How Stomach Flu Is Diagnosed

The medical term for diarrhea and vomiting caused by a virus is viral gastroenteritis, but it is often called the stomach flu. With viral gastroenteritis, the digestive system is inflamed, which leads to symptoms such as loose stools and vomiting. The symptoms usually last a few days and pass on their own. For this reason, most people don’t wind up seeing a doctor for the stomach flu or getting an official diagnosis.

The stomach flu is not related to the influenza virus (“the flu”), which is a contagious upper respiratory condition.

Self-Check/At-Home

Diagnosing viral gastroenteritis is usually done after a review of the symptoms. Most people will be able to decide by themselves from prior experience, and from knowing that an illness is “going around," that symptoms are being caused by a common virus.

If a trip to a doctor is made, a diagnosis is most often made based on the symptoms and medical history. No formal testing is usually done.

The exception to this would be if there’s a reason to believe that there’s another condition responsible, such as if the symptoms are severe or go on for more than a few days.

Labs and Tests

In most cases, a physician probably won’t order any particular tests to diagnose viral gastroenteritis.

There is a test for rotavirus, which is a viral disease that also causes vomiting and diarrhea. It is more common in children. In a case where rotavirus may be suspected, a test might be done to diagnose that condition.

In some cases, if there is an outbreak of a viral disease such as in a hospital, testing might be done to find out which strain of the virus is causing it, but this is not common. 

Medical History

A doctor will take a careful medical history to see if there is another reason why someone might have diarrhea and vomiting. Some of the questions a physician might ask about recent and past medical history include:

• Any current medications (prescription and over-the-counter) being taken
• Any recent travel (especially overseas)
• Diet over the last few days
• How often diarrhea/vomiting is happening (how many times a day)
• If anyone else in the household is or has been sick
• Medical history, including other diseases and conditions
• What symptoms are occurring 
• When the symptoms started

Physical Exam

A physical exam may also be done. Physical exams could include:

• Blood pressure check
• Digital rectal exam
• Listening to the abdomen with a stethoscope
• Listening to the lungs with a stethoscope
• Looking for signs of dehydration
• Palpating or tapping on the abdomen to check for pain or tenderness
• Pulse
• Temperature to check for a fever

Digital Rectal Exam

Digital rectal exam is used to check for blood or mucus in the rectum and to look for any problems around the anus. This exam can cause a bit of anxiety and embarrassment but it should be painless and the doctor will get it completed as quickly as possible.

There are a few different positions in which the patient might get into to prepare for the exam:

• Bending over at the waist and resting arms on the exam table 
• Lying on one side on an exam table with knees up to the chest
• Lying on the back on an exam table with feet up in stirrups

A physician will insert a gloved, lubricated finger into the anus to check for blood in the stool. Patients may feel pressure or discomfort but it should not cause any pain. This test might also be used to feel for any abnormalities such as hemorrhoids or a mass.

Stool Test

Usually, stool tests won’t be used for diagnosing viral gastroenteritis. There may be situations, however, where a stool test is ordered.

It is a fairly simple test to complete, although many people do not turn in their stool sample out of embarrassment. If a physician orders this test, it’s important to get it done to see if there is anything present in the stool that might be causing symptoms.

The physician’s office will give instructions and a clean container for catching stool. When there is diarrhea, holding the container under the bottom during a bowel movement may be the easiest way to get a stool sample.

The sample will need to be sent to a lab and tested to see if there is anything in it that can cause an infection or inflammation.  

Other Tests

It’s not common for blood tests or imaging tests to be done when the stomach flu is the presumed diagnosis. However, if another disease or condition is suspected, blood tests or imaging such as ultrasonography, computed tomography, or magnetic resonance imaging may be done to confirm or rule it out.  

Differential Diagnoses

A list of conditions that might be the cause of the signs and symptoms a patient is having is called the differential diagnoses. In some cases, it might be suspected that there is another condition causing the symptoms, and that would need to be ruled out.

This is especially true when there are symptoms like blood or mucus in the stool, black stool, severe abdominal pain, or a high fever.

Some of the diseases and conditions a physician might look for include:

• Appendicitis: An inflammation of the appendix (a small organ located at the end of the colon).
• Bacterial infection: Infection with bacteria such as Salmonella, Shigella, Campylobacter, Yersinia, or Clostridioides difficile can cause symptoms similar to stomach flu.
• Celiac disease: A disease of the small intestine where the consumption of gluten (a protein found in some foods) can cause gastrointestinal symptoms similar to gastroenteritis.
• Diabetes: A condition called classical diabetic ketoacidosis may have symptoms similar to gastroenteritis.
• Pancreatic insufficiency: A condition where the pancreas stops producing certain enzymes.
• Rotavirus: A vaccine-preventable infectious disease that is the most common cause of diarrhea in infants and children.
• Short bowel syndrome: The small bowel not absorbing enough nutrients (which can occur after surgery or damage).
• Inflammatory bowel disease: Crohn’s disease or ulcerative colitis, which cause inflammation in the digestive tract.
• Laxative use: Using laxatives too often can lead to symptoms such as persistent diarrhea.
• Urinary tract infection: In children especially, a urinary tract infection can cause diarrhea and other symptoms.
• Volvulus: When the bowel has an abnormal twist in it.
• Whipple disease: A rare bacterial infection that prevents the body from absorbing nutrients properly.

In most cases, it’s enough to diagnose the stomach flu from a physical exam and a careful medical history. The majority of the time, people start to feel better in a few days and so they never see a doctor. When a doctor is consulted about the stomach flu, treatment is mostly supportive while the virus runs its course.