The Gold standard investigation for Gastro-Oesophageal reflux disease is:
24 hour Esophageal pH recording provides a quantitative & sensitive documentation of acidic reflux episodes.
Endoscopy
24 hour esophageal pH recording
Oesophageal manometry
Measurement of length of lower oesophageal sphincter
Disorders of Gastrointestinal System Including Diarrhea