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Background: The 13C-urea breath test (13C-UBT) is preferred for non-invasive detection of Helicobacter pylori (H. pylori); however, its accuracy drops when results fall between 2‰ and 6‰ (called the gray zone). This study aimed to evaluate the accuracy of 13C-UBT (cut-off point 4‰) between 2‰ and 6‰, find a more appropriate gray zone, and identify the factors influencing 13C-UBT. Methods: Patients with 13C-UBT results 2‰–6‰, over an eight-year period, were studied. H. pylori infection was diagnosed if patients were positive for either Warthin–Starry staining or quantitative real-time polymerase chain reaction (real-time PCR), and excluded if both were negative. Accuracy of 13C-UBT under different cut-off points was calculated, and the factors affecting 13C-UBT were analyzed. Results: A total of 208 patients were included, of whom 129 were H. pylori–positive. Sensitivity, specificity, negative predictive value (NPV), and positive predictive value (PPV) of 13C-UBT were 71.32%, 83.54%, 64.08%, and 87.62%, respectively. When the cut-off point was changed to 2.15‰, the NPV of 13C-UBT reached a maximum (76.47%); when the cut-off point was changed to 4.95‰, PPV reached its maximum (93.22%). Therefore, the original gray zone (2‰–6‰) was adjusted to 2‰–4.95‰. Gastric antral intestinal metaplasia (OR = 3.055, 95% CI: 1.003–9.309) was an independent risk factor for false-negative 13C-UBT. Conclusions: Accuracy of 13C-UBT over 2‰–6‰ was poor, and the gray zone was changed to 2‰–4.95‰. 13C-UBT results over 2‰–4.95‰ should be interpreted with caution during mass screening of H. pylori, especially for patients with gastric antral intestinal metaplasia. [ABSTRACT FROM AUTHOR]