To diagnose GERD , in 24 hour ambulatory pH testing the percentage of the day that the pH was less than 4 is:

The percentage of the day that the pH was less than 4 with values exceeding 5% indicative of GERD.Intraluminal impedance monitoring can be added to pH monitoring to detect reflux events irrespective of whether or not they are acidic. Ref : Harrison, E-18, P- 2428