Sixty years of conjecture over a urinary biomarker: a step closer to understanding the proposed link between anxiety and urinary pyrroles.

Objective: For over 60 years there has been conjecture about the identity of an Ehrlich's test positive pyrrole (Mauve Factor) reputed to be a biomarker for psychological disorders, including anxiety. We reviewed studies that attempt to identify Mauve Factor and subjected authentic standards of the 2 main candidates, kryptopyrrole and hydroxypyrrole, to the Ehrlich's reaction.

Methods: Modified Ehrlich's test for kryptopyrrole and hydroxypyrrole were applied to urine samples from 10 volunteers, anxious and nonanxious.

Results: Based on the mechanistic chemistry of Ehrlich's reaction and reactions of the 2 compounds, Mauve Factor cannot be hydroxypyrrole. Analyses of urine samples from volunteers, identified by the Generalized Anxiety Disorder - 7 item scale (GAD-7 ≥10; n = 5) and control urine samples (GAD-7 <10; n = 5) using a kryptopyrrole calibration graph, show that concentrations are similar in both groups.

Conclusion: Kryptopyrrole may be the elusive Mauve Factor. Its possible origin from stercobilin via gut microbiome-mediated metabolism, its link to gut-mediated neurological effects via γ-aminobutyric acid (GABA) receptors, and its predicted interaction with Zn2+ and consequent impact on zinc homeostasis are discussed. The GAD-7 scale does not differentiate between state and trait anxiety and as such, the minimal difference in pyrrole levels between volunteer groups requires further study.