Validation and evaluation of two porphobilinogen deaminase activity assays for diagnosis of acute intermittent porphyria.

BACKGROUND: Acute intermittent porphyria (AIP) is caused by diminished activity of porphobilinogen deaminase (PBGD). The purpose of this study was to validate and compare two assays for PBGD activity. The clinical sensitivity of the PBGD activity assays in AIP diagnosis was also evaluated.

METHODS: This study included 74 subjects from 18 Taiwanese families including symptomatic patients with AIP, asymptomatic carriers, and healthy family members. The specific mutations in AIP patients were identified by DNA sequencing. PBGD activity was measured in erythrocytes by quantifying formation of coproporphyrin or uroporphyrin by the enzyme using porphobilinogen (PBG) as a substrate and fluorimetry for detection.

RESULTS: The calibration curves obtained with pure coproporphyrin or uroporphyrin were linear with correlation coefficients >0.99 in the range of 0-200nM for coproporphyrin and 0-150nM for uroporphyrin. The coefficients of variation for within-run and between-day imprecision were <9.8% for both assays. The three groups of subjects were used to establish the best cut-off of PBGD activity for identifying symptomatic AIP patients by using area under receiver operating characteristic curve analysis. The symptomatic AIP patients and asymptomatic carriers had significantly lower PBGD activity compared with the healthy family members (all p<.001).

CONCLUSION: Two different PBGD activity assays were validated. The best cut-off for coproporphyrin was derived as 46.4nmol/h/mL RBC with corresponding sensitivity of 100% and specificity of 100% and the best cut-off for uroporphyrin was derived as 43.7nkat/L RBC with corresponding sensitivity of 100% and specificity of 97.4%.