The Rome III Criteria for functional constipation must include two or more of the following and present for the past three months, with symptoms starting for at least 6 months prior to diagnosis.

- Straining during defecation for at least 25% of bowel movements

- Lumpy or hard stools in at least 25% of defecations

- Sensation of incomplete evacuation for at least 25% of defecations

- Sensation of anorectal obstruction/blockage for at least 25% of defecations

- Manual maneuvers to facilitate at least 25% of defecations

- Fewer than 3 defecations per week

- Loose stools are rarely present without the use of laxatives

- There are insufficient criteria for irritable bowel syndrome

Functional constipation is common and does not warrant diagnostic testing. Imaging and laboratory tests are typically recommended for those with alarm signs or symptoms.

The laboratory tests performed depends on the suspected underlying cause of the constipation. Tests may include CBC (complete blood count), thyroid function tests, serum calcium, serum potassium, etc.

Abdominal X-rays are generally only performed if bowel obstruction is suspected, may reveal extensive impacted fecal matter in the colon, and may confirm or rule out other causes of similar symptoms.

Colonoscopy may be performed if an abnormality in the colon like a tumor is suspected. Other tests rarely ordered include anorectal manometry, anal sphincter electromyography, and defecography.

Colonic propagating pressure wave sequences (PSs) are responsible for discrete movements of the bowel contents and are vital for normal defecation. Deficiencies in PS frequency, amplitude, and extent of propagation are all implicated in severe defecatory dysfunction (SDD). Mechanisms that can normalize these aberrant motor patterns may help rectify the problem. Recently the novel therapy of sacral nerve stimulation (SNS) has been utilized for the treatment of severe constipation.

A 2014 meta-analysis of three small trials evaluating probiotics showed a slight improvement in management of chronic idiopathic constipation, but well-designed studies are necessary to know the true efficacy of probiotics in treating this condition.

Children with functional constipation often claim to lack the sensation of the urge to defecate, and may be conditioned to avoid doing so due to a previous painful experience. One retrospective study showed that these children did indeed have the urge to defecate using colonic manometry, and suggested behavioral modification as a treatment for functional constipation.

Attempts must be made to determine whether there is a secondary cause amenable to treatment.

Primary (idiopathic) intestinal pseudo-obstruction is diagnosed based on motility studies, x-rays and gastric emptying studies.

There is considerable research into the causes, diagnosis and treatments for FGIDs. Diet, microbiome, genetics, neuromuscular function and immunological response all interact. Heightened mast cell activation has been proposed to be a common factor among FGIDs, contributing to visceral hypersensitivity as well as epithelial, neuromuscular, and motility dysfunction.

Secondary chronic intestinal pseudo-obstruction is managed by treating the underlying condition.

There is no cure for primary chronic intestinal pseudo-obstruction. It is important that nutrition and hydration is maintained, and pain relief is given. Drugs that increase the propulsive force of the intestines have been tried, as have different types of surgery.

There is no single, specific test for malabsorption. As for most medical conditions, investigation is guided by symptoms and signs. A range of different conditions can produce malabsorption and it is necessary to look for each of these specifically. Many tests have been advocated, and some, such as tests for pancreatic function are complex, vary between centers and have not been widely adopted. However, better tests have become available with greater ease of use, better sensitivity and specificity for the causative conditions. Tests are also needed to detect the systemic effects of deficiency of the malabsorbed nutrients (such as anaemia with vitamin B12 malabsorption).

Diagnosis is achieved mainly by plain and contrasted radiographical and ultrasound imaging. Colonic marker transit studies are useful to distinguish colonic inertia from functional outlet obstruction causes. In this test, the patient swallows a water-soluble bolus of radio-opaque contrast and films are obtained 1, 3 and 5 days later. Patients with colonic inertia show the marker spread throughout the large intestines, while patients with outlet obstruction exhibit slow accumulations of markers in some places. A colonoscopy can also be used to rule out mechanical obstructive causes. Anorectal manometry may help to differentiate acquired from congenital forms. Rectal biopsy is recommended to make a final diagnosis of Hirschsprung disease.

