The longitudinal stability of the urea breath test (UBT), which measures urease as a biomarker for infection with Helicobacter pylori (a major risk factor for gastric cancer), was evaluated in the environs of Tsukuba, Japan. 13C-UBT measurements were monitored at four time points in 46 free-living, H. pylori-infected, asymptomatic volunteers over a period of 7 weeks. Subjects were asked to refrain from eating cruciferous vegetables, which might confound interpretation of results. Their

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... e was monitored using both dietary records and direct biochemical testing of overnight urine. There was large between-subject UBT variation in this population (logUBT mean, 3.34; SD, 0.67). Within-subject (longitudinal) UBT values were remarkably stable in about one-quarter of the subjects (coefficients of variations for these individuals were <21%), whereas coefficients of variations in the highest quartile of variability ranged from 40% to 80%. About half of the sequential UBTs (63 of 138 such measurement pairs) changed >10 per thousand "delta over baseline" between measurements. This study provides the elements to optimize the design of a clinical trial in this population to examine the efficacy of a dietary intervention to reduce H. pylori infection. The number of subjects required to detect a 30% difference in average UBT value is highly dependent on the baseline stability of UBT measurements. For the least variable quartile, as few as 12 subjects would be needed; for the most variable quartile, at least 147 subjects would be required in each arm.