Blood Tests & Biomarkers

Blood and stool tests help track inflammation, nutrition, and treatment safety in inflammatory bowel disease (IBD). The most used blood tests are C‑reactive protein (CRP), complete blood count (CBC), iron studies, vitamin levels, and liver and kidney panels. Results guide when to adjust therapy and when to schedule scopes. Trends over time matter more than a single number, and targets differ by disease and setting. (gastro.org)

Key takeaways

• CRP and fecal calprotectin change with gut inflammation. Aim for normal values in remission, using both when possible. (gastro.org)

• Iron lack is common. Check ferritin and transferrin saturation, and treat deficiency even without anemia. (gastro.org)

• Vitamin D and B12 are often low, especially with small‑bowel Crohn’s or after ileal surgery. Replete and re‑check. (academic.oup.com)

• Albumin falls with inflammation and illness. It is not a reliable stand‑alone nutrition marker. (colab.ws)

• In stable remission, many care teams check biomarkers every 6 to 12 months. Test more often during flares or therapy changes. (gastro.org)

What these tests show

Blood and stool biomarkers offer an objective view that complements symptoms and endoscopy. Treat‑to‑target care uses them to confirm control of inflammation and to decide when to escalate or de‑escalate therapy. Short‑term targets include normal serum and stool markers, with endoscopic healing as a longer‑term goal. (pubmed.ncbi.nlm.nih.gov)

Core inflammation markers: CRP, ESR, and fecal calprotectin

• C‑reactive protein (CRP) rises when inflammation is active, then falls quickly as it improves. Normalization supports remission. Some people are “low CRP producers,” so a normal CRP does not always exclude active disease. (gastro.org)

• Erythrocyte sedimentation rate (ESR) changes more slowly and is affected by anemia and other factors. It is less specific for gut inflammation than CRP or fecal calprotectin. (pubmed.ncbi.nlm.nih.gov)

• Fecal calprotectin (stool test) reflects intestinal inflammation and often tracks best with mucosal healing. Many programs pair it with CRP to guide when endoscopy is needed. See the companion article on fecal calprotectin for details on cutoffs. (pubmed.ncbi.nlm.nih.gov)

Blood counts and iron studies

• Complete blood count (CBC) detects anemia, infection, and platelet changes that accompany inflammation. Platelets often rise with active disease, and hemoglobin falls with blood loss or iron lack. (pubmed.ncbi.nlm.nih.gov)

• Iron deficiency is very common in IBD. Use ferritin and transferrin saturation (TSAT) to diagnose and classify anemia. Typical diagnostic anchors: ferritin less than 30 micrograms per liter without inflammation, ferritin up to 100 micrograms per liter can still indicate deficiency when CRP is elevated, and TSAT less than 16 to 20 percent supports iron deficiency. (academic.oup.com)

• When to treat and re‑test. Iron lack, even without anemia, impairs energy and quality of life. Oral iron fits mild, inactive disease, while intravenous iron is preferred in active disease, severe anemia, or oral intolerance. Recheck blood counts and iron indices about 4 to 8 weeks after treatment, then periodically. (frontiersin.org)

Vitamins, minerals, and albumin

• Vitamin B12 is absorbed in the last part of the small intestine, so levels can fall with ileal Crohn’s or after ileal resection. Screen at diagnosis and at least yearly in those at risk. If serum B12 is borderline, methylmalonic acid or holotranscobalamin can confirm deficiency. (academic.oup.com)

• Vitamin D deficiency is frequent in IBD. Many clinicians aim to correct low 25‑hydroxyvitamin D, often to at least the low‑normal range, to support bone health, although a single universal target is debated in newer endocrine guidance. Measure and replete, then re‑check to confirm sufficiency. (academic.oup.com)

• Folate, zinc, and magnesium may also be low, especially with small‑bowel disease, diarrhea, or restrictive diets. Check when risk or symptoms suggest a problem. (academic.oup.com)

• Albumin falls with inflammation and illness. Low values may predict worse outcomes and affect drug levels, but albumin alone should not define malnutrition. (colab.ws)

Liver and kidney panels

• Liver enzymes (ALT, AST), alkaline phosphatase, and bilirubin monitor medicine safety and detect liver conditions linked to IBD, such as primary sclerosing cholangitis, which often presents with a cholestatic pattern, high alkaline phosphatase and gamma‑glutamyltransferase. Persistent abnormalities warrant evaluation. (journals.lww.com)

• Kidney function (creatinine, eGFR, electrolytes) can change with dehydration, diarrhea, or medicines. Mesalamine is a rare cause of interstitial nephritis, so periodic creatinine checks are advised, especially in the first years on therapy. (pubmed.ncbi.nlm.nih.gov)

Medication‑related monitoring, briefly

• Thiopurines. Baseline TPMT and NUDT15 testing helps prevent severe bone marrow suppression. Regular CBC and liver tests are required during therapy. See the Thiopurines article for a schedule. (cpicpgx.org)

• JAK inhibitors. Lipids often rise by 4 to 8 weeks after starting tofacitinib or upadacitinib. Check a lipid panel after initiation and manage per cardiovascular guidelines. Monitor CBC and liver enzymes as directed. (pfizer.com)

• Details for other drugs, including methotrexate, biologics, and S1P modulators, appear in their medication pages.

How often to test

• In symptomatic remission, many guidelines suggest checking noninvasive biomarkers every 6 to 12 months, with stool tests often preferred for ulcerative colitis. Increase frequency with new symptoms, treatment changes, or rising markers. (gastro.org)

• During flares or after a major medication change, labs are often repeated about every 1 to 3 months until stable, then spaced out. Iron and vitamin levels are rechecked after repletion to confirm response. (academic.oup.com)

Biomarkers at a glance

FAQs

Why is my CRP normal even when I feel worse

A small group of people have a limited CRP response, sometimes due to genetics. In that case, stool tests, imaging, or endoscopy are more informative than CRP alone. (pubmed.ncbi.nlm.nih.gov)

Should iron be treated if hemoglobin is still normal

Yes, iron deficiency without anemia can cause fatigue and poorer function. Treat and re‑test to confirm replenishment. (frontiersin.org)

What is a reasonable cadence for labs in remission

Many teams monitor biomarkers every 6 to 12 months in stable remission, and sooner if symptoms change. Your plan may be more frequent with prior complications or recent medication changes. (gastro.org)

Do albumin and prealbumin diagnose malnutrition

No. These proteins fall with inflammation and illness. Nutrition status should be assessed clinically, often with a dietitian. (colab.ws)

Editor note: pair this article with “Stool Tests: Fecal Calprotectin” and “Therapeutic Drug Monitoring (TDM)” for complete monitoring guidance.