Diagnostic Accuracy of Ultrasound for the Evaluation of 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 Feb;36(2):234-242.

Study Population: 1178 patients suspected of having bowel obstruction across 11 trials

Small bowel obstruction (SBO) comprises 2% of patients presenting to the emergency department (ED) with abdominal pain, with over 300,000 hospitalizations per year.1^, 2 SBO occurs when bowel contents cannot pass through the small bowel, due to either mechanical or functional etiology. If not appropriately diagnosed, SBO can result in intestinal ischemia, necrosis, and perforation.3 Traditional means of diagnosis such as plain film radiography suffer from poor sensitivity and specificity.4 Other imaging modalities include computed tomography (CT) or magnetic resonance imaging (MRI). Ultrasound has demonstrated promise in evaluation for SBO, as it is rapid, repeatable, inexpensive, does not expose the patient to radiation, and can be performed at the bedside.5^, 6

The meta-analysis discussed here included trials that evaluated the accuracy of ultrasound for diagnosing SBO.7 The gold standard confirmatory test was determined by individual study definition and included CT, enteroclysis, diagnosis at surgery or discharge, or clinical follow up. The primary outcome was diagnostic accuracy of ultrasound for SBO, with subgroup analysis based on specific clinical setting (ED vs non-ED setting). The authors of the meta-analysis also conducted a sensitivity analysis that categorized inconclusive ultrasound examinations as false negatives.

The meta-analysis included 11 prospective, observational studies (n = 1178 patients). These studies enrolled patients suspected of acute SBO, with most studies using a convenience sample. Five studies were conducted in the ED, and 6 were conducted in other settings. ED providers performed the ultrasound in 3 studies. The mean age of the patients included in the analysis was 50 years, and 74% of patients were male. Overall, ultrasound was 92% sensitive (95% confidence interval [CI], 89% to 95%) and 97% specific (95% CI, 88% to 99%), with a positive likelihood ratio (LR+) of 27 (95% CI, 8 to 98) and a negative likelihood ratio (LR-) of 0.08 (95% CI, 0.06 to 0.11).

Sixteen examinations were non-diagnostic. Assuming all of these examinations were false negatives (worst case scenario), a sensitivity analysis demonstrated a slightly lower sensitivity of 91% (95% CI 86% to 94%) and +LR of 25 (95% CI 7 to 84), though specificity and -LR remained similar.

While ultrasound can be a valuable tool in diagnosis of SBO, it depends on the individual provider’s ability to obtain and interpret appropriate images. This meta-analysis included studies with differences in sonographer experience and diagnostic criteria, which limit the applicability of the data. However, previous literature has suggested good diagnostic accuracy for identifying SBO after a 10-minute training session and 5 practice ultrasound examinations.5 ED providers were the primary sonographers in only 3 studies, limiting the generalizability of the findings to ED. Most studies enrolled patients based on a convenience sample, which could be a source of selection bias. Several studies did not explicitly state whether providers were blinded to the CT results, which may have led to potential bias. Inclusion of studies from different clinical settings can also contribute to spectrum bias; however, test characteristics for ultrasound performed by ED providers did not differ significantly from other providers based on the authors’ subgroup analysis. A recent prospective observational study published in 2019 suggests that the diagnostic accuracy of ultrasound for SBO may be lower, with a sensitivity of 88% and specificity of 54% (95% CI 45% to 63%).8 Of note, this study utilized providers with variable ultrasound experience and was performed at centers which did not regularly use ultrasound for SBO.8 Therefore, further studies evaluating the diagnostic accuracy among ED providers are needed.

Included studies in the meta-analysis were at low to moderate risk of bias, and limited data were present concerning pediatric patients.7 Moderate heterogeneity was present regarding patient population and outcome assessment, and studies demonstrated mild heterogeneity for sensitivity and moderate heterogeneity for specificity.

Although ultrasound offers several advantages compared to CT including the ability to rapidly diagnose SBO and assess progression of SBO over time, CT may still be necessary to determine the underlying etiology, which can influence the decision for operative management.

The existing evidence indicates that ultrasound is sensitive and specific in diagnosis of SBO. While the exact test characteristics for diagnosing SBO in an ED setting performed by ED physicians are not clear, this modality is rapid, repeatable, inexpensive, easily available, noninvasive, and not associated with radiation. Therefore, we have assigned a color recommendation of Green (Benefit > Harm) to the use of ultrasound for evaluation of SBO.

May 3, 2019

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)