are not attributable to reflux. On-therapy study is usually used while patients are on PPIs twice daily, intending to investigate the hypothesis of residual acid reflux (55, 56] The threshold acid exposure time for an abnormal pH study done on PPI therapy proposed to be lowered to the level of 1.6% (57). As the diagnostic yield of on PPIs study is limited [58, 59], inclusion of symptom indices (symptom index, symptom association probability) adds an important dimension to the interpretation of pH monitoring. While the time pH 4 indicates the presence of abnormal acid, the symptom indices help to identify the causality of a particular symptom with episodes of acid reflux regardless of whether the total esophageal acid exposure is normal or abnormal.