Blood Tests & Biomarkers

Blood tests and other biomarkers give a window into inflammation, anemia, nutrition, and organ health in inflammatory bowel disease (IBD). These tests are used at diagnosis and then regularly over time. They help the care team see whether Crohn’s disease or ulcerative colitis is active, whether treatments are working, and whether problems like iron or vitamin deficiencies are developing in the background.

Key Takeaways

• Blood tests in IBD track inflammation, anemia, vitamin and mineral levels, and organ function over time.

• C‑reactive protein (CRP) and ESR are general inflammation markers, but they cannot show exactly where inflammation is.

• A complete blood count (CBC) and iron studies check for anemia, which is very common in IBD and often linked to iron deficiency.

• Vitamin D, B12, and folate levels, along with albumin and other nutrition markers, help identify malnutrition and absorption problems.

• Liver, kidney, and electrolyte tests are important both for safety monitoring of medications and for complications like dehydration.

• Blood biomarkers complement, but do not replace, stool tests and endoscopy in treat‑to‑target care.

What are blood tests and biomarkers in IBD?

A biomarker is a measurable sign of what is happening in the body, such as inflammation or nutrient levels. In IBD, biomarkers come from:

• Blood tests (the focus of this article)

• Stool tests, especially fecal calprotectin (covered in a separate article)

Guidelines for ulcerative colitis recommend a full blood count, CRP, electrolytes, liver and kidney tests, iron studies, vitamin D, and fecal calprotectin at diagnosis. (academic.oup.com)
Similar testing is common in Crohn’s disease, especially when following a treat‑to‑target plan.

Inflammation markers: CRP and ESR

C‑reactive protein (CRP)

CRP is made by the liver when there is inflammation anywhere in the body. In many people with Crohn’s or more extensive ulcerative colitis, higher CRP levels tend to match active disease, and falling levels can signal healing.

CRP is useful to:

• Support a suspected flare

• Track response to new treatment

• Help decide when further tests, like colonoscopy or imaging, are needed

However:

• Some people are “low CRP producers,” so CRP may stay normal even with active disease.

• Infections, injuries, and other illnesses can raise CRP, so the result is not specific to IBD.

Current guidelines for Crohn’s and ulcerative colitis recommend using serum CRP, often together with stool biomarkers like fecal calprotectin, to monitor disease activity. (gastro.org)

Erythrocyte sedimentation rate (ESR)

ESR measures how fast red blood cells settle in a tube over one hour. It also reflects inflammation but changes more slowly than CRP and is affected by age and anemia.

Clinicians may order both ESR and CRP, but CRP is often favored for short‑term changes.

Complete blood count: anemia and more

A complete blood count (CBC) includes:

• Red blood cells, hemoglobin, hematocrit, MCV (size of red cells)

• White blood cells (WBCs)

• Platelets

Key points in IBD:

• Anemia is very common in IBD, often affecting one‑third or more of patients at any given time, and is linked to lower quality of life. (pubmed.ncbi.nlm.nih.gov)

• A low MCV (small red cells) suggests iron deficiency, while a high MCV suggests vitamin B12 or folate deficiency or effects from some medications. (wjgnet.com)

• High platelet counts can be another sign of chronic inflammation. (academic.oup.com)

• High WBCs may point to active inflammation, infection, or steroid use.

CBC results are interpreted together with iron studies, vitamin levels, and inflammation markers to understand the cause of anemia or other abnormalities.

Iron studies: understanding iron deficiency anemia

Why iron testing matters

Iron deficiency and iron‑deficiency anemia are among the most frequent complications of IBD. Chronic intestinal blood loss, reduced iron absorption, and inflammation can all lower iron stores. Reviews and guidelines highlight that iron deficiency is common yet often under‑treated in IBD. (pmc.ncbi.nlm.nih.gov)

Common iron‑related blood tests

Typical iron studies include:

• Ferritin: reflects iron stores, but rises with inflammation

• Serum iron

• Transferrin or total iron‑binding capacity (TIBC)

• Transferrin saturation (TSAT): percentage of transferrin that is carrying iron

Patterns that may be seen:

• Iron‑deficiency anemia

• Low hemoglobin

• Low ferritin (cut‑offs depend on whether inflammation is present)

• Low TSAT

• Anemia of chronic inflammation

• Low hemoglobin

• Normal or high ferritin, because ferritin rises with inflammation

• Low TSAT

Because ferritin can look “normal” even when iron stores are low in active IBD, clinicians often interpret ferritin together with CRP and TSAT, and sometimes with more specialized tests.

