A calprotectin blood test measures a protein complex released mainly by activated neutrophils and monocytes. These cells are early responders in infection and inflammation, so circulating calprotectin can rise rapidly when the innate immune system is active. Researchers and some specialist clinics use serum or plasma calprotectin to study rheumatoid arthritis, severe infection, sepsis, inflammatory bowel disease, and other neutrophil-driven conditions. However, blood calprotectin is not yet as standardized or routinely used as C-reactive protein. Results depend strongly on whether serum or plasma was tested, how quickly the sample was processed, and which assay the laboratory used. A high level supports active inflammation but cannot identify the cause by itself. This article explains what circulating calprotectin represents, why high values occur, how it differs from fecal calprotectin, and why clinical context and laboratory-specific reference information are essential.
- Neutrophil-linked marker: Calprotectin is abundant in neutrophils and is released during activation or cell damage.
- Serum is not stool: Blood and fecal calprotectin answer different clinical questions and use different units.
- No universal cutoff: Assays, specimen type, and sample handling can produce substantially different values.
- Promising but specialized: Evidence is growing in arthritis, infection, and sepsis, but routine use remains limited.
- High is nonspecific: The result must be interpreted with symptoms, blood counts, CRP, cultures, imaging, and other findings.
Table of Contents
- Calprotectin and Neutrophil Activation
- Blood Versus Fecal Calprotectin
- Reference Ranges, Units, and Sample Type
- Causes of High Blood Calprotectin
- Infection, Sepsis, and Acute Illness
- Arthritis, IBD, and Chronic Inflammation
- How to Interpret and Follow Up a Result
Calprotectin and Neutrophil Activation
Calprotectin is a complex formed mainly from two S100-family proteins, S100A8 and S100A9. It is also called MRP8/14 or S100A8/A9. Neutrophils contain large amounts of it in their cytoplasm, and monocytes contain smaller amounts. When these cells are activated, migrate into tissues, release granules, form extracellular traps, or break down, calprotectin can enter blood and other body fluids.
The protein has both defensive and inflammatory functions. It can bind metals such as zinc and manganese, making them less available to microorganisms. This process, sometimes called nutritional immunity, helps limit microbial growth. Outside cells, calprotectin can also act as a danger signal. It interacts with receptors such as Toll-like receptor 4 and the receptor for advanced glycation end products, amplifying innate immune activity.
That dual role explains why calprotectin can be both a useful marker and part of the inflammatory process itself. A high circulating concentration may indicate that many neutrophils are activated, that inflamed tissues are releasing S100A8/A9, or that cell turnover is increased. It does not reveal which organ is affected.
The relationship to neutrophil count is important but not simple. A person can have a normal number of neutrophils with high calprotectin if the cells are strongly activated. Another person can have neutrophilia but only a modest calprotectin increase. The test therefore reflects cell activity and release, not merely cell quantity.
This differs from a neutrophil count test, which reports how many neutrophils are circulating. Measuring both can sometimes distinguish an increase in cell number from a stronger activation signal, although that interpretation remains condition- and assay-specific.
Calprotectin can rise earlier than liver-derived acute-phase proteins because it is released directly from innate immune cells. CRP must first be synthesized by the liver after cytokine signaling. This potential speed has made blood calprotectin attractive for research in emergency medicine and sepsis. It has not eliminated the need for CRP, procalcitonin, cultures, or clinical assessment.
Blood Versus Fecal Calprotectin
The word “calprotectin” often refers to a stool test, but blood and fecal testing are not interchangeable.
Fecal calprotectin measures protein released into the intestinal lumen when neutrophils migrate through the gut wall. It is used mainly to detect and monitor intestinal inflammation, especially in inflammatory bowel disease. The concentration is usually reported in micrograms per gram of stool.
