Ward et al developed the Symptom Index (SI) to help correlate clinical symptoms with evidence of acid reflux in patients undergoing esophageal pH monitoring. This can help establish a cause and effect relationship for what can be vague complaints.



(1) A patient undergoes 24 hour esophageal pH monitoring.

(2) Each time a symptom was experienced the patient records the time.

(3) Upon completion of the monitoring the symptom log is compared to the record of low esophageal pH readings (< 4). A symptom was considered to be associated with a low pH if the symptom occurs within 2 minutes of a reflux episode (4 minute window).


Symptoms to evaluate:

(1) heartburn

(2) chest pain

(3) any other symptom


symptom index =

= (number of times symptom noted when pH was < 4) / (total number of times symptom noted) * 100%



• minimum SI: 0%

• maximum SI: 100%

• The higher the SI the greater the apparent linkage between acid reflux and symptoms

Symptom Index


< 25%

low SI

25 - 49%


50 - 74%


>= 75%

high SI



• Johnston et al (1994) stated that the index does not take into account the number of episodes with pH < 4 nor the total time period that the pH < 4. Interestingly the lead author found the SI useful for management of patients with GERD in 1992.


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