Which of the following is a qualitative screening test for porphobilinogen to aid in porphyrias diagnosis?

• A. Caraway test
• B. Gutmann test
• C. Jendrassik-Grof test
• D. Watson-Schwartz test

Answer: (D)

This item is testing knowledge of a qualitative urine screening method for porphobilinogen to help diagnose porphyrias. The Watson-Schwartz test is designed for this purpose. It uses Ehrlich’s reagent, which forms a colored complex with porphobilinogen. To distinguish PBG from other Ehrlich-reactive substances in urine, the mixture is partitioned between two solvents (commonly butanol and chloroform). Porphobilinogen preferentially moves into the butanol layer, so a persistent color in that organic layer indicates the presence of porphobilinogen. This makes it a simple, qualitative screen to raise suspicion of porphyrias, with positive results prompting confirmatory testing and clinical correlation. The other tests listed are not the standard qualitative screening for porphobilinogen.