Reflux symptoms can be experienced secondary to a range of problems that are not always acid related. Even in those related to acid reflux the OGD can be macroscopically normal in 85%.
15-30% of case are due to true gastro-oesophageal reflux disease,
>70% is related to non erosive reflux disease (NERD)
A recent trial by E. Savarino has helped differentiate these patients further by using assessments of their acid exposure time (AET) and symptom association probability (SAP).
Gut 2009;58:1185-1191

Of the 200 patients reviewed with presumed NERD, they found 3 main clinical catagories; 
1) NERD pH+ive   – 41%
                       – +ive AET
2) Hypersensitive – 32%
                       – Norm AET / +ive SAP
                       – secondary to non/acid reflux
3) Functional       – 27%
                      – Norm AET / -ive SAP
                      
The Rome III criteria for Functional Reflux also suggests that these patients should also have no response to anti acid / proton pump inhibitors (PPIs). Typically these patients also suffer from associated nausea, early satiety, bloating and postprandial fullness. It was felt that these patients have more in common with those suffering functional dyspepsia, than those with true NERD.