Blood calprotectin measures circulating S100A8/A9 in serum or plasma. It reflects systemic neutrophil and monocyte activation. It may be elevated because of inflammation in the joints, lungs, bloodstream, gut, or another site. Results are often reported in micrograms per milliliter, milligrams per liter, or nanograms per milliliter, depending on the assay.
| Feature | Blood calprotectin | Fecal calprotectin |
|---|---|---|
| Sample | Serum, EDTA plasma, or another specified plasma type | Stool |
| Main biological signal | Systemic myeloid-cell activation | Neutrophil migration into the intestinal tract |
| Common use | Specialist or research assessment of inflammatory activity | Routine support for distinguishing inflammatory from noninflammatory bowel symptoms and monitoring IBD |
| Typical units | Assay-dependent, such as µg/mL, mg/L, or ng/mL | Usually µg/g stool |
| Key limitation | No widely harmonized universal reference interval | Can rise with infection, NSAID use, bleeding, age, and other intestinal conditions |
A normal fecal result does not guarantee a normal blood result, and a high blood result does not prove intestinal inflammation. Someone with active rheumatoid arthritis may have high circulating calprotectin but no bowel disease. Someone with localized intestinal inflammation may have a markedly high stool level while the systemic signal is smaller.
When a report simply says “calprotectin,” identify the specimen before interpreting it. The fecal calprotectin test has better-established clinical pathways. Blood calprotectin remains an emerging complementary marker in many settings.
Reference Ranges, Units, and Sample Type
There is no universal normal range for circulating calprotectin. Published studies have used different commercial immunoassays, laboratory-developed methods, sample tubes, processing times, storage conditions, and patient populations. A cutoff from one study should not be applied directly to a result produced by another laboratory.
Serum and plasma deserve special attention. During clotting, neutrophils can release calprotectin. Serum values may therefore be higher than plasma values, even when samples come from the same person. Delayed centrifugation can add further release from blood cells. EDTA plasma, heparin plasma, and serum are not automatically equivalent.
Preanalytical factors include:
- Time between collection and centrifugation
- Temperature before processing
- Type of collection tube and anticoagulant
- Hemolysis or difficult venipuncture
- Storage duration and freeze-thaw cycles
- Recent strenuous exercise, acute stress, or infection
Analytical factors include antibody specificity, calibration material, detection range, and whether the assay reports total S100A8/A9 complexes or related forms. Some manufacturers provide a reference limit, but this is method-specific.
A useful laboratory report should include the specimen type, unit, method or platform, and reference interval or decision threshold. If any of these are missing, the ordering clinician may need to contact the laboratory. Converting units is possible only when the measured analyte and calibration are comparable. For the same mass concentration, 1 µg/mL equals 1 mg/L and 1,000 ng/mL, but numerical conversion does not correct for different assays.
Avoid labeling a value “slightly high” based on an internet range. Instead ask whether it exceeds the reporting laboratory’s validated limit and whether the difference is large enough to exceed expected biological and analytical variation. Trends are most meaningful when measured using the same specimen type and platform.
Causes of High Blood Calprotectin
High circulating calprotectin indicates increased release from neutrophils, monocytes, or inflamed tissues. It is not specific to one disease. The likely explanation depends on symptoms, duration, comorbidities, and other results.
Acute infection can produce a rapid increase. Bacterial infections often activate neutrophils strongly, but viral, fungal, and parasitic infections can also elevate calprotectin. The concentration cannot identify the organism.
Inflammatory arthritis is one of the best-studied chronic settings. In rheumatoid arthritis, circulating calprotectin may correlate with swollen joints, ultrasound synovitis, and treatment response, sometimes even when CRP is low. It remains an adjunct rather than a diagnostic criterion.
Autoinflammatory and autoimmune disease may raise levels through innate immune activation. Examples studied include juvenile idiopathic arthritis, adult-onset Still disease, vasculitis, systemic lupus erythematosus, and spondyloarthritis. Performance differs among diseases and treatments.
