A Quantitative Assay for Urinary

A procedure has been developed for the rapid, quantitative determination of urinary porphobilinogen. Interfering substances are removed by selectively adsorbing the porphobilinogen with an ion-exchange resin using a batch method. After acid elution of the porphobilinogen, Ehrlich’s reagent is added to produce the chromophore, which is then measured spectrophotometrically. Obtaining absorption at two wave-lengths permits a differentiation of porphobilinogen from interfering reactants not removed by the resin treatment. This assay is relatively simple, dependable, highly reproducible for increased amounts, and more specific than the qualitative assay. PORPHYRIAS are a group of metabolic diseases characterized chemically by au excessive excretiomi of porphyrins, and in some cases the porphyrin precursors-aminolevulinic acid and porphobilinogen (1, 2). A differemitial diagno&s for acute intermittent porphyria, the most common type of porphyria, is dependent upon demonstrating the presence of porphobilinogen (Pbg) in the urine, a liroceclure which for many years has beeui performed using the qualitative Watson-Schwartz test (3). While this test is adequate for uncomplicated situa-tions, it often yields false positive or questionable results due to the presence of interfering substamices amid to ti1e limited specificity of the reagemit. The substance that most commonly imiterferes is urohilino-gen, but this cami be effectively renioved with chloroform after the color development. Extractiomi with n-butanol has been suggested as a modification which will remove some additiomial interfering substances (4,5). However, even with these modifications, it sometimes becomes difficult to interpret the qualitative test