Blood Tests

Reviewed by: HU Medical Review Board | Last reviewed: June 2016. | Last updated: March 2022

When a patient presents with symptoms indicative of irritable bowel syndrome (IBS), such as diarrhea, constipation, abdominal pain and cramps, bloating and gas, a doctor may perform blood tests.

Types of blood tests for IBS

The most common blood tests are more often indicated in patients who experience diarrhea, which is indicative of IBS-D. The blood tests often given to patients in the diagnosis of IBS are:

  • Complete blood count (CBC)
  • C-reactive protein (CRP) test
  • Erythrocyte sedimentation rate (ESR)1

Complete blood count (CBC)

The complete blood count is a commonly performed lab test that is used to detect or monitor many different health conditions. The CBC test measures:2

  • The number of red blood cells (RBC) – RBC carry oxygen to the cells
  • The number of white blood cells (WBC) – WBC help fight infections
  • The total amount of hemoglobin in the blood – Hemoglobin is the protein in red blood cells that carries the oxygen
  • The percentage of blood that is red blood cells (hematocrit)
  • The platelet count – platelets are responsible for clotting

Normal values for the CBC may vary slightly among different labs but are generally as follows:

Normal Range
RBC count
Male: 4.7 to 6.1 million cells/mcL
Female: 4.2 to 5.4 million cells/mcL
WBC count
4,500 to 10,000 cells/mcL
Male: 13.8 to 17.2 gm/dL
Female: 12.1 to 15.1 gm/dL
Male: 40.7 to 50.3%
Female: 36.1 to 44.3%

In the diagnosis of IBS, a CBC is used to rule out any infections or other conditions that may be causing the abnormal digestive symptoms the patient is experiencing.1

C-reactive protein (CRP)

C-reactive protein is a product of the liver that rises when there is inflammation in the body. The CRP test is used in the diagnosis of IBS to rule out inflammatory conditions, which may have similar symptoms to IBS. The CRP test is a general test – it does not specify where inflammation in the body may be or what the cause is. Generally, patients with IBS have a normal CRP result, which would show that there is no CRP present in the blood.2

Erythrocyte sedimentation rate (ESR)

Erythrocyte sedimentation rate – commonly called a “sed rate” – is a test that measures the amount of inflammation in the body. It is used to detect and monitor inflammatory diseases or cancer, and it is not a specific diagnostic tool by itself. In the diagnosis of IBS, the ESR is used to eliminate possible inflammatory causes of abnormal digestive symptoms. The normal range of ESR in adults is:

  • Women under 50 years old: less than 20 mm/hr
  • Women over 50 years old: less than 30 mm/hr
  • Men under 50 years old: less than 15 mm/hr
  • Men over 50 years old: less than 20 mm/hr2

Patients with IBS generally have a normal ESR.1

When blood tests are not done in IBS

Blood tests are not always done in the diagnosis of IBS, particularly if a patient’s symptoms are consistent with IBS – for example, abdominal pain, bloating, constipation and diarrhea – and they meet the Rome III criteria for frequency and duration of symptoms. A doctor may decide not to order tests if there are no warning signs of other possible conditions found on the physical exam. Reviews of the data show that patients with IBS symptoms are not more likely than healthy patients to have a physical, structural disorder of the intestines, such as Crohn’s disease, ulcerative colitis, colorectal cancer, and thyroid disease.1

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