Gastroesophageal reflux disease (GERD) is defined solely based on clinical symptoms. According to the Montreal consensus definition, GERD is defined as the reflux of gastric contents that cause troublesome symptoms or complications 1. While upper endoscopy is considered the first step in evaluation of refractory GERD, this diagnosis can often be difficult because majority of the patients have a normal appearing distal esophagus endoscopically 2. Whether symptoms correlate with episodes of acid reflux of gastric contents into the esophagus can be evaluated by ambulatory pH study. There are several statistical and non-statistical tests to evaluate for correlation between acid reflux and symptoms. Previously we have reported on the strengths and weaknesses and have compared each of these statistical tests3. Symptom Association Probability (SAP) is the most commonly used statistical tool. The duration of the pH study is divided into 2 minute intervals. The occurrence of drop in pH and symptoms are recorded in these two-minute interval blocks. SAP utilizes Fisher’s exact test to evaluate for true association for fall in pH below 4 and reflux symptoms in each of these blocks4.
While previous studies have evaluated the different association tests to detect correlation between foregut symptoms and acid reflux episodes, the other important variable in evaluation of a pH study, pH cut-off threshold, has not been evaluated. All pH studies use a conventional pH cut off of 4 based on the premise that inactive pepsinogen is converted to active pepsin at this hydrogen ion concentration. In this model, the hydrogen ions and the active pepsin exert a corrosive action on the esophageal epithelial lining leading to destruction of the epithelial barrier function 5, 6. This consequently leads to inflammation and GERD symptoms 7. However, in a double blind esophageal acid-perfusion study involving 25 patients, 40% patients experienced heartburn at a pH of 6 8. Since patients may experience symptoms even at a higher pH, using a strict cut off of 4 during the analysis of a pH study may not capture all patients with true GERD symptoms associated with acidic reflux. Therefore, the aim of this study was to evaluate the impact of dynamic pH thresholds on gains in SAP in a large ambulatory pH study database. In addition we evaluated for the predictors of gains in SAP positivity with dynamic pH thresholds.
Prospective esophageal physiology database at our motility center was queried for eligible subjects based on predetermined inclusion and exclusion criteria. All consecutive patients that were referred for ambulatory pH testing at our tertiary care academic medical center between 2001 and 2008 were included. Other inclusion criteria were: a) presence of at least one reported symptom during the pH study b) a completed clinical symptom questionnaire c) being off of proton pump inhibitor for seven days, and d) age more than 18 years at the time of...