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

Background 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. Methods 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. Key Results 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 30 mL had a PPV 77.3%, MRGTA of 10 cm 2 had a PPV 75.0%, and RFAS of 9 cm 2 had a PPV of 68.8% for identifying constipated patients with RED . Conclusions & Inferences Rectal gas measurements on abdominal imaging may indicate RED in patients with constipation. At ~90% specificity for RED , RGV of 20 or 30 mL or MRGTA of 10 cm 2 on CT has PPV ~75%, and RASF of >9 cm 2 has PPV of ~69%.