Rectal gas volume: Defining cut-offs for screening for evacuation disorders in patients with constipation.

Abstract

Diagnosis of rectal evacuation disorders (RED) is currently based on anorectal manometry (ARM) and evacuation tests in specialized laboratories; we recently showed higher rectal gas volume (RGV) and maximum rectal gas transaxial area (MRGTA) measured on abdominal and pelvic computed tomography (CT) in patients with documented RED.The aim of this study was to obtain cut-off values of RGV, MRGTA, and rectal area on scout film (RASF) to differentiate constipated patients with RED from those without RED, based on ARM, balloon expulsion test (BET), and colon transit test. We identified 118 constipated patients (65 with RED) with prior record of CT. Using standard CT software, we used a variable region of interest (ROI) program to measure RGV, MRTGA, and RASF, as previously described. We constructed receiver operating characteristics curves based on different values, and we estimated AUC, specificity, and positive predictive value (PPV) to detect RED in patients with constipation. Receiver operating characteristics of the models to predict RED showed AUC 0.751 for RGV and 0.737 for MRGTA (both P<.001), and 0.623 for RASF (P=.029). At specificity of 90%, RGV of 30mL had a PPV 77.3%, MRGTA of 10cm2 had a PPV 75.0%, and RFAS of 9cm2 had a PPV of 68.8% for identifying constipated patients with RED. Rectal gas measurements on abdominal imaging may indicate RED in patients with constipation. At ~90% specificity for RED, RGV of 20 or 30mL or MRGTA of 10cm2 on CT has PPV ~75%, and RASF of >9cm2 has PPV of ~69%.