery measurements in samples with different porphyrin patterns varied between 73% and 59% (n = 12) for the spectrophotometric method and between 82% and 116% (n = 12) for the Bio-Rad Column Test as compared to HPLC. Between batch precision measurements revealed coefficients of variation for spectrophotometric and Bio-Rad methods for 2%-4% and 4%-10%, respectively. The recovery of the porphyrins illustrates the Bio-Rad Column Test to be more susceptible to variation in urine porphyria composition. Both methods will show satisfactory results in cases of overt porphyria because of the high urine porphyrin concentration.

Evaluation of a first-line spectrophotometric screening test for increased faecal porphyrin concentration.

We compared a spectrophotometric screening test measuring faecal porphyrin concentration with an HPLC method. There was a good overall correlation between both methods although some scatter was observed. ROC plot analysis of the screening test leads to a cut-off value of 35 nmol porphyrin per g faeces, wet weight with a sensitivity of 97% and a specificity of 96%. These results indicate that the screening test is quite useful for detection of increased total faecal porphyrin concentration, but less useful in accurate measurement of increased total faecal porphyrin concentration.

Evaluation of a first-line spectrophotometric screening test for increased urine porphyrin excretion.

We compared a spectrophotometric screening test for urine porphyrin concentration with a high performance liquid chromatography (HPLC) method. The screening test gave lower values than those obtained by HPLC, but the overall correlation was good. Occasionally, spectrophotometry failed to detect porphyrins in the urine which were detected by HPLC. The type of porphyria had no influence on the efficacy of the screening method. Receiver operating characteristic plot analysis of the screening test led to a cut-off value of 110 nmol/24 h with a sensitivity of 96% and a specificity of 86%. We conclude that the spectrophotometric screening method is useful for detection of increased total urine porphyrin concentration.