What this test measures
Interleukin-6 (IL-6) is a cytokine produced by macrophages, T cells, and many other cells in response to infection, tissue injury, and inflammatory stimuli. It is the main driver of the hepatic acute-phase response — it stimulates the liver to produce CRP, fibrinogen, haptoglobin, and other acute-phase proteins. The serum IL-6 test quantifies free IL-6, reported in pg/mL.
IL-6 rises much earlier than CRP — within 1–4 hours of an inflammatory trigger, versus 6–12 hours for CRP. It is therefore a more sensitive early marker of inflammation and sepsis, although the assay is slower and more expensive than CRP.
Why it matters
IL-6 testing has become more widely used during and after the COVID-19 pandemic. Very high IL-6 (> 80–100 pg/mL) in severe COVID-19 predicts respiratory failure and was used to identify patients who benefited from tocilizumab — an anti-IL-6 receptor monoclonal antibody. The same approach applies to cytokine release syndrome after CAR-T cell therapy and bispecific antibodies in haematology, where IL-6 monitoring guides tocilizumab dosing.
Beyond infection, IL-6 is a key driver of rheumatoid arthritis, juvenile idiopathic arthritis, Castleman disease, and giant cell arteritis — diseases successfully treated with anti-IL-6 therapy (tocilizumab, sarilumab). IL-6 is also used as a research and clinical marker of disease activity in these conditions. In neonatal and adult sepsis, IL-6 supports early diagnosis and risk stratification when used alongside procalcitonin and CRP.
How to prepare
No fasting required. Sample stability matters — IL-6 should be separated from cells within 4 hours of collection and stored frozen if not measured immediately. Disclose any biological therapy (especially tocilizumab — it raises measured IL-6 paradoxically by blocking the receptor and preventing clearance) and any active infection.
Markers & reference ranges
Reference ranges below are typical adult values. Your lab's reported range may differ slightly based on the assay platform and patient demographics — always read your report against the range printed on it.
| Marker | Normal range | If low | If high |
|---|---|---|---|
| Interleukin-6 (pg/mL)[1][2][3] | < 7 pg/mL (assay-specific) | < 7 pg/mL: no significant systemic inflammation at the time of sampling. A normal IL-6 with high clinical suspicion of sepsis or severe inflammation should be repeated — IL-6 can fluctuate rapidly. | 7 – 50 pg/mL: mild to moderate inflammation — infection, autoimmune flare, post-operative state. 50 – 200 pg/mL: significant inflammation — sepsis, severe COVID-19, active rheumatoid arthritis, cytokine release syndrome. > 200 pg/mL: severe inflammation / cytokine storm — severe sepsis with shock, severe COVID-19 progressing to ARDS, severe cytokine release syndrome — consider IL-6 receptor blockade in appropriate settings. |
IL-6 ranges and clinical context
| IL-6 (pg/mL) | Category | Clinical relevance |
|---|---|---|
| < 7 | Normal | No active systemic inflammation |
| 7 – 50 | Mild–moderate | Localised infection, post-op, mild flare |
| 50 – 200 | Significant | Sepsis, severe COVID-19, active RA, CRS grade 1–2 |
| > 200 | Cytokine storm | Severe sepsis, ARDS, severe CRS — consider IL-6R blockade |
Frequently asked questions
How is IL-6 different from CRP?
IL-6 is the cytokine that drives the liver to produce CRP. IL-6 rises within 1–4 hours of inflammation; CRP follows in 6–12 hours. IL-6 is more sensitive for very early inflammation but more expensive and slower to report. In day-to-day practice CRP is usually sufficient; IL-6 is reserved for specific situations like severe COVID-19, cytokine release syndrome, and research.
When is IL-6 important in COVID-19?
In severe COVID-19, very high IL-6 predicts respiratory failure and identifies patients who may benefit from tocilizumab (an IL-6 receptor blocker). Most patients with mild COVID-19 do not need IL-6 testing.
Why does tocilizumab raise measured IL-6?
Tocilizumab blocks the IL-6 receptor, so IL-6 cannot bind cells and stays in circulation longer. Measured serum IL-6 therefore rises paradoxically after tocilizumab — this is expected and does not mean worsening inflammation.
What is cytokine release syndrome (CRS)?
A potentially life-threatening systemic inflammatory response that occurs after certain immune therapies (CAR-T, bispecific antibodies) and severe infections. IL-6 is a central driver. Severe CRS often requires tocilizumab, corticosteroids, and ICU care.
Is IL-6 used to diagnose rheumatoid arthritis?
Not as a primary diagnostic test — anti-CCP, RF, ESR, CRP, and clinical criteria are used for diagnosis. IL-6 is sometimes used to monitor disease activity and may guide anti-IL-6 biologic therapy choice in refractory cases.
How is IL-6 used in sepsis?
IL-6 is a sensitive early marker of sepsis, rising hours before CRP. It is used alongside procalcitonin and CRP in some sepsis protocols, especially in neonatal sepsis where rapid diagnosis is critical.
Why is IL-6 not a routine test?
Routine inflammation can be assessed with CRP — cheaper and faster. IL-6 is reserved for specific clinical questions: severe COVID-19 monitoring, cytokine release syndrome, suspected very early sepsis, anti-IL-6 biologic candidate selection, and certain research applications.
Related Immunology tests
Tests commonly ordered alongside INTERLEUKIN-6, or that help interpret an unexpected result.
Sources & references
- NCBI StatPearls — Interleukin 6 · accessed 2026-05-30T00:00:00.000Z
- Mayo Clinic Labs — Interleukin-6 · accessed 2026-05-30T00:00:00.000Z
- WHO — Therapeutics and COVID-19 Living Guideline · accessed 2026-05-30T00:00:00.000Z
- NIH PMC — IL-6 in sepsis and cytokine storm · accessed 2026-05-30T00:00:00.000Z
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