Background

Acute abdominal pain is one of the most common presenting complaints to the emergency department (ED), accounting for up to 8.8% of all ED visits in 2018.¹ A study using the National Hospital Ambulatory Medical Care Survey (NHAMCS)² estimated 23 million visits for abdominal pain in 2013. Computerized tomography (CT) scans were used in 30.1% of the visits in 2010 and 28.6% in 2013. More than 50% of the visits received diagnostic imaging, and less than 20% became hospital admissions.²

Developments in imaging techniques have dramatically changed the ED evaluation of abdominal pain, with CT and ultrasound (US) widely used in current practice. Most presentations of acute abdominal pain fall into one of eight diagnoses: appendicitis, bowel obstruction, cholecystitis, renal colic, peptic ulcer disease, pancreatitis, diverticular disease, and nonspecific abdominal pain. With such a wide array of potential etiologies for abdominal pain, the clinician must decide how to rationally use diagnostic tests and not to miss any serious diagnoses against concerns for costs of testing, length of stay, and adverse effects of imaging. Concerns for overuse, incidental findings, and radiation risks have been raised, and a consensus conference was convened in 2015 resulting in a research agenda to optimize diagnostic imaging in the ED.³

Laboratory tests, such as complete blood count, urinalysis, liver function tests, and tests for pancreatic enzymes are ordered on approximately 70% of ED patients with acute nontraumatic abdominal pain.^2,4 While many studies have evaluated the usefulness of laboratory tests in the diagnosis of specific conditions (e.g., appendicitis), few studies evaluated laboratory tests among all acute undifferentiated abdominal pain. None of these studies reliably determined whether a laboratory test changed the diagnosis, so we will not discuss lab testing for undifferentiated abdominal pain patients in this chapter.

Imaging modalities available include plain abdominal radiography, CT (with or without intravenous and oral contrast), US, and magnetic resonance imaging (MRI), leaving clinicians with a choice of which tests to order and in what sequence. While it seems that a clinical decision rule could be helpful with this decision, nontraumatic abdominal pain is a heterogeneous presenting complaint, and to date, no rule has been validated.⁵ In this chapter, we will address the role of imaging in acute undifferentiated abdominal pain, including comments on special populations such as older adults. In subsequent chapters, we address tests for the most common causes of acute nontraumatic abdominal pain.

Clinical question

Which diagnostic imaging modality is most sensitive in diagnosing patients with acute undifferentiated abdominal pain?

Several studies have evaluated how abdominal CT changes diagnoses, the admission decision, and the need for surgery in ED patients. One randomized study concluded that early CT scanning in patients with acute abdominal pain improves diagnostic accuracy, and might reduce the duration of hospitalization and mortality.⁶ A prospective study evaluated the added value of CT in 536 consecutive patients with nontraumatic acute abdominal pain.⁷ Physicians responded to five questions prior to ordering the CT scan, including their pretest diagnostic impression and level of certainty about intended management before CT results were available. Pre and posttest diagnoses were concordant for only 37% of the patients. CT scanning was associated with a reduction in rates of planned immediate surgical intervention from 13% to 5% and reduced the perceived need for admission by 17%. The authors concluded that ED use of CT was associated with increased diagnostic certainty, reduced unnecessary surgery, and hospital admissions, and that the value of CT is greater than its direct costs. Barksdale et al.⁸ in a prospective cohort of 547 ED patients with nontraumatic abdominal pain, found that 54% had a significant change in their diagnosis based on the CT scanning.

Plain abdominal radiography has limited value in the ED setting and several studies have demonstrated its lower accuracy than other diagnostic modalities. A small study compared noncontrast abdominal CT with three‐view plain abdominal radiography in patients with abdominal pain within the previous 7 days.⁹ Among 103 patients, CT had a sensitivity of 96% (95% confidence interval [CI] 86–100%), specificity of 95% (95% CI 83–99%), and accuracy of 96%. The abdominal X‐ray series had a sensitivity of 30% (95% CI 18–45%), specificity of 88% (95% CI 74–96%), and accuracy of 56%. Noncontrast abdominal CT revealed the cause of acute abdominal pain, including many of the concerning surgical or medically emergent or urgent causes, significantly better than plain films, and CT led to the discovery of pathology that was not identified using plain radiography. This study also supported that noncontrast CT has sufficiently high sensitivity, specificity, and accuracy to make it a clinically useful study.⁹ Another study of undifferentiated acute abdominal pain compared the diagnostic yield of abdominal plain radiography and abdominal CT scanning.¹⁰ Out of 1000 patients, only 120 had both plain abdominal imaging and abdominal CTs performed. Plain abdominal radiography had sensitivities of 0% (95% CI ranges 0–84%) for the diagnosis of urolithiasis, appendicitis, pyelonephritis, pancreatitis, and diverticulitis. X‐rays had sensitivity of 33% (95% CI 25–42%) for bowel obstruction. The specificity for all the diagnoses was 100% (95% CI range 96–100%). The authors concluded that plain abdominal radiography was insufficiently sensitive in the evaluation of acute nontraumatic abdominal pain.¹⁰ Gerhardt et al.¹¹ examined 165 patients with acute nontraumatic nonspecific abdominal pain undergoing abdominal imaging. The need for urgent intervention within 24 hours was 13%, and 34% underwent elective interventions that mitigated morbidity or mortality. They found that noncontrast abdominal CT was the most accurate variable for prediction of an acute medical or surgical intervention. In a classification and regression tree analysis, the combination of history, physical exam, acute abdominal series imaging, and noncontrast abdominal CT imaging yielded the best test characteristics to predict the need for medical or surgical intervention (sensitivity 92%, specificity 90%, a LR+ of 9.2, and a LR− of 0.09). Other models that did not include CT imaging had lower sensitivities and specificities and were felt to be clinically unacceptable. The authors of this derivation study concluded that noncontrast abdominal CT was useful and the imaging study of choice when faced with nonspecific abdominal pain.¹¹

