Small Bowel Obstruction: When Is X-Ray Enough?

Abstract

Abdominal x-rays are frequently ordered in patients with suspected small bowel obstruction (SBO). However, this practice is discouraged by the American College of Radiology due to poor sensitivity. Abdominal x-rays have insufficient sensitivity (75%) and specificity (66%) to rule in or rule out a diagnosis of SBO. Further, x-rays lack reliable information on the etiology and level of obstruction, almost always warranting further imaging. In most cases, alternative diagnostic modalities such as ultrasound and computed tomography should be used to diagnose SBO.

FormalPara Pearls and Pitfalls

• Abdominal x-rays have poor sensitivity for small bowel obstruction (SBO). They are poor screening tests for SBO.

• Abdominal x-rays cannot reliably differentiate between ileus and SBO.

• Computed tomography and/or ultrasound are better tests when SBO is suspected.

Abdominal radiographs are often ordered in the initial workup of suspected small bowel obstruction (SBO), but their clinical utility is limited due to poor sensitivity and specificity. The historical argument for obtaining radiographs to screen for SBO is predicated on their widespread availability, low ionizing radiation, and low cost. However, the American College of Radiology (ACR) recommends that other diagnostic modalities such as computed tomography (CT) are more appropriate for patients with suspected SBO [1].

Studies assessing the performance of x-ray for diagnosing SBO yield variable results. Early studies demonstrated sensitivities as high as 86% but did not list confidence intervals [2, 3]. A meta-analysis found abdominal radiographs had a pooled sensitivity of 75% (95% CI 68–80%) in all-comers [4]. The sensitivity of abdominal x-rays may be higher in high-grade obstruction. A study reporting an overall sensitivity of 69% for SBO found that low-grade SBO was detected on only 50% of plain films, while high-grade SBO was detected in 86% of plain films [3].

The specificity of abdominal x-ray for the diagnosis of SBO is also poor, reported at 66% (95% CI 55–76%) [4]. This lack of specificity may stem partially from an inability to differentiate between ileus and mechanical obstruction [5]. Radiographs identified only 13–19% of postoperative SBOs, even when clinical context was taken into consideration [4].

In addition to difficulties reliably diagnosing the presence of SBO, abdominal x-rays often fail to identify the level and etiology of obstruction. One study found that x-ray identified the etiology of obstruction in only 7% of cases and the level of obstruction in only 66–78% of cases [6, 7]. Because x-rays rarely find the cause of obstruction and only intermittently find the level of obstruction, positive x-rays are frequently followed by more advanced imaging.

Based on available data, abdominal x-rays have insufficient sensitivity to rule out SBO and often yield inconclusive results. X-ray does not perform well enough to be used as a screening test due to the frequency of both inconclusive and false-negative results. Other diagnostic modalities such as CT and ultrasound perform far better in diagnosing SBO. Table 47.1 shows the sensitivity and specificity of x-ray, point-of-care ultrasound, and CT for diagnosing SBO.

Table 47.1 Performance of diagnostic modalities to diagnose Small Bowel Obstruction [8, 9]

In some institutions, patients presenting with a clinical picture of recurrent SBO with known etiology may receive only x-ray and/or ultrasound if conservative management is planned.

FormalPara Suggested Resources

• EM Lyceum GI Imaging, “Answers” (May 2014: https://emlyceum.com/2014/05/07/gi-imaging-answers/)

• FOAMcast Episode 23 (January 2015: http://foamcast.org/tag/small-bowel-obstruction/)

• Gottlieb M, Peksa GD, Pandurangadu AV, Nakitende D, Takhar S, Seethala RR. Utilization of ultrasound for the evaluation of small bowel obstruction: a systematic review and meta-analysis. Am J Emerg Med. 2018;36(2):234–42.