Twenty-four-hour pH monitoring is considered the “ gold standard” for detection of gastroesophageal reflux disease (GERD).However, reference pH can drift during studies using antimony pH electrodes. Posttest calibration of antimony probes to assess a “ drift” has been advocated but is not routinely preformed in clinical practice. We undertook this study to determine the frequency and degree of drift that occurs during pH studies using single-use antimony pH electrodes. One hundred consecutive 24-hr ambulatory esophageal pH studies were reviewed for presence and degree of pH drift. Separately, 10 in vitro studies using antimony electrodes exposed to solution of constant pH and temperature were recorded to determine timing of drift.Drift of at least 0.1 pH unit occurred in 88 (P < 0.001). Drift of 0.1, 0.2, 0.3, 0.4, and >0.4 pH unit occurred in 38, 28, 15,4, and 5 of the 100 studies, respectively. In six studies the adjustment for the poststudy calibration drift changed the overall interpretation of the test. Small degrees of drift occurred atvarious times during the 10 in vitro studies. Drift in pH during 24-hr pH studies using antimony is common, but large degrees of drift are not. When the threshold of pH 4.0 was adjusted to reflect the drift, a small but significant number of interpretations changed. Drift artifact may impact the results of the pH study,and calibration should be repeated after all studies. Twenty-four-hour pH monitoring is considered the “gold standard” for detection of gastroesophageal reflux disease (GERD). Less than reference pH can drift during studies using antimony pH electrodes. Posttest calibration of antimony probes to assess a “drift” has been we undertook this study to determine the frequency and degree of drift that occurs during pH studies using single-use antimony pH electrodes. One Hundred & Twenty-four-hour ambulatory esophageal pH studies were reviewed for presence and degree of pH drift. Separately, 10 in vitro studies using antimony electrodes exposed to solution of constant pH and temperature were recorded to determine timing of drift. Drift of at least 0.1 pH unit occurred in 88 (P <0.001). Drift of 0.1, 0.2, 0.3, 0.4, and> 0.4 pH unit occurred in 38, 28, 15, 4, and 5 of the 100 studies, respectively. In six studies the adjustment for the poststudy calibration drift changed the over all interpretation of the test. Small degrees of drift occurred at variable times during the 10 in vitro studies. Drift in pH during 24-hr pH studies using antimony is common, but large degrees of drift are not. When the threshold of pH 4.0 was adjusted to reflect the drift, a small but significant number of interpretations changed. Drift artifact may impact the results of the pH study, and calibration should be repeated after all studies.