Blood and Fecal Tests for IBD

These can help diagnose and manage Crohn's disease or ulcerative colitis

Blood tests and fecal (stool) tests are an important part of the diagnosis and management of inflammatory bowel disease (IBD). Blood tests play a key role by identifying characteristics of IBD, including anemia and inflammation, and can be used to determine if you are a candidate for certain drug therapies. Fecal tests detect inflammation as well, and they help determine whether direct imaging of the intestine is needed.

As a set of inflammatory diseases, which includes Crohn's disease and ulcerative colitis, IBD is diagnosed by exclusion, meaning that other causes are ruled out before a diagnosis can be made. These tests can help with that and with directing treatment.

Blood tests and fecal tests are also performed periodically to manage IBD once it has been diagnosed and treated.

Red Blood Cell Count

A red blood cell count is part of a larger blood panel known as a complete blood count (CBC). The test is used to measure how many red blood cells (RBC), also known as erythrocytes, are present in a sample of your blood.

The test is important because red blood cells contain hemoglobin, an iron-rich protein that transports oxygen to the body's tissues. If you have a low RBC count, you are said to have anemia.

Anemia is a common feature of IBD and one that is frequently overlooked as a disease complication. People with IBD tend to have iron deficiency anemia due to chronic intestinal bleeding and the impaired absorption of iron due to persistent bowel inflammation.

Hemoglobin Test

A hemoglobin test is another blood test used to detect and measure the extent of anemia. When both RBC and hemoglobin levels are low, it may be the result of a condition called anemia of chronic disease (ACD), also known as anemia of inflammation.

With ACD, you can have normal or even high levels of iron in tissues but a low level of iron in the blood. Because of this, the amount of hemoglobin in red blood cells—which relies on iron to bind to oxygen molecules—will be reduced.

ACD is characteristic of diseases that cause chronic inflammation, including many autoimmune diseases.

Crohn's disease and ulcerative colitis are believed to be either autoimmune (meaning that the immune system attacks itself) or immune-mediated (meaning that the immune system is overresponsive).

Hematocrit Test

A hematocrit test doesn't measure a specific protein or cell within the blood. Rather, it is a calculation of the volume of red blood cells.

The hematocrit blood test determines the percentage of RBC in a sample of blood, with low percentages indicating anemia and high percentages suggesting dehydration, oxygen deprivation, or other causes.

In addition to diagnosing anemia, a hematocrit test can aid in the management of IBD. A sudden drop in a hematocrit percentage may be an early warning sign of disease complications or predict the risk of complications following IBD surgery.

By contrast, people with stable hematocrit percentages generally have few complications and better control of IBD symptoms.

White Blood Cell Count

White blood cells are key components of your immune system. They not only target and neutralize disease-causing microorganisms (pathogens) but "learn" to identify specific pathogens so they can launch a targeted assault should that pathogen return.

White blood cells (WBC), also known as leukocytes, are comprised of various types of cells, each with its specific purpose. They include granulocytes (neutrophils, eosinophils, and basophils), monocytes, and lymphocytes (T-cells and B-cells).

A white blood cell count is a panel of tests that measures each of these cell types. Elevations of WBC is generally a sign of an infection or inflammation. A doctor can begin to investigate the most likely causes of the inflammation or infection based on which cells are elevated.

White blood cell counts are important because many of the drugs used to treat IBD are immunosuppressive, meaning that they blunt the immune system's response to disease. If the WBC is too low, a person may be at a high risk of infection.

C-Reactive Protein and ESR Tests

There are two blood tests commonly used to measure the degree of generalized inflammation in the body:

  • C-reactive protein (CRP) measures a type of protein secreted by the liver in response to inflammation.
  • Erythrocyte sedimentation rate (ESR) measures the rate in which red blood cells (erythrocytes) settle to the bottom of a test tube in one hour. People experiencing inflammation will have a faster rate of sedimentation than those who don't.

CRP is a more direct method of measuring inflammation and is better at detecting acute inflammation, such as might occur during exacerbations (flare-ups) of Crohn's disease or ulcerative colitis. ESR measures inflammation indirectly and tends to remain elevated with a chronic inflammatory disorder like IBD.

CRP and ESR are only generalized markers for inflammation but can be useful in predicting the development of exacerbations in people with IBD, allowing for early treatment and better control of symptoms.

Fecal Calprotectin Test

A fecal calprotectin test is used to differentiate between inflammatory bowel diseases and non-inflammatory bowel diseases like diverticular disease and ischemic colitis. The test detects and measures the amount of a protein called calprotectin in a stool sample.

Calprotectin is a byproduct of neutrophils. When someone's neutrophil count is abnormally elevated, a sign of inflammation, the calprotectin count would also be elevated.

A calprotectin value above 50 to 60 micrograms per milligram (mcg/mg) is suggestive of IBD.

The test helps patients avoid unnecessary procedures like endoscopy by confirming the presence of inflammation within the intestine itself (rather than just the blood). In this respect, the fecal calprotectin test is more qualitative than blood tests like ESR or CRP.

A fecal calprotectin test may also be performed if there is an IBD flare to characterize its severity or used to monitor your response to treatment until your condition is fully stabilized.

In addition to calprotectin, a stool sample may be used to check for hidden blood in stools (fecal occult blood test) or cultured to rule out bacterial or parasitic infections as the cause of your symptoms.

TPTM Testing

A thiopurine methyltransferase (TPMT) test may be ordered if a doctor is considering Purixan (mercaptopurine) or Azasan (azathioprine) as a treatment for severe IBD. The TMPT test can detect the deficiency of the enzyme thiopurine and determine your risk of severe side effects if using immune-suppressing thiopurine drugs.

Depending on the level of thiopurine methyltransferase in your blood, your doctor will be able to calculate the proper dose to avoid drug side effects. In some cases, thiopurine levels may be so low as to contraindicate the use of this class of drugs.

Specialized Blood Tests

There are specialized blood tests used by some doctors that may help support the diagnosis of Crohn's disease or ulcerative colitis. They detect specific proteins, called antibodies, produced by the immune system in response to certain inflammatory diseases. They include:

  • Anti-Cbir1 antibody (CBir1)
  • Anti-Saccharomyces cerevisiae antibody (ANSA)
  • Anti-outer-membrane porin C antibody (OmpC)
  • Perinuclear anti-neutrophil cytoplasmic antibody (pANCA)

Around 80% of people with IBD may have these and other biomarkers for IBD. Having the antibodies, however, does not mean that you have IBD since the antibodies may be present even in people without IBD.

Because these tests have low specificity (meaning that they can occur with many diseases), they have varying degrees of acceptance in the medical community. Still, the blood tests may help support a diagnosis since the presence of these antibodies may help exclude other conditions.

A Word From Verywell

If you suspect you have IBD, it is important to remember that blood tests cannot diagnose IBD but instead help characterize your symptoms and point the doctor in the direction of the correct diagnosis.

If you have IBD, it is important to have routine blood works performed so that any complications or side effects can be spotted early and treated before they become severe.

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