Zonulin (Faecal) — Add-On

Zonulin is offered as a faecal add-on marker of intestinal barrier function. The underlying biology is legitimate in outline: intestinal tight junctions are dynamically regulated, paracellular permeability is a genuine and measurable physiological property, and zonulin was described by Fasano and colleagues as a protein — identified as pre-haptoglobin-2 — that reversibly loosens those junctions in response to gliadin and enteric bacterial exposure. Practitioners order it to characterise barrier function in patients with gastrointestinal symptoms, autoimmune disease, food sensitivity, or suspected increased permeability, typically as an add-on to a broader stool profile. Practitioners should understand that the evidence supporting faecal zonulin as a clinical test is weak, and that the problems are analytical rather than merely interpretive. Independent work using mass spectrometry examined what commercial zonulin immunoassays actually detect and found that they do not measure pre-haptoglobin-2, instead cross-reacting with several structurally unrelated proteins including complement C3 and properdin. Different commercial kits produce poorly correlated results when run on the same specimens, which is difficult to reconcile with all of them measuring a single defined analyte. Correlation between zonulin results and direct permeability measurement, such as the lactulose-mannitol ratio, is inconsistent across studies. These are not marginal calibration concerns; they bear on whether the assay measures the intended molecule at all. The clinical construct the test is usually ordered to support requires similar care. Increased intestinal permeability is a real phenomenon, demonstrable in coeliac disease, inflammatory bowel disease, and critical illness, and is an active area of research. It is generally observed as a consequence of established disease or injury rather than as an independently treatable primary cause, and 'leaky gut syndrome' as a standalone diagnosis is not recognised by mainstream gastroenterology. Where a patient's symptoms suggest a specific diagnosable condition — coeliac disease, inflammatory bowel disease, or enteric infection — validated testing directed at that condition is the appropriate route, since those diagnoses carry established management while a zonulin result does not. Zonulin is best regarded as exploratory, interpreted alongside validated markers rather than in place of them, and should not be used to establish a diagnosis, to justify a treatment regimen, or as a value to be normalised.