GOALS: We aim to look at the prevalence of multichannel intraluminal impedance-pH (MII-pH) studies, which are difficult to interpret secondary to low distal baseline impedance (DBI), and characterize them by their respective diagnosis and DBI. BACKGROUND: Some patients exhibit low DBI because of fluid retention in the esophagus or acute or chronic mucosal changes. Low DBI can make MII-pH difficult to interpret. STUDY: We reviewed MII-pH reports from patients studied from January 2002 to December 2009. We conducted a computerized search of the final interpretation for the terms "low," "low baseline," "difficult," and "unable." Reflux reports stating difficult or unable to interpret were analyzed. The associated manometry studies were reviewed to obtain the DBI (mean value at 5 and 10 cm above the lower esophageal sphincter in the pretest esophageal resting state). RESULTS: Of 2809 MII-pH tracings, 38 (1.4%) were classified as difficult to interpret because of low DBI. The most common underlying manometric diagnosis was ineffective esophageal motility at 36.8%, followed by 28.9% with achalasia, and 10.5% with scleroderma esophagus. An additional 15.8% of patients had increased gastroesophageal reflux on MII-pH. In only 7.9% of patients was no obvious reason for the low DBI identified. Of the 38 patients, 92% had a DBI <1000 Ω, and 58% had a DBI <500 Ω. CONCLUSIONS: These findings indicate that difficulty in interpreting MII-pH due to low baseline is very infrequent, and they suggest that it is unadvisable to perform MII-pH testing on patients with a DBI <500 Ω on prior MII-esophageal manometry. If needed, pH only testing off acid-suppressing therapy may be more advisable in these patients.