Inflammatory bowel disease can increase circulating calprotectin, especially with substantial systemic activity, but blood testing does not replace stool calprotectin or endoscopic evaluation.
Tissue injury and critical illness can trigger neutrophil activation without infection. Surgery, trauma, burns, pancreatitis, ischemia, and other forms of sterile inflammation may raise the result.
Cancer and tumor-associated inflammation may elevate S100A8/A9 through immune-cell recruitment and tissue signaling. Blood calprotectin is not a general cancer screening test.
Chronic cardiometabolic or pulmonary inflammation is being investigated. Obesity, smoking-related lung disease, and vascular inflammation may contribute to modest elevations, but cutoffs and clinical uses are not established.
A high result can also be affected by sample handling. If serum was left in contact with cells for too long, the measured concentration may overstate the person’s in-vivo level. Before launching an extensive investigation for a borderline result, it may be reasonable to confirm that collection and processing met the laboratory’s requirements.
Infection, Sepsis, and Acute Illness
One major area of research is whether blood calprotectin can help identify infection and sepsis earlier than traditional markers. Because neutrophils release it rapidly, calprotectin may increase near the beginning of innate immune activation. Prospective emergency-department studies have reported promising diagnostic accuracy for bacterial infection, sepsis, and severe outcomes.
Promising does not mean definitive. Infection can be localized, systemic, mild, or severe. Noninfectious inflammation can create a similar signal. A high level cannot establish sepsis, which requires suspected or confirmed infection plus life-threatening organ dysfunction from a dysregulated host response.
In emergency care, calprotectin may be compared with:
- CRP: A liver-produced marker that often rises over several hours and is widely available.
- Procalcitonin: Used selectively to support bacterial infection assessment and antibiotic decisions.
- White blood cell and neutrophil counts: Show cell numbers but not activation directly.
- Lactate: Helps assess impaired perfusion and severity but is not infection-specific.
- Cultures and molecular tests: Seek the causative organism but may take time or remain negative.
- Vital signs and organ function: Determine urgency and are essential for sepsis recognition.
A 2024 prospective study concluded that blood calprotectin could improve diagnosis and management of severe infections when combined with existing biomarkers. A 2025 critical-care study also assessed calprotectin against CRP for distinguishing sepsis from nonsepsis at intensive-care admission. These findings support further use and validation, not replacement of established sepsis pathways.
Anyone with confusion, severe shortness of breath, mottled or blue skin, very low urine output, fainting, rapidly worsening illness, or extreme weakness needs urgent medical care. Waiting for a calprotectin result would be unsafe. The procalcitonin test and calprotectin are laboratory aids, not substitutes for immediate clinical recognition.
Arthritis, IBD, and Chronic Inflammation
In rheumatoid arthritis, calprotectin has attracted interest because neutrophils and monocytes are active inside inflamed joints. Studies show associations between serum or plasma levels and clinical disease activity, ultrasound-detected synovitis, and response to treatment. Some evidence suggests it may reveal residual joint inflammation in patients receiving interleukin-6 inhibitors or JAK inhibitors, treatments that can suppress CRP production.
This potential advantage is biologically plausible. CRP is downstream of liver signaling, while calprotectin is released closer to the inflamed joint by myeloid cells. However, the marker is not yet part of standard rheumatoid arthritis classification or routine treatment targets. Differences between assays and study populations remain barriers.
In inflammatory bowel disease, circulating calprotectin may correlate with systemic activity, but stool testing more directly reflects neutrophil movement into the gut. Blood testing may be convenient when stool collection is difficult, yet it can be elevated by inflammation outside the bowel. A clinician may use symptoms, CRP, blood counts, albumin, fecal calprotectin, imaging, and endoscopy together.
In autoinflammatory disease, S100A8/A9 biology may be especially relevant. Very active innate immune conditions can produce high values, and related S100 proteins are studied as disease-activity markers. Still, there is no single blood-calprotectin threshold that diagnoses adult-onset Still disease, juvenile arthritis, or another condition.
