CT findings and outcomes of acute cholecystitis: is additional ultrasound necessary?

Background: While ultrasound is often the preferred imaging modality for suspected acute cholecystitis (AC), CT is often the first line study when patients are being evaluated for abdominal pain. The diagnostic value of CT in the setting of AC is controversial, given the prevalent use of additional imaging.

Purpose: To evaluate the positive predictive value (PPV) of CT for diagnosing acute cholecystitis (AC) when used as a first line imaging study for evaluating abdominal pain and assess if additional imaging with ultrasound studies provides additional clinically useful information.

Materials and methods: Abnormal gallbladder findings in CT imaging studies were queried in a retrospective study over a 25-month period within a large urban health system. Sonographic (US) studies performed within 72 h of the initial CT were also included. Outcomes were determined by surgical pathology, fluid analysis, and clinical outcomes. Cases were stratified by the interpreting radiologist's subjective confidence level of diagnosing AC, and the PPVs were compared between cases using CT without US and cases with both CT and US.

Results: Of the 468 CT studies meeting criteria, 192 were read as concerning for AC. PPV of CT was 44.7% without US and 50.5% when US was positive, which amounted to an insignificant gain (p = 0.41). When subdividing by confidence level, high-confidence positive CTs demonstrated no significant difference without ultrasound (80%) compared to with ultrasound (75%). Less confident reads in CT demonstrated potential gain from ultrasound; in the case of a "probable" CT impression, PPV increased from 45% without US to 90% with a high-confidence ultrasound impression.

Conclusion: Based on current practice within a large health system, CT examinations with high suspicion for AC demonstrated little gain from follow-up ultrasound. However, ultrasound may be of benefit when CT interpretations are less confident but still suspicious for AC.