Approach Considerations
No specific laboratory studies are indicated in the evaluation of a patient with a perianal or anorectal abscess. Certain patients, such as individuals with diabetes and those who are immunocompromised, are at high risk for developing bacteremia and possibly sepsis, as a result of an anorectal abscess. In such cases, complete laboratory evaluation is important. Laboratory evaluation of the septic patient is not the focus of this article.
When an anorectal abscess is not obviously apparent but a high degree of clinical suspicion exists, imaging (eg, computed tomography [CT], ultrasonography [US], or magnetic resonance imaging [MRI]) may be necessary for the diagnosis. Plain films are of little utility for this purpose and should not be obtained unless other diagnoses are being considered.
Laboratory Studies
Many patients with perianal abscesses present as outpatients and need no laboratory blood tests; incision and drainage will be sufficient. For patients with such abscesses who present to the emergency department (ED) with no signs of systemic disease, this management strategy may be adequate, in that laboratory data will yield very little additional information of practical use.
In patients with suspected perirectal abscess or systemic disease, a complete blood count with differential may be obtained and may show leukocytosis or a left shift, but the absence of these findings does not preclude either of these entities. As many as 23% of patients with diagnosed perirectal abscesses have a normal temperature and a normal white blood cell count with a normal differential. [2]
Even though presumptive antibiotics are not required with routine incision and drainage of uncomplicated perianal abscesses, [21, 22] wound cultures should be collected in all patients in whom incision and drainage is performed; newer strains of bacteria (eg, methicillin-resistant S aureus) are being recognized as causes of perirectal abscesses, [8, 9] and recurrence rates are as high as 10%. [1] Blood cultures may be obtained but may have little to no diagnostic yield; one study reported no growth on blood cultures. [2]
CT, Ultrasonography, and MRI
Although imaging studies usually are not necessary in the evaluation of patients with an anorectal abscess (which will be perianal in the majority of cases), clinical suspicion of an intersphincteric or supralevator abscess may require confirmation by means of CT, anal US, or MRI. [23, 24] As a rule, use of anal US is limited to confirming the presence of an intersphincteric abscess, though this modality can also be used intraoperatively to help identify a difficult abscess or fistula.
CT is readily available in most EDs and is commonly used in the diagnosis of perirectal abscesses. In one retrospective study, CT for perirectal abscesses confirmed by surgical drainage yielded a sensitivity of 77%, with the false-negative scans being significantly more likely to come from immunocompromised patients. [25]
Although not readily used for this purpose in the ED, transperineal US has shown good results for the detection of fistulous tracts and fluid collections in preoperative planning, with sensitivities ranging from 85% [26, 27, 28] to 100% [29] or the detection of surgically significant disease.
MRI is the criterion standard for imaging of perirectal abscesses; its 91% sensitivity makes it useful in preoperative planning. [30] In the ED, however, its use is restricted.
Endoscopy
Evidence suggests that the use of endoscopic visualization (transrectal and transanal) is an excellent way of evaluating complex cases of perianal abscess and fistula. With the endoscopic technique, the extent and configuration of the abscess and fistulas can be clearly visualized. Endoscopic visualization has been reported to be as effective as fistulography. [31]
In experienced hands, endoscopic evaluation is the preferred diagnostic procedure in patients with perirectal pathology because of the low risk of bacterial dissemination and the low incidence of patient discomfort. Performing an endoscopic evaluation after nonsurgical treatment is also effective for documenting the patient’s response to therapy.
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Illustration of major types of anorectal abscesses (submucosal type not pictured).
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Goodsall rule for anorectal fistulas. Fistulas that exit in posterior half of rectum generally follow curved course toward posterior midline, whereas those that exit in anterior half of rectum usually follow radial course to dentate line.
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Goodsall rule for anorectal fistulas. Note curved nature of posterior fistulas and radial (straight) orientation of anterior fistulas.
