-Context -The standard doses of 13 C-urea in 13 C-urea breath test is 75 mg. Objective -To assess the diagnostic accuracy of 13 C-urea breath test containing 25 mg of 13 C-urea comparing with the standard doses of 75 mg in the diagnosis of Helicobacter pylori infection. Methods -Two hundred seventy adult patients (96 males, 174 females, median age 41 years) performed the standard 13 C-urea breath test (75 mg 13 C-urea) and repeated the 13 C-urea breath test using only 25 mg of 13 C-urea within a 2 week interval. The test was performed using an infrared isotope analyzer. Patients were considered positive if delta over baseline was >4.0‰ at the gold standard test. Results -One hundred sixty-one (59.6%) patients were H. pylori negative and 109 (40.4%) were positive by the gold standard test. Using receiver operating characteristic analysis we established a cut-off value of 3.4% as the best value of 25 mg 13 C-urea breath test to discriminate positive and negative patients, considering the H. pylori prevalence (95% CI: 23.9-37.3) at our setting. Therefore, we obtained to 25 mg 13 C-urea breath test a diagnostic accuracy of 92.9% (95% CI: 88.1-97.9), sensitivity 83.5% (95% CI: 75.4-89.3), specificity 99.4% (95% CI: 96.6-99.9), positive predictive value 98.3% (95% CI: 92.4-99.4), and negative predictive value 93.0% (95% CI: 88.6-96.1). Conclusions -Low-dose 13 C-urea breath test (25 mg 13 C-urea) does not reach accuracy sufficient to be recommended in clinical setting where a 30% prevalence of H. pylori infection is observed. Further studies should be done to determine the diagnostic accuracy of low doses of 13 C-urea in the urea breath test. HEADINGS -Helicobacter infections. Breath tests.

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