MRI use has become increasingly common for diagnosis of appendicitis in children and pregnant patients due to the radiation dosage that, while of nearly negligible risk in healthy adults, can be harmful to children or the developing fetus. In pregnancy, it has been found to be more useful during the second and third trimester, particularly as the enlargening uterus displaces the appendix, making it difficult to find by ultrasound. The periappendiceal stranding that is reflected on CT by fat stranding on MRI appears as increased fluid signal on T2 weighted sequences. First trimester pregnancies are usually not candidates for MRI, as the fetus is still undergoing organogenesis, and there are no long-term studies to date regarding its potential risks or side effects.

A diagnosis of peritonitis is based primarily on the clinical manifestations described above. Rigidity (involuntary contraction of the abdominal muscles) is the most specific exam finding for diagnosing peritonitis (+ likelihood ratio: 3.9). If peritonitis is strongly suspected, then surgery is performed without further delay for other investigations. Leukocytosis, hypokalemia, hypernatremia, and acidosis may be present, but they are not specific findings. Abdominal X-rays may reveal dilated, edematous intestines, although such X-rays are mainly useful to look for pneumoperitoneum, an indicator of gastrointestinal perforation. The role of whole-abdomen ultrasound examination is under study and is likely to expand in the future. Computed tomography (CT or CAT scanning) may be useful in differentiating causes of abdominal pain. If reasonable doubt still persists, an exploratory peritoneal lavage or laparoscopy may be performed. In patients with ascites, a diagnosis of peritonitis is made via paracentesis (abdominal tap): More than 250 polymorphonucleate cells per μL is considered diagnostic. In addition, Gram stain is almost always negative, whereas culture of the peritoneal fluid can determine the microorganism responsible and determine their sensitivity to antimicrobial agents.

In general, plain abdominal radiography (PAR) is not useful in making the diagnosis of appendicitis and should not be routinely obtained from a person being evaluated for appendicitis. Plain abdominal films may be useful for the detection of ureteral calculi, small bowel obstruction, or perforated ulcer, but these conditions are rarely confused with appendicitis. An opaque fecalith can be identified in the right lower quadrant in fewer than 5% of people being evaluated for appendicitis. A barium enema has proven to be a poor diagnostic tool for appendicitis. While failure of the appendix to fill during a barium enema has been associated with appendicitis, up to 20% of normal appendices do not fill.

In normal conditions, the peritoneum appears greyish and glistening; it becomes dull 2–4 hours after the onset of peritonitis, initially with scarce serous or slightly turbid fluid. Later on, the exudate becomes creamy and evidently suppurative; in dehydrated patients, it also becomes very inspissated. The quantity of accumulated exudate varies widely. It may be spread to the whole peritoneum, or be walled off by the omentum and viscera. Inflammation features infiltration by neutrophils with fibrino-purulent exudation.

Stable patients presenting to A&E (accident and emergency department) or ER (emergency room) with severe abdominal pain will almost always have an abdominal x-ray and/or a CT scan. These tests can provide a differential diagnosis between simple and complex pathologies. However, in the unstable patient, fluid resuscitation and a FAST-ultrasound are done first, and if the latter is positive for free fluid, straight to surgery. They may also provide evidence to the doctor whether surgical intervention is necessary.

Patients will also most likely receive a complete blood count (or full blood count in the U.K.), looking for characteristic findings such as neutrophilia in appendicitis.

Traditionally, the use of opiates or other painkillers in patients with an acute abdomen has been discouraged before the clinical examination, because these would alter the examination. However, the scientific literature does not reveal any negative results from these alterations.

There may be signs of septic shock. A physical examination reveals abdominal tenderness and possible loss of bowel sounds. An abdominal radiography shows colonic dilation. White blood cell count is usually elevated. Severe sepsis may present with hypothermia or leukopenia.

On x-rays, gas may be visible in the abdominal cavity. Gas is easily visualized on x-ray while the patient is in an upright position. The perforation can often be visualised using computed tomography. White blood cells are often elevated.

Diagnosis is made by paracentesis (needle aspiration of the ascitic fluid). SBP is diagnosed if the fluid contains neutrophils (a type of white blood cell) at greater than 250 cells per mm (equals a cell count of 250 x10/L) fluid in the absence of another reason for this (such as inflammation of one of the internal organs or a perforation). The fluid is also cultured to identify bacteria. If the sample is sent in a plain sterile container 40% of samples will identify an organism, while if the sample is sent in a bottle with culture medium the sensitivity increases to 72–90%.

Surgical intervention is nearly always required in form of exploratory laparotomy and closure of perforation with peritoneal wash. Occasionally they may be managed laparoscopically.

