Abdominal pain: myths & misdiagnoses CONTINUED
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The third myth is that administration of an opioid analgesic contaminates the examination. Trials consistently demonstrate that giving morphine doesn't alter the physical exam,6,7,8 and in one study at Brigham and Women's Hospital in Boston, administration of intravenous morphine actually enhanced diagnostic accuracy.

Routine investigations
The purpose of a diagnostic test is to distinguish patients who have a disease from those who don't. A positive likelihood ratio (LR+) indicates how much a positive test result is associated with the disease in question, whereas a negative likelihood ratio (LR-) connotes absence of the illness. The higher the LR, the more powerful the test is at discriminating whether to rule the disease in or out.

The WBC count is one of the screens most frequently ordered on patients with abdominal pain, yet study after study demonstrates that it's poor at separating those who have serious disease from the others. In one report that looked at 241 patients with undifferentiated abdominal pain, an elevated WBC was found in about 25% of subjects. The outcomes, however, were no different from patients whose WBC was normal.9 In a case series where appendicitis was diagnosed by laparotomy, the WBC count had a sensitivity of 76% and a specificity of only 52%.10

Plain abdominal x-rays also have limited use. Most researchers conclude that they give a low diagnostic yield, generate incidental and misleading findings and don't change patient management.11 Even among radiologists, there's poor inter-observer reliability. A clear indication, though, is a suspected bowel obstruction.

Urinalysis is cheap, simple and readily available. Either the dipstick test or routine analysis with microscopy exhibits high yield when the results fit with the clinical scenario. A screening urine pregnancy test is recommended for all women of child-bearing potential.

Clearly, not every patient with abdominal pain needs laboratory investigation. The decision to subject someone to testing ought to be driven by the severity or persistence of the pain and the presence of high-risk findings, e.g. advanced age, abnormal vital signs, right lower quadrant tenderness, etc.

Likelihood ratios
The incidence of appendicitis among all patients presenting to the ED with acute abdominal pain of < 1 week s duration is in the range of 12-26%. it's higher, though, in younger individuals < age 50. Most patients with appendicitis don't have the classic history or physical findings and, unfortunately, there are no pathognomonic features. Fortunately, clinicians don't usually rely on a single symptom or physical sign, but must use a combination of the two.

On history, the two features that have the highest positive LRs are pain in the RLQ, LR+ = 7.3-8.5, and migration of initial periumbilical pain, LR+ = 3.2. See Table 3 for other signs and symptoms.12 Only one physical finding showed a relatively high LR+ across all studies, and this was rigidity (LR+ = 3.8). Rebound tenderness had too wide a range of variance to make it a useful discriminating physical sign.

In ruling out appendicitis, a few features of the history proved to be useful absence of RLQ pain and having experienced previous episodes of the same pain, LR-: 0.0-0.3 and 0.3, respectively.

Microscopic hematuria and pyuria are present in 20-30% of patients with appendicitis, but they also occur in many other conditions and asymptomatic individuals as well. Elevated WBC (> 11.0 x 109/L) has poor sensitivity and specificity for the diagnosis.

Contrast-enhanced CT
Contrast-enhanced CT of the abdomen has become the king of tests to diagnose most intra-abdominal surgical conditions. Though it's less accurate than ultrasound in the evaluation of hepatobiliary or adnexal disorders, it's highly sensitive and specific for detecting appendicitis, diverticulitis, perforation, abdominal aortic aneurysm, abscess formation and mesenteric ischemia. Appendiceal CT has a sensitivity approaching 100% in certain hands and LR+ values are as high as 26.13

The problem with routine use of CT is that the radiation exposure increases the risk of cancer, especially in younger patients. Ultrasound has similar sensitivity to CT in children and it's almost as good as CT in adults. Consider this option when radiation exposure is a concern.

 
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References:

  1. Staniland JR et al. Clinical presentation of acute abdomen: study of 600 patients. BMJ 1972;3(5823):393-8.
  2. Liddington MI, Thomson WH. Rebound tenderness test. Br J Surg 1991;78(7):795-6.
  3. Brewster GS, Herbert ME. Medical myth: a digital rectal examination should be performed on all individuals with possible appendicitis. West J Med 2000;173(3):207-8.
  4. Manimaran N, Galland RB. Significance of routine digital rectal examination in adults presenting with abdominal pain. Ann R Coll Surg Engl 2004;86(4):292-5.
  5. Rusnak RA et al. Misdiagnosis of acute appendicitis: common features discovered in cases after litigation. Am J Emerg Med 1994;12(4):397-402.
  6. Brewster GS et al. Medical myth: Analgesia should not be given to patients with an acute abdomen because it obscures the diagnosis. West J Med 2000;172(3):209-10.
  7. Wolfe JM et al. Does morphine change the physical examination in patients with acute appendicitis? Am J Emerg Med 2004;22(4):280-5.
  8. Thomas SH et al. Effects of morphine analgesia on diagnostic accuracy in Emergency Department patients with abdominal pain: a prospective, randomized trial. J Am Coll Surg 2003;196(1):18-31.
  9. Lukens TW et al. The natural history and clinical findings in undifferentiated abdominal pain. Ann Emerg Med 1993;22(4):690-6.
  10. Cardall T et al. Clinical value of the total white blood cell count and temperature in the evaluation of patients with suspected appendicitis. Acad Emerg Med 2004;11(10):1021-7.
  11. Ahn SH et al. Acute nontraumatic abdominal pain in adult patients: abdominal radiography compared with CT evaluation. Radiology 2002;225(1):159-64.
  12. Wagner JM et al. Does this patient have appendicitis? JAMA 1996;276(19):1589-94.
  13. Rao PM et al. Helical CT combined with contrast material administered only through the colon for imaging of suspected appendicitis. AJR Am J Roentgenol 1997;169(5):1275-80.
 
Joseph H. Finkler, MD, CCFP(EM) is a clinical associate professor in the Department of Emergency Medicine at the University of British Columbia and an emergency physician at St. Paul s Hospital.

 
 
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