Vitamin and nutrition blood tests

Vitamin B12 and folate

Vitamin B12 is absorbed in the last part of the small intestine (terminal ileum). People with Crohn’s disease affecting this area, or with an ileal resection, have higher risk of B12 deficiency. Folate can be low due to poor intake, small‑bowel disease, or certain medicines, such as older forms of 5‑ASA like sulfasalazine.

Both B12 and folate deficiency can cause macrocytic anemia (large red blood cells), fatigue, and, for B12, nerve‑related symptoms if not corrected.

Vitamin D

Low vitamin D levels are very common in IBD. Guidelines suggest measuring vitamin D at diagnosis in ulcerative colitis, and many centers do the same for Crohn’s. (academic.oup.com)

Research in IBD links low vitamin D with higher inflammation and also with a higher chance of iron deficiency, possibly through effects on iron‑regulating hormones. (pubmed.ncbi.nlm.nih.gov)
Vitamin D is also important for bone health, which can be affected by long‑term inflammation and steroid use.

Albumin and protein levels

Albumin is a protein made by the liver. Low albumin can reflect:

• Ongoing inflammation

• Protein loss from the gut

• Poor nutrition or weight loss

In severe colitis, low albumin together with high CRP and anemia can signal a higher risk of complications and may influence treatment decisions. (academic.oup.com)

Liver, kidney, and electrolyte tests

Routine panels often include:

• Liver enzymes and bilirubin

• Creatinine and blood urea nitrogen (BUN) for kidney function

• Electrolytes such as sodium, potassium, and bicarbonate

These help to:

• Detect side effects of medicines such as thiopurines, methotrexate, JAK inhibitors, and some biologics

• Monitor dehydration or salt losses from diarrhea and vomiting

• Guide decisions about drug dosing and safety

ECCO guidance for ulcerative colitis lists electrolytes, liver, and renal function among initial investigations. (academic.oup.com)

Drug‑related and special blood tests

Depending on the treatment plan, additional blood tests may be used:

• Thiopurine monitoring

• TPMT or NUDT15 genetic testing before azathioprine or 6‑MP

• Regular CBC and liver tests while on therapy

• Therapeutic drug monitoring (TDM)

• Drug levels and antibody tests for some biologics, especially anti‑TNF medicines

• Used to guide dose changes or switching therapies

These tests are part of broader medication‑specific protocols, described in the medication articles.

How often are labs checked?

There is no single schedule that fits everyone. In practice, many teams:

• Run a full lab panel at diagnosis and before starting new therapies

• Repeat labs more frequently during flares or when changing treatment

• Check core tests (CBC, CRP, basic chemistry, sometimes iron and vitamin levels) at regular intervals in stable remission, often every 3 to 12 months

The exact plan depends on disease type, severity, past surgery, and medications.

Quick reference: common blood tests in IBD

*Interpretation depends on the whole clinical picture.

FAQs

Do normal blood tests mean IBD is in remission?

Not always. Some people have normal CRP and other labs even when there is active inflammation on colonoscopy or imaging. Blood tests are one piece of the puzzle and are usually interpreted together with symptoms, stool tests like fecal calprotectin, and, when needed, scopes or scans.

Can blood tests tell whether it is Crohn’s disease or ulcerative colitis?

Standard blood tests cannot clearly separate Crohn’s from ulcerative colitis. They show patterns such as anemia or inflammation but do not define the exact diagnosis. The type of IBD is usually confirmed by endoscopy, biopsies, and imaging, sometimes with help from specialized serologic tests.

Why are iron and vitamin levels checked even when symptoms seem mild?

Iron, B12, folate, and vitamin D levels can fall slowly and may cause fatigue, weakness, or bone problems long before gut symptoms change. In IBD, iron deficiency and vitamin deficiencies are common and often silent, so periodic checks help catch and treat them early.

Will blood biomarkers replace colonoscopy?

Current guidelines recommend using blood and stool biomarkers to reduce how often invasive tests are needed, not to replace them entirely. (gastro.org)
When biomarkers and symptoms are stable, endoscopy may be spaced out. If results are unclear or worrying, colonoscopy or imaging is still important.