Acute intermittent porphyria: diagnostic conundrums.

Acute intermittent prophyria is a genetic disorder of haem biosynthesis caused by defects in the gene encoding hydroxymethylbilane synthase on the long arm of chromosome 11. Every effort should be made to identify gene carriers amongst the relatives of patients known to have acute intermittent porphyria as they are at risk of developing potentially fatal neurogenic attacks if exposed to precipitating factors. Erythrocyte hydroxymethylbilane synthase activity was determined in 46 members of two large well characterised families by assaying enzyme activity by both high performance liquid chromatography (HPLC) and fluorimetric assays. Additionally, hydroxymethylbilane synthase immunoreactivity was determined by a sandwich-type ELISA. Statistically significant correlations were observed between erythrocyte hydroxymethylbilane synthase activity assayed by HPLC and by the fluorimetric assay, and enzyme protein concentration (r = 0.85, p < 0.001 and r = 0.80, p < 0.001, respectively). The assay of hydroxymethylbilane synthase immunoreactive concentration in erythrocytes was useful in excluding acute intermittent porphyria in one patient in whom unequivocal assignment of porphyric status was not possible by assaying enzyme activity alone. Erythrocyte hydroxymethylbilane synthase activity assayed by HPLC and fluorimetry showed approximately equal diagnostic performances, both giving rise to a dichotomic distribution of values, with overlap zones of 6% (1/16) and 22% (2/9), respectively, at the "cut off" applied.