The Stool Test Doctors Are Debating Before a New Biologic

On August 10, JAAD, the Journal of the American Academy of Dermatology, published a formal reply to a proposal most patients will never hear about: adding a stool test to the safety checklist before starting a new biologic for a painful skin condition.

The piece it answers, by Du and colleagues, is titled, by its own words, an argument for integrating fecal calprotectin testing into a multimodal safety strategy before starting bimekizumab in patients with hidradenitis suppurativa, a chronic disease of recurring boils and tunnels under the skin. The August 10 reply, from Emily G. Summers, Hans Herfarth, Christina Crater, Delphine Deherder, and Christopher J. Sayed, sits behind the same paywall as the original. Neither piece has surfaced anywhere public yet, so the specific arguments on either side are not independently checkable at this writing. What is checkable is the context that explains why this disagreement exists at all.

Why gut health enters a skin drug conversation

Bimekizumab, sold as Bimzelx, blocks two related immune signals, IL-17A and IL-17F. The FDA approved it for hidradenitis suppurativa in November 2024. Its US prescribing label already carries an explicit caution about the drug class it belongs to: cases of inflammatory bowel disease have been reported in patients on IL-17 inhibitors, and the label instructs doctors to avoid the drug in anyone with active IBD and to watch for new or worsening gut symptoms. Hidradenitis suppurativa and inflammatory bowel disease, Crohn's disease especially, also turn up in the same patients more often than chance would predict, which is why gastroenterologists and dermatologists increasingly share a byline.

That overlap shows up plainly in this one. Herfarth directs the University of North Carolina's Multidisciplinary IBD Center. Sayed directs UNC's hidradenitis suppurativa clinic and has served as an investigator on bimekizumab's pivotal HS trials. Crater and Deherder are both listed elsewhere as employees of UCB, the company that manufactures bimekizumab. That pairing is disclosed and ordinary in clinical correspondence, not hidden, but worth naming: a reply weighing how cautiously to screen for a drug's own labeled risk includes people paid by the company that sells it, standing alongside two independent specialists who run the relevant clinics at the same university.

What the numbers already show

The practical question, whether a stool test catches disease that symptom questions alone would miss, already has a documented answer. A 2025 study of 150 hidradenitis suppurativa patients found that symptom questionnaires alone would have flagged 44.7 percent of the group for a gastroenterology referral, while fecal calprotectin testing across 88 previously undiagnosed patients caught exactly one additional case of occult Crohn's disease. Specialists can read that number, one diagnosis per 88 tests, in opposite directions: too low a yield to justify routine testing, or precisely the kind of rare catch worth having before a drug that carries a labeled gut warning.

The label itself states plainly: "Cases of inflammatory bowel disease (IBD) have been reported in patients treated with IL-17 inhibitors, including BIMZELX."

Anyone starting or already taking bimekizumab for hidradenitis suppurativa has a reasonable set of questions for their next visit: whether the clinic screens for bowel symptoms as routine practice, whether stool testing is offered and on what basis, and what a borderline result would actually change about their care.

Sources: DailyMed (FDA label) · PMC