The diagnosis is mainly established based on the characteristic symptoms. Stomach pain is usually the first signal of a peptic ulcer. In some cases, doctors may treat ulcers without diagnosing them with specific tests and observe whether the symptoms resolve, thus indicating that their primary diagnosis was accurate.

More specifically, peptic ulcers erode the muscularis mucosae, at minimum reaching to the level of the submucosa (contrast with erosions, which do not involve the muscularis mucosae).

Confirmation of the diagnosis is made with the help of tests such as endoscopies or barium contrast x-rays. The tests are typically ordered if the symptoms do not resolve after a few weeks of treatment, or when they first appear in a person who is over age 45 or who has other symptoms such as weight loss, because stomach cancer can cause similar symptoms. Also, when severe ulcers resist treatment, particularly if a person has several ulcers or the ulcers are in unusual places, a doctor may suspect an underlying condition that causes the stomach to overproduce acid.

An esophagogastroduodenoscopy (EGD), a form of endoscopy, also known as a gastroscopy, is carried out on people in whom a peptic ulcer is suspected. By direct visual identification, the location and severity of an ulcer can be described. Moreover, if no ulcer is present, EGD can often provide an alternative diagnosis.

One of the reasons that blood tests are not reliable for accurate peptic ulcer diagnosis on their own is their inability to differentiate between past exposure to the bacteria and current infection. Additionally, a false negative result is possible with a blood test if the person has recently been taking certain drugs, such as antibiotics or proton-pump inhibitors.

The diagnosis of "Helicobacter pylori" can be made by:

- Urea breath test (noninvasive and does not require EGD);

- Direct culture from an EGD biopsy specimen; this is difficult to do, and can be expensive. Most labs are not set up to perform "H. pylori" cultures;

- Direct detection of urease activity in a biopsy specimen by rapid urease test;

- Measurement of antibody levels in the blood (does not require EGD). It is still somewhat controversial whether a positive antibody without EGD is enough to warrant eradication therapy;

- Stool antigen test;

- Histological examination and staining of an EGD biopsy.

The breath test uses radioactive carbon to detect H. pylori. To perform this exam the person will be asked to drink a tasteless liquid which contains the carbon as part of the substance that the bacteria breaks down. After an hour, the person will be asked to blow into a bag that is sealed. If the person is infected with H. pylori, the breath sample will contain radioactive carbon dioxide. This test provides the advantage of being able to monitor the response to treatment used to kill the bacteria.

The possibility of other causes of ulcers, notably malignancy (gastric cancer) needs to be kept in mind. This is especially true in ulcers of the "greater (large) curvature" of the stomach; most are also a consequence of chronic "H. pylori" infection.

If a peptic ulcer perforates, air will leak from the inside of the gastrointestinal tract (which always contains some air) to the peritoneal cavity (which normally never contains air). This leads to "free gas" within the peritoneal cavity. If the person stands erect, as when having a chest X-ray, the gas will float to a position underneath the diaphragm. Therefore, gas in the peritoneal cavity, shown on an erect chest X-ray or supine lateral abdominal X-ray, is an omen of perforated peptic ulcer disease.

A gastric peptic ulcer is a mucosal perforation which penetrates the muscularis mucosae and lamina propria, usually produced by acid-pepsin aggression. Ulcer margins are perpendicular and present chronic gastritis. During the active phase, the base of the ulcer shows 4 zones: fibrinoid necrosis, inflammatory exudate, granulation tissue and fibrous tissue. The fibrous base of the ulcer may contain vessels with thickened wall or with thrombosis.

The most confirmatory investigation is endoscopy of upper gastrointestinal tract.

Laboratory

- Individuals with gastric outlet obstruction are often hypochloremic, hypokalemic, and alkalotic due to loss of hydrogen chloride and potassium. High urea and creatinine levels may also be observed if the patient is dehydrated.

Abdominal X-ray

- A gastric fluid level may be seen which would support the diagnosis.

Barium meal and follow through

- May show an enlarged stomach and pyloroduodenal stenosis.

Gastroscopy

- May help with cause and can be used therapeutically.

The differential diagnosis of gastric outlet obstruction may include: early gastric carcinoma hiatal hernia, gastroesophageal reflux, adrenal insufficiency, and inborn errors of metabolism.

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.

Given that ascending cholangitis usually occurs in the setting of bile duct obstruction, various forms of medical imaging may be employed to identify the site and nature of this obstruction. The first investigation is usually ultrasound, as this is the most easily available. Ultrasound may show dilation of the bile duct and identifies 38% of bile duct stones; it is relatively poor at identifying stones farther down the bile duct. Ultrasound can help distinguish between cholangitis and cholecystitis (inflammation of the gallbladder), which has similar symptoms to cholangitis but appears differently on ultrasound. A better test is magnetic resonance cholangiopancreatography (MRCP), which uses magnetic resonance imaging (MRI); this has a comparable sensitivity to ERCP. Smaller stones, however, can still be missed on MRCP depending on the quality of the hospital's facilities.