The main diagnostic tools are blood tests, X-rays of the abdomen, CT scanning, and/or ultrasound. If a mass is identified, biopsy may determine the nature of the mass.

Radiological signs of bowel obstruction include bowel distension and the presence of multiple (more than six) gas-fluid levels on supine and erect abdominal radiographs.

Contrast enema or small bowel series or CT scan can be used to define the level of obstruction, whether the obstruction is partial or complete, and to help define the cause of the obstruction.

According to a meta-analysis of prospective studies by the Cochrane Collaboration, the appearance of water-soluble contrast in the cecum on an abdominal radiograph within 24 hours of oral administration predicts resolution of an adhesive small bowel obstruction with a pooled sensitivity of 97% and specificity of 96%.

Colonoscopy, small bowel investigation with ingested camera or push endoscopy, and laparoscopy are other diagnostic options.

The two key features of obstructed defecation are:

1. An inability to voluntarily evacuate rectal contents

2. Normal colonic transit time

A 5 item questionnaire was validated for diagnosis and grading of obstructed defecation syndrome. The parameters were:

1. Excessive straining

2. Incomplete rectal evacuation

3. Use of enemas and/or laxatives

4. Vaginal-anal-perineal digitations (needing to press in the back wall of the vagina or on the perineum to aid defecation)

5. Abdominal discomfort and/or pain

Chronic idiopathic constipation is similar to constipation-predominant irritable bowel syndrome (IBS-C); however, people with CIC do not have other symptoms of IBS, such as abdominal pain. Diagnosing CIC can be difficult as other syndromes must be ruled out as there is no physiological cause for CIC. Doctors will typically look for other symptoms, such as blood in stool, weight loss, low blood count, or other symptoms.

To be considered functional constipation, symptoms must be present at least a fourth of the time.

Possible causes are:

- Anismus

- Descending perineum syndrome

- Other inability or unwillingness to control the external anal sphincter, which normally is under voluntary control

- A poor diet

- An unwillingness to defecate

- Nervous reactions, including prolonged and/or chronic stress and anxiety, that close the internal anal sphincter, a muscle that is not under voluntary control

- Deeper psychosomatic disorders which sometimes affect digestion and the absorption of water in the colon

There is also possibility of presentation with other comorbid symptoms such as headache, especially in children.

It is important to note that both barium enema and colonoscopy are contraindicated during acute episodes of diverticulitis, as the barium may leak out into the abdominal cavity, and colonoscopy can cause perforations of the bowel wall.

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.

Ileus is a cause of colic in horses due to functional obstruction of the intestines. It most commonly seen in horses postoperatively, especially following colic surgery. Horses experiencing ileus are at risk for gastric rupture due to rapid reflux build-up, and require intense medical management with frequent nasogastric intubation. Ileus may increase adhesion formation, because intestinal segments have more prolonged contact and intestinal distention causes serosal injury and ischemia. It is usually treated with aggressive fluid support, prokinetics, and anti-inflammatories.

Physical examination can rule out anismus (by identifying another cause) but is not sufficient to diagnose anismus.

Fetal and neonatal bowel obstructions are often caused by an intestinal atresia, where there is a narrowing or absence of a part of the intestine. These atresias are often discovered before birth via an ultrasound, and treated with using laparotomy after birth. If the area affected is small, then the surgeon may be able to remove the damaged portion and join the intestine back together. In instances where the narrowing is longer, or the area is damaged and cannot be used for a period of time, a temporary stoma may be placed.

Traditionally, nothing by mouth was considered to be mandatory in all cases, but gentle feeding by enteral feeding tube may help to restore motility by triggering the gut's normal feedback signals, so this is the recommended management initially. When the patient has severe, persistent signs that motility is completely disrupted, nasogastric suction and parenteral nutrition may be required until passage is restored. In such cases, continuing aggressive enteral feeding causes a risk of perforating the gut.