A systematic review by Alshamari et al.¹² compared the use of low‐dose CT scanning with abdominal radiography in diagnosing patients who present with nontraumatic acute abdominal pain. The sensitivity of low‐dose CT ranged from 75% to 96%, while the sensitivity of abdominal radiography ranged from 30% to 77%.

A prospective study in the Netherlands evaluated eleven possible diagnostic strategies in 1021 nonpregnant adults with acute abdominal pain.¹³ The strategies included clinical evaluation alone or in combination with plain X‐rays, US, and/or CT scanning. Physician reviewers later classified the patients as having had an urgent or a nonurgent condition (treatment within 24 hours not required) based on 6‐month follow‐up. The prevalence of urgent conditions was 65%. Clinical evaluation alone was 88% sensitive and 41% specific for an “urgent” condition. US to all patients would have been less sensitive (70%), but more specific (85%). Two strategies that involved doing US in all patients were the most sensitive: US followed by CT scanning if the US is negative or inconclusive (sensitivity 94%, specificity 68%), or CT only if the US is negative (sensitivity 85%, specificity 76%). These two strategies would result in the lowest use of CT (49% and 27% of patients, respectively). The authors concluded that in nonpregnant adults with acute abdominal pain, if a clinician thinks imaging is required, then US should be the initial modality, followed by CT, as US has high sensitivity and can reduce the use of CT by about half.¹³

Subsequent studies concluded that CT and US have been shown to increase diagnostic accuracy substantially and have decreased the added diagnostic value of plain abdominal radiography and that there is no place for plain abdominal radiography in the workup of ED adult patients with acute nonspecific abdominal pain in current practice.¹⁴

The decision to order a CT should be based on clinical judgment. However, there is a high variability in the use of CT between emergency physicians. Cross et al.¹⁵ performed a retrospective study including 6409 ED visits for nontraumatic abdominal pain to evaluate physician‐ and patient‐level factors. They found that among physician‐level factors, only those with more than 21 years of experience after medical school graduation were less likely to order a CT. After adjusting for physician‐level factors, they found that patient‐visit factors (e.g., age, arrival model, admission team, prior CT, ED arrival time, US, and white blood cell [WBC] count) were key reasons behind CT ordering practice.

Diagnostic US has been widely studied for specific abdominal diagnoses (e.g., gallstones). However, its use in acute nonspecific abdominal pain by emergency physicians has not been comprehensively studied. Hasani et al.¹⁶ prospectively compared the diagnostic accuracy of emergency physicians with radiologists when using point‐of‐care US in patients presenting to the ED with acute nontraumatic abdominal pain. Emergency physicians had a diagnostic accuracy of 78%, sensitivity of 76%, and specificity of 81%. The radiologists had similar performance with accuracy of 78.6%, sensitivity of 82%, and specificity of 73%. They found that emergency physicians had better results in diagnosing some entities (abdominal aortic aneurysm [AAA] and renal stones) than others (acute appendicitis, cholelithiasis, and cholecystitis). A systematic review of the diagnostic characteristics of point‐of‐care US in the ED for diagnosing AAA, included seven high‐quality studies¹⁷ and showed excellent performance (pooled sensitivity 99% [95% CI 96–100%], specificity 98% [95% CI 97–99%]). In unstable patients presenting with undifferentiated abdominal pain in which AAA is suspected, point‐of‐care US should be the initial study of choice.

Jang et al.¹⁸ performed a pilot study to assess the impact of emergency physician‐performed US on diagnostic testing and decision‐making in ED patients presenting with nonspecific abdominal pain. A total of 128 patients were included, of which 58 (45%) had an improvement in diagnostic accuracy and planned diagnostic workup with emergency physician‐performed US.

While different imaging modalities are used to evaluate the undifferentiated patient with abdominal pain in the ED, a systematic review published in 2018 found 10 studies evaluating diagnostic pathways to be used in this population.¹⁹ Only five studies developed their pathways based on prospective studies, however, none of these studies have evaluated the impact on complication rates, hospital length of stay, or cost. Although based on low‐quality evidence, most studies recommend the use of routine US followed by CT scan when there is still diagnostic uncertainty. The authors highlight the fact that sensitivity and accuracy of US are operator and hospital dependent, and this should be considered when developing diagnostic protocols in the ED.

Lastly, there are few diagnoses that are infrequent as the cause of acute nontraumatic abdominal pain in the ED. Acute mesenteric ischemia is such example and it has an estimated annual incidence of 0.09–0.2% per patient‐year. However, the mortality associated with this condition is very high and it is often a concern of emergency physicians. This diagnosis is not further discussed in other chapters, and we will briefly discuss it here. A systematic review and meta‐analysis by Cudnik et al.²⁰

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