Abdominal Abscess Workup

Delayed diagnosis and treatment can lead to increased mortality rates and have a significant economic impact. Accordingly, an efficient and well-directed workup is important.

Appropriate hematologic studies should be done. Hematologic parameters suggestive of infection (eg, leukocytosis, anemia, abnormal platelet counts, abnormal liver function) frequently are present, although patients who are debilitated and/or elderly often fail to mount reactive leukocytosis or fever.

Blood cultures indicating persistent polymicrobial bacteremia strongly implicate the presence of an intra-abdominal abscess. Because more than 90% of intra-abdominal abscesses contain anaerobic organisms, particularly B fragilis, postoperative Bacteroides bacteremia suggests intra-abdominal sepsis.

Plain abdominal radiographs, though rarely diagnostic, frequently indicate the need for further investigation.^[6] Abnormalities on plain abdominal films may include a localized ileus, extraluminal gas, air-fluid levels, mottled soft-tissue masses, absence of psoas outlines, or displacement of viscera.

In subphrenic or even subhepatic abscesses, the chest radiograph may show pleural effusion, elevated hemidiaphragm, basilar infiltrates, or atelectasis.

Ultrasonography is readily available, portable, and inexpensive. The findings can be quite specific when correlated with the clinical picture. In experienced hands, ultrasonography has an accuracy rate greater than 90% for diagnosing intra-abdominal abscesses. Bedside ultrasonography is particularly useful for immobile, critically ill ICU patients.

A drawback of ultrasonography is that marked obesity, bowel gas, intervening viscera, surgical dressings, open wounds, and stomas can create problems with definition. In addition, the quality of the procedure is operator-dependent. These disadvantages may limit the efficacy of this modality in postoperative patients.

Computed tomography (CT) scanning has greater than 95% accuracy and is the best diagnostic imaging method for abdominal abscess. The presence of ileus, dressings, drains, or stomas does not interfere with reliability.

For good anatomic resolution, use oral and intravenous (IV) contrast (see the images below). Oral contrast may help to differentiate a fluid-filled extraluminal structure from a normal intestine. Extravasation of oral contrast indicates a fistula or an anastomotic leak. IV contrast may enhance the abscess by concentrating the contrast material within the abscess wall. The use of oral and IV contrast may be limited by ileus, allergy to contrast material, and renal insufficiency.

Identify any occult abscesses using serial images obtained from the diaphragm to the pelvis. The appearance of an air bubble within a fluid collection or a low-attenuation extraluminal mass is diagnostic of an intra-abdominal collection. CT scans can document inflammatory edema in the adjacent fat (obliteration of fat plane) and hyperemia in the abscess wall (enhancement).

Drawbacks of CT scanning include nonportability, relative difficulty in diagnosing intraloop abscesses, and, possibly, poor patient cooperation.

Recent intra-abdominal surgery also may pose a diagnostic problem in patients in whom intra-abdominal abscesses are suspected. CT scanning is not recommended for use in diagnosing such abscesses until approximately postoperative day 7, by which time postoperative tissue edema is reduced and nonsuppurative fluids (eg, hematoma, seroma, intraoperative irrigation fluid) should be reabsorbed. In most postoperative patients, signs of intra-abdominal abscesses do not develop within the first 4-5 days.

A literature review from the Netherlands indicated that CT scanning is superior to graded-compression ultrasonography in the diagnosis of acute appendicitis, a potential cause of abdominal abscess.^[7]

Scans using radioactive agents, such as leukocytes labeled or tagged with gallium-67 or indium-111, may localize the area of inflammation. Such scans are time consuming, and they have a substantial false-positive rate resulting from nonpyogenic inflammatory conditions, bowel accumulation of tagged leukocytes, surgical drains, and incisions.

Typically, radioisotope scans provide no pertinent information that is not found with a CT scan. The disadvantages of these scans limit their use to cases in which intra-abdominal abscesses are strongly suspected in a patient but ultrasonography or CT scanning has failed to provide adequate diagnostic information.