The gold standard test for biliary obstruction is still endoscopic retrograde cholangiopancreatography (ERCP). This involves the use of endoscopy (passing a tube through the mouth into the esophagus, stomach and thence to the duodenum) to pass a small cannula into the bile duct. At that point, radiocontrast is injected to opacify the duct, and X-rays are taken to get a visual impression of the biliary system. On the endoscopic image of the ampulla, one can sometimes see a protuberant ampulla from an impacted gallstone in the common bile duct or the frank extrusion of pus from the common bile duct orifice. On the X-ray images (known as cholangiograms), gallstones are visible as non-opacified areas in the contour of the duct. For diagnostic purposes, ERCP has now generally been replaced by MRCP. ERCP is only used first-line in critically ill patients in whom delay for diagnostic tests is not acceptable; however, if the index of suspicion for cholangitis is high, an ERCP is typically done to achieve drainage of the obstructed common bile duct.

If other causes rather than gallstones are suspected (such as a tumor), computed tomography and endoscopic ultrasound (EUS) may be performed to identify the nature of the obstruction. EUS may be used to obtain biopsy (tissue sample) of suspicious masses. EUS may also replace diagnostic ERCP for stone disease, although this depends on local availability.

Diagnosis is generally made by endoscopy with biopsy to evaluate histology. Review of symptoms and associated conditions is important.

Routine blood tests show features of acute inflammation (raised white blood cell count and elevated C-reactive protein level), and usually abnormal liver function tests (LFTs). In most cases the LFTs will be consistent with obstruction: raised bilirubin, alkaline phosphatase and γ-glutamyl transpeptidase. In the early stages, however, pressure on the liver cells may be the main feature and the tests will resemble those in hepatitis, with elevations in alanine transaminase and aspartate transaminase.

Blood cultures are often performed in people with fever and evidence of acute infection. These yield the bacteria causing the infection in 36% of cases, usually after 24–48 hours of incubation. Bile, too, may be sent for culture during ERCP (see below). The most common bacteria linked to ascending cholangitis are gram-negative bacilli: "Escherichia coli" (25–50%), Klebsiella (15–20%) and Enterobacter (5–10%). Of the gram-positive cocci, Enterococcus causes 10–20%.

The tests that are considered to evaluate of the passage of blood in the stool are based on the characteristics of bleeding (color, quantity) and whether or not the person passing blood has a low blood pressure with elevated heart rate, as opposed to normal vital signs. The following tests are combined to determine the causes of the source of bleeding.

- Digital rectal exam (DRE) and fecal occult blood test (FOBT)

- Colonoscopy

- Anoscopy

- Esophagogastroduodenoscopy (EGD)

- Capsule endoscopy

- CT Scan

Melena is defined as dark, tarry stools, often black in color due to partial digestion of the RBCs.

Hematochezia is defined as bright red blood seen in the toilet either inside of, or surrounding the stool.

Hematochezia is typically presumed to come from the lower portion of the GI tract, and the initial steps of diagnosis include a DRE with FOBT, which if positive, will lead to a colonoscopy. If the person has a large amount of blood in their stool, an EGD test may be necessary. If no source of active bleeding is found on these examinations, a capsule endoscopy may be performed, in order to more closely examine the small bowel, which cannot be seen with the other types of studies. With melena, a DRE with FOBT is often also performed, however the suspicion for a source from the upper GI tract is higher, leading first to the use of EGD with the other tests being required if no source is identified. The anoscopy is another type of examination, which can be used along with a colonoscopy, which exams the rectum and distal portion of the descending colon.

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.

Stress ulcer is suspected when there is upper gastrointestinal bleeding in the appropriate clinical setting, for example, when there is upper gastrointestinal bleeding in elderly patients in a surgical intensive care unit (ICU) with heart and lung disease, or when there is upper gastrointestinal bleeding in patients in a medical ICU who require respirators.

Stress ulcer can be diagnosed after the initial management of gastrointestinal bleeding, the diagnosis can be confirmed by upper GI endoscopy.

It is important to differentiate DPI from small intestinal obstruction, since obstruction may require surgical intervention, but this can at times be difficult. Horses suffering from DPI usually have a higher protein concentration in their peritoneal fluid compared to horses with small intestinal obstruction, often without a concurrent increase in nucleated cell count. They usually have some relief and decrease in pain after gastric decompression, while horses with an obstruction often still act colicky after nasogastric intubation. Distention of the small intestine may be less than what is felt on rectal examination of horses with obstruction, especially after gastric decompression. Horses with DPJ usually produce larger volumes of reflux (usually greater than 48 liters in the first 24 hours) than those with obstruction, and are often pyretic (temperatures of 101.5–102.5) and have alterations in white blood cell levels, while those with obstructions usually have a normal or lower than normal temperature and normal leukocyte levels.

