<prompt> Best test to diagnose gastroesophageal reflux disease and quantity acid output isEsohagogramEndoscopyManometry24 Hr pH monitoring  <response>: .for GERD, THE INVESTIGATIONS can be, 24 hours oesophageal pH monitoring is regarded as the gold standard.PPI should be stopped for 3 weeks prior to pH monitoring. barium study in head down position. endoscopy to exclude other diseases. oesophagal manometry to assess the state of lower oesophagal sphincter. Ref: SRB;s manual of surgery,5th ed, pg no 792 