Background &Aims:The efficacy of proton-pump inhibitor (PPI) therapy often is assessed to determine whether patients' symptoms are acid-related and if patients have gastroesophageal reflux disease (GERD), although the accuracy of this approach is questionable. We evaluated the diagnostic performance of the PPI test, in conjunction with other tests, for the diagnosis of GERD.Methods:We analyzed data from the DIAMOND study, a multinational trial that compared the ability of the reflux disease questionnaire with that of symptom-based clinical diagnosis to identify GERD in primary care patients with frequent upper-gastrointestinal symptoms. Patients (n = 308) were given placebo and further evaluated by endoscopy, wireless esophageal pH-metry, and symptom association monitoring. Those with GERD (n = 197) were identified based on the presence of reflux esophagitis, esophageal pH level less than 4 for more than 5.5% of 24 hours, or positive results from symptom association monitoring (or a positive result from the PPI test in patients with borderline levels of esophageal acidity). All patients then were given single-blind therapy with esomeprazole (40 mg once daily) for 2 weeks and symptoms were recorded daily.Results:A positive response to the PPI test was observed in 69% of patients with GERD and in 51% of those without GERD. Response to placebo did not influence the diagnostic ability of the subsequent PPI test. More patients with reflux esophagitis had a positive result from the PPI test than patients without GERD (57% vs 35%;P= .002) or patients with GERD but no esophagitis. A clinical diagnosis by the primary care physician of an acid-related disease was not associated with response to PPIs.Conclusions:In a well-characterized population of primary care patients with frequent upper-gastrointestinal symptoms of any type, the PPI test has limited ability to identify patients with GERD, diagnosed by current standard tests.