Area under pH 4: advantages of a new parameter in the interpretation of esophageal pH monitoring data in infants.

Twenty-four-hour continuous esophageal pH monitoring has become the preferred test to quantify acid gastroesophageal reflux. Agreement has been achieved that acid gastric content reflux into the esophagus constitutes a major cause of reflux esophagitis; we therefore calculated the "area under pH 4" (A) in 560 consecutive pH monitorings in infants 1-12 months old and related this parameter (A) to the reflux index (RI): RI less than 10%: A 185 +/- 295 (mean +/- 1 SD); RI 10-19%: A 1,046 +/- 1,206; RI 20-29%: A 1,967 +/- 2,038; RI 30-39%: A 3,307 +/- 2,955; RI greater than 40%: A 7,977 +/- 7,227. A higher RI is associated with a greater area under pH 4 (p less than 0.001); the high SD obtained in each group, however, illustrates the great variability in surface (or acidity of the reflux episodes) in all groups. Esophagoscopy and biopsy were performed in 112 infants (20%). Specificity in the prediction of (mild) esophagitis was higher for the area under pH 4 (88%) compared with the RI (50%). The sensitivity of both parameters was comparable (96 versus 93%). Our results strongly suggest that in young infants, the acidity of the reflux episodes (the area under pH 4) is a determining factor in the prediction of esophagitis. These data need to be confirmed by more studies before general application of this parameter can be advised.