Blood tests are routine in managing inflammatory bowel disease, yet the abbreviations and numbers can feel like a foreign language. Understanding these markers can help you make sense of what is happening inside your body and guide conversations about your treatment.
What blood tests measure in IBD
Blood tests in IBD assess inflammation levels, check for complications such as anaemia or nutrient deficiencies, monitor how organs cope with medication, and track disease activity over time. No single test can diagnose IBD or confirm a flare, but patterns across several markers give your clinical team a clearer picture.
These tests do not replace colonoscopy, imaging, or faecal calprotectin measurements, but they are less invasive and can be repeated frequently.
Common inflammatory markers
C-reactive protein (CRP)
C-reactive protein is made by the liver in response to inflammation. When tissues are inflamed, immune cells release cytokines, which stimulate CRP production. Levels rise quickly during active inflammation and fall when it subsides.
In IBD, elevated CRP often correlates with disease activity, particularly in Crohn’s disease. However, around 25% of people with active IBD have normal CRP, meaning a low result does not rule out a flare. CRP is also non-specific: it can be elevated due to infections, other inflammatory conditions, or stress.
Normal CRP is typically below 5 milligrams per litre, though ranges vary by laboratory.
Erythrocyte sedimentation rate (ESR)
ESR measures how quickly red blood cells settle in a test tube. Inflammation changes blood proteins, causing red blood cells to clump and fall faster. ESR rises and falls more slowly than CRP, reflecting longer-term inflammation.
Like CRP, ESR is not specific to IBD and can be influenced by age, anaemia, and other conditions. It is often used alongside CRP for a fuller picture.
Markers of anaemia
Haemoglobin (Hb)
Haemoglobin carries oxygen around your body. Low haemoglobin (anaemia) is common in IBD, resulting from chronic inflammation suppressing red blood cell production, blood loss from inflamed gut lining, or deficiencies in iron, vitamin B12, or folate.
Anaemia causes fatigue, breathlessness, dizziness, and pale skin. Normal levels are roughly 130 to 180 grams per litre in men and 120 to 160 in women.
Haematocrit
Haematocrit measures the proportion of blood made up of red blood cells. It mirrors haemoglobin and is low in anaemia. Low haematocrit alongside low haemoglobin reinforces anaemia diagnosis, while high haematocrit can indicate dehydration.
Mean corpuscular volume (MCV)
MCV indicates average red blood cell size. Small cells suggest iron deficiency; large cells suggest vitamin B12 or folate deficiency. In IBD, both patterns can occur, making MCV a helpful diagnostic clue.
White blood cell count and differential
Total white blood cell count
White blood cells are part of your immune system. Elevated counts can signal inflammation, infection, or corticosteroid use. Low counts may indicate bone marrow suppression from immunosuppressive drugs such as azathioprine or methotrexate, requiring close monitoring.
Neutrophils, lymphocytes, and eosinophils
The differential breaks down white cell types. Neutrophils rise during bacterial infections and active inflammation. Lymphocytes are involved in longer-term immune responses and can be suppressed by some IBD medications. Eosinophils may be elevated in drug reactions but are not commonly a focus in routine monitoring.
Platelet count
Platelets help blood clot. In active IBD, platelet counts often rise as part of inflammatory response. Persistently elevated platelets suggest ongoing inflammation, while very high levels may slightly increase clotting risk. Low counts are less common but can occur with certain medications.
Markers of liver and kidney function
Liver enzymes (ALT, AST, ALP, GGT)
Many IBD medications are processed by the liver, so monitoring is essential. ALT and AST are released when liver cells are damaged. ALP and GGT can indicate bile duct inflammation, which can occur in primary sclerosing cholangitis, associated with ulcerative colitis.
Mild elevations are common, but significant or rising levels may prompt medication review or specialist input.
Creatinine and estimated glomerular filtration rate (eGFR)
Creatinine is a waste product filtered by kidneys. Elevated creatinine or low eGFR can indicate reduced kidney function from dehydration, medications, or other conditions. Regular monitoring is important if you take drugs affecting the kidneys.
Nutritional markers
Ferritin
Ferritin reflects iron stores. Low ferritin indicates iron deficiency, common in IBD. However, ferritin can be artificially elevated during inflammation even when iron stores are low, so it is interpreted alongside other markers.
Vitamin B12 and folate
Vitamin B12 is absorbed in the terminal ileum, commonly affected by Crohn’s disease. Deficiency causes macrocytic anaemia and neurological symptoms. Folate deficiency can occur due to poor absorption, inadequate intake, or medication side effects. Both are routinely monitored.
Albumin
Albumin is the main blood protein, made by the liver. Low albumin can indicate malnutrition, chronic inflammation, or protein loss through inflamed gut lining. It helps assess nutritional status and disease severity.
Practical takeaways
- Blood tests are one tool among many and should be interpreted alongside symptoms, stool tests, and imaging.
- Trends over time are more informative than single results. Keep a record of key markers.
- Normal CRP or ESR does not rule out active inflammation, particularly in ulcerative colitis.
- Anaemia is common in IBD and often multifactorial. Identifying the cause guides treatment.
- Medication monitoring is essential. Attend all scheduled blood tests even when you feel well.
- Discuss any results you do not understand with your IBD team.
Conclusion
Blood tests provide valuable insight into inflammation, nutritional status, and medication safety in IBD, but they are not the whole story. Results should always be considered in the context of your symptoms, clinical examination, and other investigations. Understanding these markers empowers you to engage more actively in your care and helps you and your team make informed decisions about treatment and monitoring.
References
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- Travis SPL, Farrant JM, Ricketts C, et al. Predicting outcome in severe ulcerative colitis. Gut. 1996;38(6):905-910. doi:10.1136/gut.38.6.905
- Lewis JD, Gelfand JM, Troxel AB, et al. Immunosuppressant medications and mortality in inflammatory bowel disease. Am J Gastroenterol. 2008;103(6):1428-1435. doi:10.1111/j.1572-0241.2008.01836.x
This article is intended for informational and educational purposes only. It does not constitute medical advice and should not be used as a substitute for professional medical guidance, diagnosis, or treatment.