Conservative treatment including intravenous fluids, antibiotics, nasogastric aspiration and bowel rest is indicated only if the person is nontoxic and clinically stable.

All people with cirrhosis might benefit from antibiotics (oral fluoroquinolone norfloxacin) if:

- Ascitic fluid protein <1.0 g/dL. Patients with fluid protein <15 g/L and either Child-Pugh score of at least 9 or impaired renal function may also benefit.

- Previous SBP

People with cirrhosis admitted to the hospital should receive prophylactic antibiotics if:

- They have bleeding esophageal varices

If the condition does not improve, the risk of death is significant. In case of poor response to conservative therapy, a colectomy is usually required.

Typhlitis is a medical emergency and requires prompt management. Untreated typhlitis has a poor prognosis, particularly if associated with pneumatosis intestinalis (air in the bowel wall) and/or bowel perforation, and has significant morbidity unless promptly recognized and aggressively treated.

Successful treatment hinges on:

1. Early diagnosis provided by a high index of suspicion and the use of CT scanning

2. Nonoperative treatment for uncomplicated cases

3. Empiric antibiotics, particularly if the patient is neutropenic or at other risk of infection.

In rare cases of prolonged neutropenia and complications such as bowel perforation, neutrophil transfusions can be considered but have not been studied in a randomized control trial. Elective right hemicolectomy may be used to prevent recurrence but is generally not recommended

"...The authors have found nonoperative treatment highly effective in patients who do not manifest signs of peritonitis, perforation, gastrointestinal hemorrhage, or clinical deterioration. Recurrent typhlitis was frequent after conservative therapy (recurrence rate, 67 percent), however," as based on studies from the 1980s

People with the above symptoms are commonly studied with computed tomography, or CT scan. The CT scan is very accurate (98%) in diagnosing diverticulitis. In order to extract the most information possible about the patient's condition, thin section (5 mm) transverse images are obtained through the entire abdomen and pelvis after the patient has been administered oral and intravascular contrast. Images reveal localized colon wall thickening, with inflammation extending into the fat surrounding the colon. The diagnosis of acute diverticulitis is made confidently when the involved segment contains diverticula. CT may also identify patients with more complicated diverticulitis, such as those with an associated abscess. It may even allow for radiologically guided drainage of an associated abscess, sparing a patient from immediate surgical intervention.

Other studies, such as barium enema and colonoscopy, are contraindicated in the acute phase of diverticulitis because of the risk of perforation.

The severity of diverticulitis can be radiographically graded by the Hinchey Classification.

The differential diagnosis includes colon cancer, inflammatory bowel disease, ischemic colitis, and irritable bowel syndrome, as well as a number of urological and gynecological processes.

Gastroscopy, or endoscopic evaluation of the stomach, is useful in chronic cases of colic suspected to be caused by gastric ulcers, gastric impactions, and gastric masses. A 3-meter scope is required to visualize the stomach of most horses, and the horse must be fasted prior to scoping.

Laparoscopy involves inserting a telescoping camera approximately 1 cm in diameter into the horse's abdomen, through a small incision, to visualize the gastrointestinal tract. It may be performed standing or under general anesthesia, and is less invasive than an exploratory celiotomy (abdominal exploratory surgery).

Twenty percent of infants born with meconium peritonitis will have vomiting and dilated bowels on x-rays which necessitates surgery.

Meconium peritonitis is sometimes diagnosed on prenatal ultrasound where it appears as calcifications within the peritoneum.

Typhlitis is diagnosed with a radiograph CT scan showing thickening of the cecum and "fat stranding".

Meconium peritonitis refers to rupture of the bowel prior to birth, resulting in fetal stool (meconium) escaping into the surrounding space (peritoneum) leading to inflammation (peritonitis). Despite the bowel rupture, many infants born after meconium peritonitis "in utero" have normal bowels and have no further issues.

Infants with cystic fibrosis are at increased risk for meconium peritonitis.

Diagnosis may be simple in cases where the patient's signs and symptoms are idiopathic to a specific cause. However this is generally not the case, considering that many pathogens which cause enteritis may exhibit the similar symptoms, especially early in the disease. In particular, "campylobacter, shigella, salmonella" and many other bacteria induce acute self-limited colitis, an inflammation of the lining of the colon which appears similar under the microscope.

A medical history, physical examination and tests such as blood counts, stool cultures, CT scans, MRIs, PCRs, colonoscopies and upper endoscopies may be used in order to perform a differential diagnosis. A biopsy may be required to obtain a sample for histopathology.

Once a child is born prematurely, thought must be given to decreasing the risk for developing NEC. Toward that aim, the methods of providing hyperalimentation and oral feeds are both important. In a 2012 policy statement, the American Academy of Pediatrics recommended feeding preterm infants human milk, finding "significant short- and long-term beneficial effects," including reducing the rate of NEC by a factor of two or more.