Several options are available in the case of paralytic ileus. Most treatment is supportive. If caused by medication, the offending agent is discontinued or reduced. Bowel movements may be stimulated by prescribing lactulose, erythromycin or, in severe cases that are thought to have a neurological component (such as Ogilvie's syndrome), neostigmine. There is also evidence from a systematic review of randomized controlled trials that chewing gum, as a form of 'sham feeding', may stimulate gastrointestinal motility in the post-operative period and reduce the duration of postoperative ileus.

If possible the underlying cause is corrected (e.g. replace electrolytes).

For severe constipation which has not responded to laxatives, dietary supplements or other lifestyle changes. Measures the patients ability to propel a stool through the anal canal. A simulated stool (balloon) is inserted into the rectum with the patient lying on his/her side, and the patient is asked to push the balloon out as they would a stool. Used to assess muscle strength and coordination.

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.

Obstructed defecation may be a cause of incomplete evacuation of stool. Normal emptying of rectal contents is 90-100%. <90% evacuation could be defined as incomplete evacuation. Incomplete evacuation is also one of the factors implicated in the cause of fecal leakage.

The diagnosis of bacterial overgrowth can be made by physicians in various ways. Malabsorption can be detected by a test called the "D-xylose" test. Xylose is a sugar that does not require enzymes to be digested. The D-xylose test involves having a patient drink a certain quantity of D-xylose, and measuring levels in the urine and blood; if there is no evidence of D-xylose in the urine and blood, it suggests that the small bowel is not absorbing properly (as opposed to problems with enzymes required for digestion).

The gold standard for detection of bacterial overgrowth is the aspiration of more than 10 bacteria per millilitre from the small bowel. The normal small bowel has less than 10 bacteria per millilitre. Some experts however, consider aspiration of more than 10 positive if the flora is predominately colonic type bacteria as these types of bacteria are considered pathological in excessive numbers in the small intestine. The reliability of aspiration in the diagnosis of SIBO has been questioned as SIBO can be patchy and the reproducibility can be as low as 38 percent. Breath tests have their own reliability problems with a high rate of false positive. Some doctors factor in a patients' response to treatment as part of the diagnosis.

Breath tests have been developed to test for bacterial overgrowth, based on bacterial metabolism of carbohydrates to hydrogen and/or methane, or based on the detection of by-products of digestion of carbohydrates that are not usually metabolized. The hydrogen breath test involves having the patient fast for a minimum of 12 hours then having them drink a substrate usually glucose or lactulose, then measuring expired hydrogen and methane concentrations typically over a period of 2–3 hours. It compares well to jejunal aspirates in making the diagnosis of bacterial overgrowth. C and C based tests have also been developed based on the bacterial metabolism of D-xylose. Increased bacterial concentrations are also involved in the deconjugation of bile acids. The glycocholic acid breath test involves the administration of the bile acid C glychocholic acid, and the detection of CO, which would be elevated in bacterial overgrowth.

Some patients with symptoms of bacterial overgrowth will undergo gastroscopy, or visualization of the stomach and duodenum with an endoscopic camera. Biopsies of the small bowel in bacterial overgrowth can mimic those of celiac disease, making the diagnosis more challenging. Findings include blunting of villi, hyperplasia of crypts and an increased number of lymphocytes in the lamina propria.

However, some physicians suggest that if the suspicion of bacterial overgrowth is high enough, the best diagnostic test is a trial of treatment. If the symptoms improve, an empiric diagnosis of bacterial overgrowth can be made.

A high-fiber diet and fiber supplements are advisable to prevent constipation. The American Dietetic Association recommends 20–35 grams each day. Wheat bran has been shown to reduce intra colonic pressure.

The US National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) says foods such as nuts, popcorn hulls, sunflower seeds, pumpkin seeds, caraway seeds, and sesame seeds have traditionally been labeled as problem foods for people with this condition; however, no scientific data exists to prove this hypothesis. The seeds in tomatoes, zucchini, cucumbers, strawberries, raspberries, and poppy seeds, are not considered harmful by the NIDDK.

One study found that nuts and popcorn do not contribute positively or negatively to patients with diverticulosis or diverticular complications.

Functional gastrointestinal disorders are very common. Globally, irritable bowel syndrome and functional dyspepsia alone may affect 16–26% of the population.

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.

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.