Special examinations. CBC with differential, serum electrolytes with renal function.Most patients with intra-abdominal infections demonstrate leukocytosis (>11,000 cells/mL)

Laboratory studies

CBC with differential, serum electrolytes with renal function.Most patients with intra-abdominal infections demonstrate leukocytosis (>11,000 cells/mL) with a shift to the immature forms on the differential cell count. Patients in severe sepsis, patients who are immunocompromised, and patients with certain types of infections (e.g., fungal, cytomegaloviral) may demonstrate absence of leukocytosis or leucopenia.

Blood chemistry may reveal dehydration and acidosis.

Liver function tests, if clinically indicated.

Amylase and lipase, if pancreatitis is suspected.

Urinalysis is essential to rule out urinary tract diseases (e.g., pyelonephritis, renal stone disease); however, patients with lower abdominal and pelvic infections often demonstrate WBC in the urine and microhaematuria.

In patients with diarrhoea, evaluate a stool for specific culture (i.e., Salmonella, Shigella, cytomegalovirus), if the patient’s history suggests infectious enterocolitis.

Aerobic and anaerobic blood cultures.

Peritoneal fluid evaluation (i.e., paracentesis, aspiration of abdominal fluid collections, intraoperative peritoneal fluid cultures).

When assessign a peritoneal fluid sample for peritoneal infection, evaluate the sample for pH, glucose, protein, lactate dehydrogenase, cell count, Gram stain, and aerobic and anaerobic cultures, a peritoneal fluid amylase.

Imaging studies

Radiographs. Plain films of the abdomen (e.g., supine, upright, and lateral decubitus positions) are often the first imaging studies obtained in patients presenting with peritonitis. Their value in reaching a specific diagnosis is limited.

Free air is present in most cases of anterior gastric and duodenal perforation but is much less frequent with perforations of the small bowel and colon. Remember that the presence of free air is not mandatory with visceral perforation and that small amounts of free air are missed easily on plain films.

Ultrasonic. Abdominal Ultrasonic may be helpful in the evaluation of right upper quadrant (e.g., perihepatic abscess, cholecystitis, biloma, pancreatitis, pancreatic pseudocyst), right lower quadrant, and pelvic pathology (e.g., appendicitis, tubo-ovarian abscess, Douglas pouch abscess), but the examination is sometimes limited because of patient discomfort, abdominal distension, and bowel gas interference.

Ultrasonography may detect increased amounts of peritoneal fluid, but its ability to detect quantities of less than 100 mL is limited. The central (perimesenteric) peritoneal cavity is not visualized well with transabdominal ultrasonography. Examination from the flank or back may improve the diagnostic yield, and providing the ultrasonographer with specific information of the patient’s condition and the suspected diagnosis before the examination is important. With an experienced ultrasonographer, a diagnostic accuracy of greater than 85% has been reported in several series.

Over the past several years, Ultrasonic-guided aspiration and placement of drains has evolved into a valuable tool in the diagnosis and treatment of abdominal fluid collections.

Computed tomography scans of the abdomen and pelvis remain the diagnostic study of choice for peritoneal abscess and the related visceral pathology. CT scan is indicated in all cases where the diagnosis cannot be established on clinical grounds and findings on abdominal plain films. Whenever possible, the CT scan should be performed with enteral and intravenous contrast. CT scans can detect small quantities of fluid, regions of inflammation and other GI tract pathology, with sensitivities that approach 100%.

Peritoneal abscesses and other fluid collections may be aspirated for diagnosis and drained under CT guidance.

Nuclear medicine scans have little use in the initial evaluation of patients with suspected peritonitis or intra-abdominal sepsis. They are most frequently used in the evaluation of fever of unknown origin or in patients with persistent fever despite adequate antibiotic treatment and negative CT scan findings.

Magnetic resonance imaging is an emerging imaging modality for the diagnosis of suspected intra-abdominal abscesses.

Contrast studies. Conventional contrast studies (i.e., upper gastrointestinal tract and colorectal contrast enema, fistulogram, contrast studies of drains and stents) are reserved for specific indications in the setting of suspected peritonitis or peritoneal abscess.