A study by researchers in Peoria, IL, published in "Pediatrics" in 2008, demonstrated that using a higher rate of lipid (fats and/or oils) infusion for very low birth weight infants in the first week of life resulted in zero infants developing NEC in the experimental group, compared with 14% with NEC in the control group. (They started the experimental group at 2 g/kg/d of 20% IVFE and increased within two days to 3 g/kg/d; amino acids were started at 3 g/kg/d and increased to 3.5.)

Neonatologists at the University of Iowa reported on the importance of providing small amounts of trophic oral feeds of human milk starting as soon as possible, while the infant is being primarily fed intravenously, in order to prime the immature gut to mature and become ready to receive greater oral intake. Human milk from a milk bank or donor can be used if mother's milk is unavailable. The gut mucosal cells do not get enough nourishment from arterial blood supply to stay healthy, especially in very premature infants, where the blood supply is limited due to immature development of the capillaries, so nutrients from the lumen of the gut are needed.

A Cochrane review published in April 2014 has established that supplementation of probiotics enterally "prevents severe NEC as well as all-cause mortality in preterm infants."

Increasing amounts of milk by 30 to 40 ml/kg is safe in infant who are born weighing very little. Not beginning feeding an infant by mouth for more than 4 days does not appear to have protective benefits.

Data from the NICHD Neonatal Research Network's Glutamine Trial showed that the incidence of NEC among extremely low birthweight (ELBW, <1000 g) infants fed with more than 98% human milk from their mothers was 1.3%, compared with 11.1% among infants fed only preterm formula, and 8.2% among infants fed a mixed diet, suggesting that infant deaths could be reduced by efforts to support production of milk by mothers of ELBW newborns.

Research from the University of California, San Diego found that higher levels of one specific human milk oligosaccharide, disialyllacto-N-tetraose, may be protective against the development of NEC.

The diagnosis is usually suspected clinically but often requires the aid of diagnostic imaging modalities, most commonly radiography. Specific radiographic signs of NEC are associated with specific Bell's stages of the disease:

Bell's stage 1/Suspected disease:

- Mild systemic disease (apnea, lethargy, bradycardia, temperature instability)

- Mild intestinal signs (abdominal distention, increased gastric residuals, bloody stools)

- Non-specific or normal radiological signs

Bell's stage 2/Definite disease:

- Mild to moderate systemic signs

- Additional intestinal signs (absent bowel sounds, abdominal tenderness)

- Specific radiologic signs (pneumatosis intestinalis or portal venous air)

- Laboratory changes (metabolic acidosis, thrombocytopaenia)

Bell's stage 3/Advanced disease:

- Severe systemic illness (hypotension)

- Additional intestinal signs (striking abdominal distention, peritonitis)

- Severe radiologic signs (pneumoperitoneum)

- Additional laboratory changes (metabolic and respiratory acidosis, disseminated intravascular coagulation)

More recently ultrasonography has proven to be useful as it may detect signs and complications of NEC before they are evident on radiographs, specifically in cases that involve a paucity of bowel gas, a gasless abdomen, or a sentinel loop. Diagnosis is ultimately made in 5–10% of very low-birth-weight infants (<1,500g).

Treatment for sigmoid volvulus may include sigmoidoscopy. If the mucosa of the sigmoid looks normal and pink, place a rectal tube for decompression, correct any fluid, electrolyte, cardiac, renal or pulmonary abnormalities and then take the person to the operating room for repair. If surgery is not performed, there is a high rate of recurrence.

For people with signs of sepsis or an abdominal catastrophe, immediate surgery and resection is advised.

In a cecal volvulus, the cecum may be returned to a normal position and sutured in place, a procedure known as cecopexy. If identified early, before presumed intestinal wall ischemia has resulted in tissue breakdown and necrosis, the cecal volvulus can be detorsed laparoscopically.

The differential diagnoses of acute abdomen include but are not limited to:

1. Acute appendicitis

2. Acute peptic ulcer and its complications

3. Acute cholecystitis

4. Acute pancreatitis

5. Acute intestinal ischemia (see section below)

6. Acute diverticulitis

7. Ectopic pregnancy with tubal rupture

8. Ovarian torsion

9. Acute peritonitis (including hollow viscus perforation)

10. Acute ureteric colic

11. Bowel volvulus

12. Bowel obstruction

13. Acute pyelonephritis

14. Adrenal crisis

15. Biliary colic

16. Abdominal aortic aneurysm

17. Familial Mediterranean fever

18. Hemoperitoneum

19. Ruptured spleen

20. Kidney stone

21. Sickle cell anaemia