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Blood Test Tracker

Interleukin-6 (IL-6)

A signaling protein your immune system releases early in infection and inflammation. It is ordered in specific situations, not at routine check-ups.

AI-verified against cited sources. Awaiting expert reviewAlso called IL-6, IL6LOINC 26881-3

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What it is

Interleukin-6 (IL-6) is a messenger protein released by immune cells, fat tissue and working muscles. It helps start the body's inflammation response and signals the liver to make C-reactive protein (CRP). In healthy people at rest, blood levels are very low and are often near the lowest amount a test can detect. IL-6 rises within hours of an infection or injury and falls quickly afterward.

Why it matters

Clinicians order IL-6 in specific situations. Mayo Clinic Laboratories lists checking for chronic inflammatory conditions (such as rheumatoid arthritis, lupus, ankylosing spondylitis or inflammatory bowel disease), Castleman disease, a suspected infection in the body or in an artificial joint, and judging the risk of needing a ventilator in severe COVID-19. In one large US study of adults without known heart disease (MESA), people with higher IL-6 had more deaths, including heart-related deaths, over the following years. That does not mean one result can predict what will happen to any one person.

If it’s high

A level above your lab's range means the inflammation signal is switched on. That can be a short-term response to infection, an injury or hard exercise, or part of a long-term inflammatory condition. IL-6 does not show the cause. Levels above 1,000 pg/mL have been reported in people who are seriously ill, which is why hospitals sometimes use it.

Common causes

  • Hard exercise shortly before the blood draw
  • Higher body weight, since body fat tissue is a main source of IL-6 at rest
  • Infections, from mild to severe
  • Recent surgery or injury
  • A flare of a long-term inflammatory condition, such as rheumatoid arthritis, lupus or inflammatory bowel disease
  • Treatment with an IL-6 blocking medicine such as tocilizumab, which raises the measured level even when a person is improving
  • Castleman disease (rare)

What can help

  • Tell the lab and your clinician if you take an IL-6 blocking medicine
  • If you are tracking results, try to draw at a similar time of day and avoid hard exercise right before
  • Ask which other tests, such as CRP, a blood count or cultures, your clinician is looking at

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 0–6.4 pg/mL. Source: Mayo Clinic Laboratories (catalog page mirrored on testcatalog.org), Annals of Hematology, Frontiers in Medicine.

What can affect your result

  • Time of day: levels tend to be lowest in the morning
  • Exercise, especially shortly before the draw
  • Body fat, since fat tissue releases IL-6 at rest
  • Acute illness, infection, surgery or injury
  • IL-6 blocking medicines such as tocilizumab
  • The assay and lab: ranges differ widely and are not interchangeable
  • Hard exercise shortly before the draw can raise IL-6.
  • IL-6 blocking medicines such as tocilizumab can make the measured IL-6 rise sharply, which can mislead if read as a sign of worsening inflammation.
  • Mayo Clinic Laboratories rejects serum that is grossly hemolyzed, lipemic or icteric.
  • Different assays and labs report different values for the same sample, so compare results only within the same lab.

Levels vary through the day, with a low point in the morning. Use a similar time of day when comparing results. Changes within hours, and faster than CRP, in both directions.

When to talk to a clinician

IL-6 is usually ordered by specialists or in hospital. Talk with the clinician who ordered it about any result above your lab's range. Call 911 or your local emergency number for fever with confusion, trouble breathing, a racing heartbeat or fainting, rather than waiting for a result.

Questions you could ask

  • Why was IL-6 chosen for me rather than CRP?
  • Is this lab's range the right one to compare my result against?
  • What do you think is behind this result, and what else are you checking?
  • Could hard exercise, a recent illness or a medicine I take explain it?
  • Should we repeat it, and at what time of day?

Questions people ask

Why do labs give such different normal values for IL-6?

Different assays measure IL-6 in slightly different ways, and most healthy people are close to the lowest amount a test can detect. One reference lab reports under 6.4 pg/mL, and others use other limits. Use the range printed on your own report, and compare results only within the same lab.

Is IL-6 the same as CRP?

No. IL-6 is a signal released early, and it tells the liver to make CRP. CRP rises later, lasts longer and is much more widely used. Clinicians often look at both.

Does a normal IL-6 mean there is no inflammation?

Not necessarily. IL-6 rises and falls quickly, so a normal result does not rule out inflammation that has already settled or has not yet started. Your clinician reads it with your symptoms and other tests.

Show the science and sources

Units

  • 1 pg/mL = 1 pg/mL
  • 1 pg/mL = 1 ng/L

For clinicians and researchers

IL-6 is a pleiotropic cytokine, and its main effect on the liver is to induce acute-phase proteins, including CRP, so CRP is widely used as a surrogate for IL-6 activity. Production comes from immune cells and, at rest, mainly from adipose tissue, with skeletal muscle adding more during exercise. A meta-analysis of 43 studies (Nilsonne 2016) found diurnal variation with a trough in the morning. IL-6 reacts earlier than CRP and procalcitonin and falls quickly, which gives it fast kinetics but lower specificity. In sepsis, levels above about 1,000 pg/mL have been linked to severe systemic inflammation and poorer outcomes, but they lack specificity and peaks can change quickly without a change in clinical status (Meisner 2014). In MESA (n = 6,622, no baseline cardiovascular disease), the top IL-6 tercile had a higher adjusted risk of all-cause mortality (HR 1.98) than the lowest. Blockade of the IL-6 pathway with tocilizumab raises measured serum IL-6 sharply (in one small Castleman disease series, from a mean of about 24 to about 749 pg/mL), which complicates interpretation on that treatment; CRP is the usual surrogate for IL-6 activity. Assays are immunoassays whose calibrators and upper limits differ, and most healthy adults sit at or near the detection limit; published upper limits range from about 2 pg/mL (a 2026 healthy-adult study using flow cytometry) to 6.4 pg/mL (Mayo) and 8.0 pg/mL (a Japanese hospital lab). Mayo Clinic Laboratories (test IL6DX) reports serum IL-6 as under 6.4 pg/mL; serum stability is 14 days refrigerated, 90 days frozen and 24 hours at room temperature.

LOINC 26881-3 (primary). Specimen: serum-or-plasma.

Sources

  1. Interleukin-6, Serum (Mayo test ID IL6DX), Mayo Clinic Laboratories (catalog page mirrored on testcatalog.org)
  2. Diurnal Variation of Circulating Interleukin-6 in Humans: A Meta-Analysis, PLOS ONE (2016)
  3. Update on Procalcitonin Measurements, Annals of Laboratory Medicine (2014)
  4. Interleukin-6 levels decrease following rituximab treatment but increase following tocilizumab treatment, Annals of Hematology (2026)
  5. A system model of the effects of exercise on plasma Interleukin-6 dynamics in healthy individuals: Role of skeletal muscle and adipose tissue, PLOS ONE (2017)
  6. Interleukin-6 and Cardiovascular Events in Healthy Adults: MESA, JACC: Advances (2024)
  7. Establishment of reference intervals for plasma IL-6, IL-8, IL-10, and IL-1β in healthy adults from Lianyungang, Jiangsu, China: a single-center flow cytometry analysis, Frontiers in Medicine (2026)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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