From the Guidelines
In 24-hour pH monitoring, the Symptom Index (SI), Symptom Sensitivity Index (SSI), and Symptom Association Probability (SAP) are calculated to correlate acid reflux events with patient symptoms, with a Symptom Index >50%, Symptom Association Probability >95% indicating a significant association between symptoms and reflux events 1.
Definitions and Calculations
The Symptom Index is calculated by dividing the number of symptoms associated with reflux episodes by the total number of symptoms, then multiplying by 100.
- A value greater than 50% is considered positive, indicating that at least half of the patient's symptoms occur during reflux events.
- The Symptom Sensitivity Index measures the percentage of reflux episodes that are associated with symptoms, calculated by dividing the number of reflux episodes associated with symptoms by the total number of reflux episodes, then multiplying by 100.
Key Considerations
- The Symptom Association Probability uses statistical analysis to determine if the association between symptoms and reflux events is coincidental, with a value above 95% considered statistically significant 1.
- These metrics are complementary and should be interpreted together, as each has limitations.
- For accurate calculation, patients must carefully record all symptoms during the monitoring period, and the timing window for associating symptoms with reflux events is typically 2 minutes following the reflux episode.
Clinical Application
- Reflux symptom association on ambulatory reflux monitoring, such as a symptom association probability >95% and symptom index >50%, increase confidence that symptoms are truly associated with reflux when esophageal acid exposure time is increased 1.
- These metrics can help guide the management of GERD, including the consideration of reflux-symptom association, integrity of the anti-reflux barrier, central obesity, esophageal physiology, visceral sensitivity, hypervigilance, and downstream gastrointestinal motility.
From the Research
Definitions and Calculations
- The Symptom Index (SI) is a numerical score that quantifies the association between symptoms and reflux, primarily assessing the specificity of a patient's reflux symptoms 2.
- The Symptom Sensitivity Index (SSI) is a score that quantifies the subject's sensitivity for reflux, providing a valuable additional parameter in 24-hour esophageal pH recording 2.
- The Symptom Association Probability (SAP) is a statistical method that calculates the probability that gastroesophageal reflux episodes and symptoms are associated, expressed as (1.0 - P) x 100%, where P is the probability that reflux and symptom episodes are unrelated 3.
Calculations and Comparisons
- The SAP is calculated by dividing the 24-hour pH signal into consecutive 2-minute periods, evaluating these periods and the 2-minute periods preceding the onset of symptoms for the occurrence of reflux, and applying Fisher's Exact Test to calculate the probability that reflux and symptom episodes are unrelated 3.
- The SSI is calculated based on the number of symptom episodes and reflux episodes, providing a measure of the subject's sensitivity for reflux 2, 4.
- The SI, SSI, and SAP are related but distinct measures, with discordance between them observed in some patients 3, 2, 4.
Clinical Applications
- The SAP, SI, and SSI are used to quantify and establish the relation between a patient's symptom and gastro-oesophageal reflux, with the SAP and SSI being preferable but still suboptimal predictors of response to high dose omeprazole 4.
- The addition of esophageal impedance monitoring to pH monitoring increases the yield of symptom association analysis in patients off PPI therapy, allowing for a higher proportion of patients to have a positive SAP 5.