How is porphyria cutanea tarda diagnosed?

Fluorescent spectrophotometric analysis of plasma is a rapid screen for porphyria. Plasma is exposed to an excitation wavelength of 400 to 410 nm, and the emission peaks are measured. A sharp emission peak at 619 nm confirms a porphyrin disorder. To differentiate the specific porphyrins further, a 24-hour urine specimen is submitted for porphyrin studies. In PCT, the major porphyrins elevated in urine are uroporphyrin I and 7-carboxylporphyrin III. In addition, stool porphyrins should be tested; PCT patients have normal levels of protoporphyrins but increased isocoproporphyrins in the stool.