Ultrasound can also be helpful to distinguish DPJ from obstruction. Horses with small intestinal obstruction will usually have an intestinal diameter of −10 cm with a wall thickness of 3–5mm. Horses with proximal enteritis usually have an intestinal diameter that is narrower, but wall thickness is often greater than 6mm, containing a hyperechoic or anechoic fluid, with normal, increased, or decreased peristalsis. However, obstructions that have been present for some time may present with thickened walls and distention of the intestine.

DPJ can only be definitively diagnosed during surgery or at necropsy, when its gross appearance of the small intestine may be evaluated.

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.

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.

Mucus may also be found in stool.

A texture described as tarry stool is generally associated with dark black stool seen in partially digested blood. This is generally associated with melena.

For diagnosis, measures of liver biochemistry and pancreatic enzymes are performed. Along with ruling out structural abnormalities, normally by performing an abdominal ultrasound and endoscopic retrograde cholangiopancreatography (ERCP). Measurements of bile transit when performing ERCP are taken to help evaluate different treatment options.

Sphincter of Oddi dysfunction is best diagnosed using manometry-an internal test done to measure the pressures within surrounding ducts to determine whether or not the muscle is functioning normally.

Treatment is aimed at removing the irritant or infection. "Helicobacter pylori" infection is usually treated with antibiotics.

Diagnosis of Pancreatic pseudocyst can be based on cyst fluid analysis:

- Carcinoembryonic antigen (CEA) and CEA-125 (low in pseudocysts and elevated in tumors);

- Fluid viscosity (low in pseudocysts and elevated in tumors);

- Amylase (usually high in pseudocysts and low in tumors)

The most useful imaging tools are:

- Ultrasonography – the role of ultrasonography in imaging the pancreas is limited by patient habitus, operator experience and the fact that the pancreas lies behind the stomach (and so a gas-filled stomach will obscure the pancreas).

- Computerized tomography – this is the gold standard for initial assessment and follow-up.

- Magnetic resonance cholangiopancreatography (MRCP) – to establish the relationship of the pseudocyst to the pancreatic ducts, though not routinely used

Postnatal diagnostic procedures include abdominal x-ray and ultrasound, CT scan, and upper GI and small bowel series.

Proximal enteritis usually is managed medically. This includes nasogastric intubation every 1–2 hours to relieve gastric pressure secondary to reflux, which often produces to 2–10 L, as well as aggressive fluid support to maintain hydration and correct electrolyte imbalances. Maintaining hydration in these patients can be very challenging. In some cases, fluid support may actually increase reflux production, due to the decreased intravascular oncotic pressure from low total protein and albumin levels, leading to loss of much of these IV fluids into the intestinal lumen. These horses will often display dependent edema (edema that collects in locations based on gravity). Colloids such as plasma or Hetastarch may be needed to improve intravascular oncotic pressure, although they can be cost prohibitive for many owners. Reflux levels are monitored closely to help evaluate fluid losses, and horses recovering from DPJ show improved hydration with decreased reflux production and improved attitude.

Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used for pain relief, reduction of inflammation, and for their anti-endotoxin effects, but care must be taken since they may produce gastrointestinal ulceration and damage the kidneys. Due to a suspected link to "Clostridial" infection, anti-microbials are often administered, usually penicillin or metronidazole. Aminoglycosides should be used with extreme caution due to the risk of nephrotoxicosis (damage to the kidney). The mucosa of the intestines is damaged with DPJ, often resulting in absorption of endotoxin and risking laminitis, so therapy to combat and treat endotoxemia is often employed. This includes treatment with drugs that counteract endotoxin such as Polymyxin B and Bio-Sponge, fluid support, and laminitis prevention such as icing of the feet. Prokinetic drugs such as lidocaine, erythromycin, metoclopramide, and bethanechol are often used to treat the ileus associated with the disease.

Horses are withheld food until reflux returns to less than 1–2 L of production every 4 hours, and gut sounds return, often requiring 3–7 days of therapy. Parenteral nutrition is often provided to horses that are withheld feed for greater than 3–4 days. It is suspected to improve healing and shorten the duration of the illness, since horses often become cachexic due to the protein losing enteropathy associated with this disease.

Surgery may need to be performed to rule out colic with similar presenting signs such as obstruction or strangulation, and in cases that are long-standing (> 7 days) to perform a resection and anastomosis of the diseased bowel. However, some horses have recovered with long-term medical support (up to 20 days).

The need for medications to prevent stress ulcer among those in the intensive care unit is unclear. As of 2014, the quality of the evidence is poor. It is unclear which agent is best or if prevention is needed at all. Benefit may only occur in those who are not being fed.