Treatment trends may be useful. A substantial fall after effective therapy can support reduced neutrophil activation. A persistent elevation may suggest ongoing disease, infection, another inflammatory source, or a preanalytical issue. The result should not trigger medication escalation without clinical evidence, because immunosuppressive treatment can carry serious risks.
How to Interpret and Follow Up a Result
Start with the report rather than a generic online chart. Confirm whether the specimen was serum or plasma, note the unit, and use the laboratory’s reference interval. Ask why the test was ordered and what decision it was intended to support.
A practical review includes five steps:
- Confirm the technical context. Check specimen type, processing requirements, assay, reference limit, and whether the sample was collected during an acute event.
- Look for a matching clinical pattern. Fever, joint swelling, diarrhea, abdominal pain, cough, a wound, or recent surgery changes the meaning.
- Compare companion results. CRP, ESR, complete blood count, neutrophils, kidney and liver tests, cultures, and disease-specific markers may agree or reveal a different process.
- Review the trend. Use the same assay and specimen type whenever possible. A clear rise or fall is usually more meaningful than a small isolated difference.
- Investigate the cause, not the marker. Treatment should target infection, inflammatory disease, injury, or another source rather than calprotectin itself.
A normal blood calprotectin does not exclude localized infection, early disease, or conditions driven by other immune pathways. A high result does not prove bacterial infection or justify antibiotics. Likewise, it does not confirm an autoimmune diagnosis or cancer.
Preparation is generally minimal. Fasting is usually unnecessary unless other blood tests require it. Tell the healthcare team about recent infection, vaccination, strenuous exercise, surgery, medications, pregnancy, and known inflammatory conditions. Do not stop prescribed drugs before testing unless instructed.
Repeat testing may be appropriate when a borderline result conflicts with the clinical picture, when sample handling was uncertain, or when a specialist is monitoring response. The interval depends on the condition and the expected speed of change.
The most accurate summary is that circulating calprotectin is a direct window into myeloid-cell activation with considerable research promise. Its interpretation is more technically sensitive and less standardized than familiar markers. Used carefully, it can add information; used alone, it can mislead.
One further limitation is that “normal” may depend on treatment. Corticosteroids, biologic medicines, antibiotics, and other therapies can change neutrophil behavior or reduce the inflammatory source. A falling calprotectin level after treatment can be encouraging, but it does not prove complete disease control. Conversely, some medicines change circulating neutrophil counts by redistributing cells without creating the same degree of tissue inflammation. This is why the marker should be paired with examination and disease-specific outcomes. In arthritis, that may mean joint swelling and ultrasound. In infection, it may mean temperature, blood pressure, oxygen level, cultures, and organ function. In bowel disease, symptoms and stool markers may be more direct. The strongest use of blood calprotectin is therefore as one piece of a deliberately chosen monitoring plan rather than as a broad screening test in people without a clear clinical indication today.
References
– Blood Calprotectin as a Biomarker for Infection and Sepsis—the Prospective CASCADE Trial 2024 – Calprotectin as a Biomarker for Infectious Diseases 2025 – Calprotectin as a Sepsis Diagnostic Marker in Critical Care 2025 – Circulating Calprotectin in Rheumatoid Arthritis: Unravelling the Impact of Preanalytical and Analytical Factors on Its Assessment 2024 – Serum Calprotectin Correlates Stronger with Inflammation and Disease Activity in ACPA Positive Than in ACPA Negative Rheumatoid Arthritis 2025 – From Bench to Bedside: Calprotectin (S100A8/S100A9) as a Biomarker in Rheumatoid Arthritis 2022
Disclaimer
This article is for education and does not establish a diagnosis from a blood calprotectin result. Circulating calprotectin methods and reference intervals are not fully standardized, and serum and plasma values may differ substantially. A qualified clinician and the reporting laboratory should interpret the result with symptoms, sample details, and other tests.
