Analysis of the acidity index and integrated intragastric acidity in 645 patients presenting with gastroesophageal reflux disease symptoms

Objective. In a recent study of patients receiving proton-pump inhibitor (PPI) therapy, a new parameter, the acidity index (AI), was described as being less complicated to calculate and of comparable accuracy (r = 0.93) to integrated intragastric acidity (IA) in assessing intragastric pH control. The aim of this study was to correlate AI with IA using a large database of ambulatory 24-h pH-metry studies in untreated patients presenting with gastroesophageal reflux disease (GERD) symptoms. Material and methods. We retrospectively analyzed 645 studies obtained from 1995 to 2001. Daytime (0800 h-2200 h), night-time (2200 h-0800 h) and 24-h IA and AI were calculated according to age, gender and the presence or absence of GERD, and correlations between these parameters were assessed using linear regression with F-statistic values, p-values and Akaike's Information Criterion values. GERD was defined as total esophageal pH time <4.0, 5 cm above the lower esophageal sphincter, for ≥4.2% of the day. IA and AI were calculated as follows: IA (mmol.h/ l) = ∑(acid in mmol/l at time "t" + acid in mmol/l at time "t - 1")/2 x ("t" - "t - 1"); AI = (%time pH <4 - %time pH <3) x 1 + (%time pH <3 - %time pH <2) x 10 + (%time pH <2 - %time pH <1) x 100 + (%time pH <1 - %time pH <0.8) x 1000. Results. Overall, the mean 24-h IA value was 882.0 ± 820.0 mmol.h/l (daytime 392.0 ± 400.0, night-time 490.0 ± 486.0). The mean 24-h AI value was 102.0 ± 87.0 (daytime 86.0 ± 80.0, night-time 120.0 ± 114.0, p <0.001). The mean 24-h IA value was 1057.0 ± 829.4 mmol.h/l (daytime 459.8 ± 406.0, night-time 597.2 ± 500.4, p <0.001) in GERD patients and 713.0 ± 775.0 mmol.h/l (daytime 326.0 ± 383.0, night-time 387.0 ± 448.5) in non-GERD patients (p <0.001). The mean 24-h AI value was 122.1 ± 88.1 (daytime 101.4 ± 82.5, night-time 145.3 ± 120.7) in GERD patients and 83.0 ± 81.0 (daytime 71.0 ± 73.9, night-time 96.4 ± 102.6) in non-GERD patients (p <0.001). Our statistical modeling demonstrated that the correlation between the acidity index and IA becomes progressively poorer with increasing values of acidity. Conclusions. We conclude that gastric acid production assessed by both IA and AI is higher during evening hours in comparison with daytime hours and the difference between night-time and daytime values is statistically significant. In addition, gastric acid production assessed by both IA and AI is significantly higher in GERD patients than non-GERD patients. This difference is primarily due to differences in nocturnal acid production. The AI correlates poorly with measured IA, especially at higher levels of gastric acidity. Therefore, AI is not an acceptable surrogate for IA in assessing gastric acid production.