Possible agents include antacids, H2-receptor blockers, sucralfate, and proton pump inhibitors (PPIs). Tentative evidence supports that PPIs may be better than H2 blockers.

Concerns with the use of stress ulcer prophylaxis agents include increased rates of pneumonia and "Clostridium difficile" colitis.

Zollinger–Ellison syndrome may be suspected when the above symptoms prove resistant to treatment, when the symptoms are especially suggestive of the syndrome, or when endoscopy is suggestive. The diagnosis is made through several laboratory tests and imaging studies:

- Secretin stimulation test, which measures evoked gastrin levels

- Fasting gastrin levels on at least three separate occasions

- Gastric acid secretion and pH (normal basal gastric acid secretion is less than 10 mEq/hour; in Zollinger–Ellison patients, it is usually more than 15 mEq/hour)

- An increased level of chromogranin A is a common marker of neuroendocrine tumors.

In addition, the source of the increased gastrin production must be determined using MRI or somatostatin receptor scintigraphy.

Treatment of accessory pancreas depends on the location and extent of the injured tissue. Surgery may be an option, or some physicians order prophylactic antibiotics.

Intestinal decompression by tube placement in a small stoma can also be used to reduce distension and pressure within the gut. The stoma may be a gastrostomy, jejunostomy, ileostomy or cecostomy, and may also be used to feed, in the case of gastrostomy and jejunostomy, or flush the intestines.

Colostomy or ileostomy can bypass affected parts if they are distal to (come after) the stoma. For instance, if only the large colon is affected, an ileostomy may be helpful. Either of these ostomies are typically placed at or a few centimeters below the patients belly button per doctor recommendation based on the affected area of the intestines as well as concerns for patient comfort and future physical growth for children.

The total removal of the colon, called a colectomy or resection of affected parts of the colon may be needed if part of the gut dies (for instance toxic megacolon), or if there is a localised area of dysmotility.

Gastric and colonic pacemakers have been tried. These are strips placed along the colon or stomach which create an electric discharge intended to cause the muscle to contract in a controlled manner.

A potential solution, albeit radical, is a multi-organ transplant. The operation involved transplanting the pancreas, stomach, duodenum, small intestine, and liver, and was performed by Doctor Kareem Abu-Elmagd on Gretchen Miller, the subject of the Discovery Channel program "Surgery Saved My Life".

The diagnosis of hemosuccus pancreaticus can be difficult to make. Most patients who develop bleeding in the gastrointestinal tract have endoscopic procedures done to visualize the bowel in order to find and treat the source of the bleeding. With hemosuccus, the bleeding is coming from the pancreatic duct which enters into the first part of the small intestine, termed the duodenum. Typical gastroscopes used to visualize the esophagus, stomach and duodenum are designed with fiber-optic illumination that is directed in the same direction as the endoscope, meaning that visualization is in the forward direction. However, the pancreatic duct orifice is located on the side of the duodenum, meaning that it can be missed on forward-viewing endoscopy. A side-viewing endoscope (known as a "duodenoscope", or "side-viewer") used for endoscopic retrograde cholangiopancreatography (ERCP), a procedure to visualize the bile ducts and pancreatic duct on fluoroscopy, can be used to localize the bleeding to the pancreatic duct. It can be confused with bleeding from the common bile duct on endoscopy, leading to the term "pseudohematobilia".

Liver function test is normal apart from an increased serum bilirubin in the event of pancreaticobiliary reflux. Serum amylase is normal outside episodes of acute pancreatitis. It is difficult to diagnose HP because the bleeding is usually intermittent. Endoscopy is essential in ruling out other causes of upper gastrointestinal bleeding and in rare cases; active bleeding can be seen from the duodenal ampulla. Even though endoscopy may be normal, it helps to rule out other causes of upper digestive bleeding (erosive gastritis, peptic ulcers, and oesophageal and gastric fundus varices, etc.). Ultrasonography can be used to visualize pancreatic pseudocysts or aneurysm of the peripancreatic arteries. Doppler ultrasound or dynamic ultrasound has been reported to be diagnostic. Contrast-enhanced CT is an excellent modality for demonstrating the pancreatic pathology and can also demonstrate features of chronic pancreatitis, pseudocysts, and pseudoaneurysms. On precontrast CT, the characteristic finding of clotted blood in the pancreatic duct, known as the sentinel clot, is seldom seen. Computed tomography may show simultaneous opacification of an aneurysmal artery and pseudocyst or persistence of contrast within a pseudocyst after the arterial phase. Again, these findings are only suggestive of the diagnosis. Ultimately, angiography is the diagnostic reference standard. Angiography identifies the causative artery and allows for delineation of the arterial anatomy and therapeutic intervention.