Lactulose/mannitol test has high efficacy for excluding organic causes of chronic diarrhea

Diagnosis in chronic diarrhea in the absence of a distinctive clinical pattern is often challenging, as biochemical tests prescribed at the first evaluation do not show enough sensitivity and specificity to tailor further investigation. Intestinal permeability to sugars is an accurate test for detecting intestinal damage. The aim of this study was to evaluate the diagnostic value of the lactulose/mannitol (L/M) test in patients with chronic diarrhea.

We conducted a prospective cohort study to evaluate the diagnostic value of the L/M test in chronic diarrhea. The test was administered to 261 consecutive patients presenting with three or more bowel movements daily for at least 3 wk. Biochemical tests including complete blood cell count, acute phase reactive proteins, serum albumin and iron, and stool cultures for bacteria, ova, and parasites were assessed at the same time. Additional diagnostic investigations were directed by clinical features as well as first-line test results.

Over 3 yr, 120 (46%) of our patients were found to have an organic cause for chronic diarrhea, whereas in 141 (54%) a functional condition was diagnosed. Multivariate logistic regression analysis revealed that the L/M test and C-reactive protein were independent predictors for the final diagnosis of organic cause of chronic diarrhea, with odds ratios of 1.5 (95% CI = 1.29-1.78) and 5.2 (95% CI = 1.90-14.12), respectively. The area under the receiver operating characteristic (ROC) curve of the adjusted model was 0.82, with positive predictive value of 80.4% and negative predictive value of 77.7%.

The L/M test is a powerful tool for workup in patients with chronic diarrhea. Introducing the L/M test as first-level test effectively improves the selection of patients who need further evaluation.