Nabbed: CT bests x-ray for detection of illegal drug packets

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Low-dose CT provides an effective and more sensitive alternative to abdominal x-ray in the screening of people suspected of carrying intracorporeal drug packets, according to a study published online Sept. 5 in Radiology. The authors cautioned recent advances in packet materials may render x-ray obsolete for this application.

The standard procedure to care for persons suspected of swallowing drug packets usually entails abdominal x-ray to confirm or discredit the suspicion and determine the need for stool analysis. However, the utility of the approach has not been investigated using CT as a reference standard. Although prior studies had reported CT as more accurate than x-ray, dose concerns had limited its use. Low-dose scanning addresses these concerns.

Thus, Pierre-Alexandre Poletti, MD, from the department of radiology at University Hospital Geneva, and colleagues designed a study to evaluate the diagnostic performance of abdominal x-ray in the detection of intracorporeal packets.

The study focused on 300 adult suspects (90 percent men) who underwent x-ray and CT imaging from July 2007 to July 2010.

Subjects first underwent abdominal x-ray, and the radiologist used a standardized electronic form to define the study as positive or negative for packets, and rated diagnostic confidence on a 1-4 scale.

Next, subjects underwent CT, with results also reported on an electronic form. Based on the CT exam, the radiologist reported the number of packets as 12 or fewer or more than 12. The packets were described as isoattenuated or hyperattentuated.

The effective dose for the x-ray studies was 2. mSv for women and 1.3 mSv in men. For CT studies, effective dose was 1.7 mSv for women and 1.2 mSv for men.

CT showed packets, which were found at stool analysis, in 53 suspects.

Radiologists suspected the presence of packets in 51 suspects on the basis of x-ray imaging. “When compared with low-dose CT, 41 abdominal radiographs showed true-positive findings, 10 had false-positive findings, 267 had true-negative findings, and 12 had false-negative findings.” X-ray was primarily limited by its 23 percent false-negative rate in this study.

Positive and negative abdominal x-ray results were more likely to be accurate when confidence was high (level 3 or 4), according to Poletti and colleagues. “This observation suggests that abdominal radiography cannot be relied on to detect packets when the radiologist is not confident in his or her interpretation.”

A further concern regarding x-ray is radiologists’ observation that the density and number of packets significantly correlated to the rate of false-negative readings.

Abdominal x-ray sensitivity was 0.50 for suspects carrying iso-opaque packets compared with 0.89 for those carrying opaque packets, which suggests radiologic attenuation plays an important role in detection. Sensitivity fell to 0.25 when the suspect carried 12 or fewer isoattenuated packets. “With improvement in packet manufacturing, it would not be surprising if a majority of intracorporeal containers became undetectable at abdominal radiography in a short time.”

“Performing low-dose CT instead of abdominal radiography will improve the detection of illegal intracorporeal packets, without increasing the radiation dose,” Poletti and colleagues concluded.