For oesophageal pH monitoring, the pH probe is usually positioned 5 cm above the lower oesophageal sphincter (LOS). This is by convention, and has not been compared with other positions in its ability to discriminate between physiological and abnormal acid reflux. Using simultaneous two level 24 hour pH monitoring (5 and 10 cm above manometrically determined LOS) in 31 controls and 51 patients with reflux oesophagitis, the significance of the precise position of the probe in the oesophagus was examined. Secondly, this study compared the discrimination between the two groups achieved at the two levels. Patients had greater acid exposure than controls at both levels. In controls, acid exposure was greater at distal than at the proximal level except the supine acid exposure, which was similar at both levels. In patients, acid exposure was greater at the distal level for all variables (median % of total time pH < 4 = 11.7 v 7.6; p = 0.001). There was excellent correlation between the two levels for all variables in controls (r = 0.883, 0.935, 0.813, and p < 0.001 for percentage of time pH < 4 for total, supine, and upright times) as well as in patients (r = 0.848, 0.848, 0.779, and p < 0.001). On discriminant and receiver operating characteristic analysis, pH threshold 4 seemed as good as or better than other pH thresholds in discriminating between controls and patients. The percentage of total time pH below 4 seemed to discriminate as well or better than other variables at both levels. The distal level (5 cm above LOS) provided slightly better discrimination than proximal level (10 cm) (percentage of subjects correctly classified=81.7 v 75.6). The critical factor for the reliability of the test is not the precise position of the pH probe relative to the LOS, but that the same position is consistently used in patients and controls.
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