Porphobilinogen is most commonly quantitated in the urine by which method?

• A. Ion-exchange column
• B. The Watson-Schwartz method
• C. Thin-layer chromatography
• D. Electrophoresis

Answer: (B)

Measuring urinary porphobilinogen relies on a colorimetric reaction that is selective for PBG, followed by cleanup to remove interfering substances. The Watson-Schwartz method is the classic approach: Ehrlich's reagent reacts with porphobilinogen in acidic urine to form a red chromogen, which is then extracted into an organic solvent to separate it from other urine components such as urobilinogen and porphyrins. The intensity of the color correlates with the PBG concentration, providing a practical quantitative result. This makes it the best answer because the other techniques listed are primarily separation methods (ion-exchange, thin-layer chromatography, electrophoresis) and do not offer the same direct, specific colorimetric quantitation for PBG in urine.