Viscous fluid expulsion in the evaluation of the constipated patient

The aim of this study was to evaluate a viscous fluid expulsion test as a method for quantifying rectal outlet obstruction.

We examined the ability of 30 severely constipated patients and 40 healthy control subjects to expel a viscous fluid simulating feces.

Constipated patients with defecographic rectal retention expelled a significant lower viscous fluid volume compared with controls (P < 0.0001). Considering the viscous fluid expulsion test to be positive if the patient expelled less than 160 ml of fluid (lower 95 percent confidence value in healthy controls) and using the presence of defecographic rectal outlet obstruction as a reference, the predictive value of a negative viscous fluid expulsion test was 100 percent, the predictive value of a positive test was 67 percent, and sensitivity and specificity was 100 percent and 82 percent, respectively. The viscous fluid test identified symptomatic outlet obstruction in almost twice as many patients as did defecography. Some patients, however are-despite their obstructive symptoms-able to evacuate their rectum when examined in the laboratory.

The viscous fluid expulsion test is a simple, reproducible, nonionizing, and low-cost method of quantifying rectal outlet obstruction. A positive test result should lead to a defecography, whereas a negative test result renders defecography dispensable and spares the patient from pelvic radiation.