Blood tests

There are several blood tests which are used to help support a diagnosis of IBD or to monitor the effects of IBD or its treatment.

Blood samples are usually taken from a vein in the arm, using a needle attached to a syringe or to a collecting container. For some specialized tests, you may be asked to fast (avoid eating) overnight or for a few hours before the sample is taken. Your doctor or nurse will let you know if this is necessary.

Some of the blood tests used most often in IBD are described below:


An FBC counts and measures the levels of the three main types of blood cells (red cells, white cells, and platelets). The number of white blood cells can indicate if there is any inflammation or infection in the body. An increase in the number of platelets (small cells involved in blood clotting) can also be a sign of inflammation. By measuring the level of hemoglobin (a molecule in red blood cells which carries oxygen through the body), an FBC test can detect anemia – for more information see Ferritin and Transferrin Tests below.

Some of the drugs given for IBD, such as azathioprine and mercaptopurine, can affect the bone marrow and reduce the levels of red and white blood cells and platelets. People on these drugs are usually given regular FBC tests to help
monitor their blood cell levels.


Inflammation can increase the levels of some types of proteins found in the blood. Blood tests such as C-Reactive Protein (CRP) and Erythrocyte Sedimentation Rate (ESR) can be used to detect inflammation by measuring the levels of these proteins or ‘inflammatory markers’.


A liver function test or LFT can help to show whether the liver is working properly. It can also be used to help diagnose some of the rare liver complications of IBD, such as Primary Sclerosing Cholangitis. 

An LFT measures the levels of a specific group of proteins and enzymes found in the blood. LFTs usually also include a test for a protein called serum albumin. Low levels of this protein can be a sign of inflammation.

Some of the drugs given for IBD, for example, azathioprine, methotrexate, and some 5ASA drugs (such as mesalazine) can affect the liver. LFTs are a good way of checking for this.


These tests measure the levels of urea, creatinine, and electrolytes (dissolved body salts such as potassium and sodium) in the blood. They are usually carried out to assess how well the kidneys are working. This test can also be used to check for dehydration (a common effect of acute diarrhea). Regular tests are often recommended for people on certain IBD drug treatments.


Iron deficiency (a lack of iron) is one of the main causes of anemia, a common complication of IBD. Symptoms of anemia include fatigue, headaches and shortness of breath.

An CBC or complete blood count (as described earlier) is one way of detecting iron deficiency. The doctors may also check your ferritin or transferrin saturation levels.

Ferritin is a protein in the blood that tends to increase with inflammation, so can be used as an inflammatory marker. It also stores iron and a low level of ferritin is usually a good indicator of low iron levels.

Transferrin, another blood protein, binds to iron to carry it from the gut. A transferrin saturation test shows how much of the transferrin in the body is actually carrying iron. A low transferrin saturation reading is another sign of iron deficiency.


Your blood may also be tested for Vitamin B12 and folic acid. These important vitamins work together to form healthy red blood cells.

Vitamin B12 is absorbed into the blood stream from the terminal ileum (the last part of the small intestine before it joins the colon). If the ileum is very inflamed or damaged, or has been removed by surgery, the body may not be able to absorb enough Vitamin B12, in which case it can be given by injection.

Low folic acid levels can be caused by poor nutrition or poor absorption from the small intestine. Some medications for IBD, such as methotrexate and sulphasalazine (a 5-ASA drug), can also affect folic acid levels.


Other blood tests that may be helpful in IBD include:

  • Tests for calcium and phosphate. Both these minerals are important for bone health. Vitamin D levels may also be measured if bone loss is suspected

  • A magnesium check. Magnesium is needed for healthy muscles and bones. Severe diarrhea may cause low magnesium levels, so a blood test may be needed to check this

  • Tests for trace elements, such as zinc, selenium, chromium and other minerals which are usually found in tiny quantities in the blood of healthy people. These may be checked if you are on intravenous feeding or long-term nutritional treatment


This test can help predict who is more likely to experience side effects from thiopurine drugs such as Azathioprine or Mercaptopurine. However, it cannot identify everyone who may be affected.


Before you start on certain Immunosuppressant or Biologic drugs, such as infliximab or adalimumab, you may need to be screened for infectious diseases such as Hepatitis B or C, tuberculosis, HIV, or chicken pox.