Interleukin-6

Overview

Interleukin‑6 (IL‑6) is a proinflammatory cytokine produced by immune cells (e.g., macrophages, dendritic cells), endothelial cells, adipocytes, and various tissues in response to infection, tissue injury, or inflammation. It stimulates the liver to produce acute‑phase proteins such as C‑reactive protein and fibrinogen, and also promotes T‑ and B‑cell activation, hematopoiesis, and bone metabolism. Elevated serum IL‑6 levels are associated with systemic inflammation, autoimmune diseases, sepsis, some malignancies, and chronic inflammatory states, whereas very low levels are not typically clinically significant. Because of its central role in the inflammatory cascade, IL‑6 is used as a research biomarker of inflammation and immune activation and is increasingly targeted clinically with IL‑6‑blocking therapies (e.g., tocilizumab, siltuximab).

Clinical Use Cases

  • Evaluating systemic inflammatory conditions (e.g., rheumatoid arthritis, systemic juvenile idiopathic arthritis, Castleman disease).
  • Research monitoring of cytokine‑driven inflammation in sepsis, COVID‑19, and critical illness.
  • Assessing disease activity and response to anti‑IL‑6 therapy in autoimmune and hematologic conditions.
  • Investigating the role of chronic low‑grade inflammation in obesity, cardiovascular disease, and metabolic syndrome.

Specimen Types

  • Serum (most commonly used for IL‑6 measurement).
  • Plasma (EDTA or heparin; results can vary slightly by anticoagulant).

Measurement Methods

  • Immunoassays (enzyme‑linked immunosorbent assay, ELISA) for individual or multiplex cytokine panels.
  • Chemiluminescent or electrochemiluminescent immunoassays on automated platforms.
  • Multiplex bead‑based flow cytometry (e.g., Luminex) for research panels including IL‑6 with other cytokines.

Test Preparation and Influencing Factors

  • Fasting is usually not required, but consistency (e.g., morning draw) is recommended for serial monitoring.
  • Acute infection, trauma, surgery, and active inflammatory flares markedly increase IL‑6.
  • Anti‑inflammatory drugs (e.g., corticosteroids, NSAIDs, targeted biologic therapies such as IL‑6 inhibitors) can suppress levels.
  • Intraindividual diurnal variation and assay variability mean that a single value should be interpreted in clinical context and often repeated for trends.

Synonyms

  • IL‑6.
  • Interleukin‑6, serum.
  • Interleukin‑6, plasma (depending on specimen).

Further Reading