A coated preparation for the detection of a H. pylori infection according to a urea breath test protocol which comprises a core composition containing at least an isotope C-labeled urea, an excipient and a lubricant in defined proportions and covered with 0.1.about.10 weight % of a coating agent based on 100 weight % of the core composition. With this preparation, the influence of the urease-producing bacteria resident in organs other than the stomach, such as the mouth and throat, is excluded to enable a diagnosis of H. pylori infection without the risk for a false-positive test and with reasonable rapidity.