Blood Tests

Reviewed by: HU Medical Review Board | Last Reviewed: January 2018.

Blood tests play a key role in helping healthcare providers to diagnose a person with inflammatory bowel disease (IBD), such as Crohn’s disease (CD) or ulcerative colitis (UC). If the physical exam and medical history suggest that IBD and its complications may be the cause of a person’s symptoms, then healthcare providers will usually recommend blood tests. These laboratory tests help providers to learn more about how the person’s body is functioning. To diagnose IBD, healthcare providers use the results of blood tests together with stool tests and diagnostic procedures, such as endoscopy and imaging.

What happens during a blood test?

Drawing blood for laboratory testing is usually a relatively simple and quick procedure.1 A healthcare provider will typically draw a small amount of blood from a vein in your arm, which is then sent to a laboratory for analysis. Blood may be drawn in your primary care physician’s office, or at a laboratory collection site.

What is the role of blood tests in diagnosing inflammatory bowel disease?

IBD cannot be diagnosed on the basis of blood testing alone.1 Though blood tests can be used to reveal signs of inflammation in a person’s body, they are not able to show precisely what the cause of the inflammation is for certain. However, the results of blood tests can be used to help healthcare providers decide what further diagnostic procedures, such as endoscopy and imaging, would be helpful in figuring out whether the cause is CD, UC, or something else.

For people who have been diagnosed with IBD, regular blood testing is also an important way that healthcare providers can monitor the activity of the disease during periods of remission as well as during flare-ups.

There are two general categories of blood tests: “routine” blood tests and specialized blood tests. The routine blood tests are commonly used to help make an initial diagnosis of IBD, as well as being used to regularly monitor the disease’s activity. Specialized blood tests are used in certain cases to help make a diagnosis of IBD, but not all healthcare providers recommend them for all patients.

What kinds of information do blood tests provide?

To help diagnose IBD, routine blood tests can be used to detect:1-4

  • Inflammation and infection
  • Anemia
  • Dehydration
  • Low albumin, or protein


One of the main functions of blood testing for IBD is to check for what is called “active” inflammation in a person’s body.5,6 There are several substances in the blood whose levels can indicate this kind of inflammation. For example, a high level of white blood cells in a person’s blood can be a sign of inflammation or infection.

A laboratory test can be used to measure the level of “C-reactive proteins” (CRPs) in a person’s blood. The liver produces CRPs and releases them into the bloodstream when there is an infection or some other source of inflammation in the body, such as a IBD flare-up.

The erythrocyte sedimentation rate (sometimes called ESR or “sed rate”) is another laboratory test that detects inflammation is in the body that may be caused by IBD.1 For this test, a sample of the patient’s blood is sent to a lab to measure how fast the red blood cells fall to the bottom of a tall tube. The ESR test results can indicate that there is inflammation in the body, which can be the result of many different causes such as CD, UC, anemia, infection, or pregnancy. For this reason, the ESR test is often used together with the CRP test to provide more information.


Anemia is a condition in which there are not enough red blood cells, which results in too little oxygen delivered by the blood through the body.6 It can be caused by a lack of iron in the blood (iron-deficiency anemia); it can also be caused by lack of vitamin B12 or folate nutrients. Because anemia is a common complication of having IBD, knowing whether or not a patient has anemia can help healthcare providers make a diagnosis.

Laboratory blood tests are able to reveal if a person has anemia, and what is causing it. The test for anemia evaluates:

  • The amount of hemoglobin in the blood
  • The size of the red blood cells
  • The percentage of red blood cells
  • The amount of iron in the blood, and how much of it is being delivered throughout the body
  • The levels of vitamin B12 and folate


A laboratory blood test called an “electrolyte panel” is used to test a patient’s levels of sodium, potassium, chloride, and carbon dioxide.1 This test can be used to check if a person’s body is dehydrated. Persistent diarrhea, a typical symptom of IBD, is a major cause of dehydration.

What are specialized blood tests?

Some healthcare providers may recommend that patients have more specialized blood tests to help determine if they have IBD.1 These blood tests are used to detect substances called “biomarkers” that are present in the bodies of many patients with IBD. These tests can also be used to help to determine if a patient has CD or UC, the two major forms of inflammatory bowel disease. However, not all people with IBD have these biomarkers in their bodies, and some people who do not have IBD do have these biomarkers. More research is needed to learn more about the role of biomarkers in diagnosing the disease.

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