Healthy Living

How Is Diarrhea Diagnosed?

How Is Diarrhea Diagnosed?

Diarrhea is a waterborne and foodborne disease that may occur due to an infection caused by viruses, bacteria, or parasites. Diarrhea can also happen as a side effect from food allergies and food intolerances. The condition is characterized by loose or watery stools, which is usually accompanied by a stomachache, stomach cramps, nausea and vomiting. Diarrhea usually lasts for two to three days and resolves on its own. If one is well hydrated during this period, then there is no cause for worry. However, if diarrhea persists beyond three days and/or is accompanied by blood or mucus in the stool, fever, and severe pain, then one should seek medical help to get a proper diagnosis and treatment.

A diagnosis needs to be done to find out the cause of diarrhea. Initially, a complete physical examination and one's medical history is required. Doctors also request for blood and stool tests for further investigation. 

Tests and Diagnosis

When people consult a doctor to get a treatment plan for diarrhea, a doctor generally asks the following questions:

  • When did the problem start?
  • What is the frequency of your bowel movement and the type of stool you pass?
  • Is your diarrhea accompanied with nausea or bloating?
  • Is there blood present in your stool?
  • Have you travelled recently?
  • Have you been in contact with someone who has diarrhea?
  • Are you on any type of medication?
  • Have you eaten out recently?

Dehydration is associated with diarrhea, and it can cause a serious problem if not treated properly. Doctors are always alert about it and its severity.

Three Stages of Dehydration

  • Early stage - There is no sign or symptom, but one starts to feel weak.
  • Moderate dehydration: Signs like thirst, dizziness, pinched face, dry tongue, and sunken eyes show up.
  • Severe dehydration: A low urine output, rapid and feeble pulse, low blood pressure, pale skin, and reduced consciousness may manifest. It may end in death if a patient is not treated with rehydration therapy immediately.

Tests for Diarrhea

In the case of acute diarrhea, the patient may need to give a stool sample, which is examined under a microscope. Severe cases may demand further tests. Parasites or their eggs are generally visible when viewed through a microscope. Thus, the microscope evaluates the parasite eggs or samples for further testing.

When diarrhea is caused by bacteria, white blood cells (WBC) appears in the stool. Hence, a WBC test is requested if there is a sign of a bacterial infection. A stool culture can also be done to know the exact pathogenic gastrointestinal bacteria that causes diarrhea.

Blood tests such as a complete blood count (CBC), electrolyte levels, blood urea nitrogen (BUN), and creatinine can also be requested by the doctor for further investigation.

The tests for chronic diarrhea usually include the following:

  • Liver function tests
  • Albumin test
  • Complete blood count to find out the presence of anemia
  • Checking the symptoms of calcium, vitamin B12, and iron malabsorption
  • Thyroid function tests
  • Antibody testing for celiac disease


Doctors will ask for a stool sample to find out the root cause of one's illness. Diagnosis becomes easier when there is a stool exam result. Most of the time, further testing is needed if a person has:

  • continuous diarrhea for more than 14 days
  • symptoms affecting the whole body e.g., fever or dehydration
  • recently been admitted to a hospital for treatment
  • been taking antibiotics
  • presence of blood or pus in the stool 

Blood Tests

Doctors may request for blood samples if they suspect that unhealthy foods are the reason of a patient's diarrhea. A blood sample is also required to find signs of inflammation or other bowel diseases.

Rectal Examination

A digital rectal examination (DRE) might be needed if there are any signs of an unexplained and continuing diarrhea. This test is mostly done on adults having an age of 50 and above.

In this test, a health care provider puts on a glove and inserts his finger into the patient's bottom to feel any abnormalities. It is useful for finding out any rectal problems that can cause diarrhea for a long time without showing any symptoms.

Further Investigations

If all tests have been done, but the doctors are still unable to find the cause, they may refer the patient to a local hospital to conduct further investigation. The patient may have to go through tests such as:

  • Sigmoidoscopy – is a procedure that uses a thin and flexible tube, which contains a small camera and light at one end. The tube is then inserted into the bottom and up into the bowel to visually check the sigmoid colon and rectum of the patient.
  • Colonoscopy - is a similar procedure to the previous one, which allows the doctor to visually inspect the parts of your large intestine i.e., the colon and rectum. This procedure utilizes a colonoscope, which is a thin and flexible tube that goes into the colon. A biopsy or tissue sample can be collected during the procedure. Abnormal growths in the colon can also be taken out. A colonoscopy procedure can help the doctor find ulcers, tumors, and colon polyps. That is why the procedure is also regarded as a screening test for colon cancer or precancerous growths in the rectum.

The Bottom Line

Diarrhea might last for two to three days and then clears up on its own. That is why there is usually nothing to worry about. A proper diagnosis should be sought if diarrhea persists for a longer period of time, and is accompanied by other severe symptoms, especially dehydration.

Diarrhea can have several causes and through the process of asking relevant questions, a physical examination and other tests such as a complete blood count (CBC) or a stool culture will help the doctor identify the cause of diarrhea. Diarrhea could be caused by a viral, bacterial, or parasitic infection, or as an autoimmune response in the case of celiac disease and Crohn's disease. A food allergy or food intolerance can also cause diarrhea. A skilled doctor will be able to make a proper diagnosis and find the root cause of the disease. 

The treatment usually depends on the type of diarrhea. Blood tests and stool examinations are the common laboratory tests needed in the diagnosis of diarrhea.