How can I tell if my heartburn is acid, non-acid, or functional?

The short version: you cannot reliably tell the difference without esophageal testing, but the pattern of your symptoms and your response to acid-blocking drugs gives strong clues.

The most useful tool is combined multichannel intraluminal impedance and pH monitoring (MII-pH). It measures both acid and non-acid reflux, and it can tell whether your symptoms actually line up with reflux events [3]. In a 2014 study of 100 patients who still had heartburn despite taking proton pump inhibitors (PPIs), only 14% had acid reflux alone. The majority, 55%, had non-acid reflux, and 30% had a mix of both [1]. A 2016 study of 39 PPI-resistant patients found that 21 out of 39 (about 54%) had functional heartburn, meaning their pain came from a sensitized esophagus, not from reflux at all [2].

The pattern helps. If your heartburn gets better with a PPI, it is likely acid-related. If it does not, non-acid reflux or functional heartburn becomes more likely [2]. Functional heartburn is diagnosed when endoscopy is normal, acid exposure on pH monitoring is normal, and there is no link between your symptoms and reflux episodes [4]. Non-acid reflux is common in people on PPIs, because the drug turns acid reflux into weakly acidic reflux without stopping the reflux itself [3].

The evidence here is thin on long-term outcomes. Most studies are small and look at short-term symptom relief, not at what happens to the esophagus over years. The ACG guideline on esophageal testing notes that the overall evidence base is of low quality [5].

My call: the distinction requires MII-pH testing, but a failed PPI trial strongly points away from acid reflux. Confidence: moderate — the diagnostic categories are well-defined, but the evidence on how to treat non-acid and functional heartburn long-term is weak.