Test Catalog

Test ID: IL6    
Interleukin 6, Plasma

Useful For Suggests clinical disorders or settings where the test may be helpful

Evaluation of patients with suspected systemic inflammation associated with infections, autoimmunity, and cancers

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Interleukin-6 (IL-6) is an important pro-inflammatory cytokine, which plays critical roles in both innate and adaptive immunity.(1,2) IL-6 is produced by a variety of different cell types, including macrophages, endothelial cells, and T cells. Its production can be initiated in response to microbial invasion or other cytokines, such as tumor necrosis factor alpha (TNF-alpha) and IL-1 beta. As part of the innate immune system, IL-6 acts on hepatocytes to induce expression of C-reactive protein, fibrinogen, and serum amyloid A, also known as the acute phase reactants. IL-6 also plays a key role in activating antibody-producing B cells to proliferate, leading to an enhanced antibody response.(1)


Under normal conditions, IL-6 plays an important role in cellular homeostasis. However, during inflammatory conditions, the concentration of IL-6 increases in several folds, highlighting its clinical relevance as a major alarm signal in humans in response to infections (sepsis/septicemia), inflammation, autoimmunity, and cancer.(2-4) Increased concentrations of IL-6 have been reported in localized (prosthetic joint infections [PJI], periodontitis), and systemic (eg, sepsis, coronavirus 2019 [COVID-19]) infections, autoimmune conditions (eg, rheumatoid arthritis (RA), systemic lupus erythematosus, ankylosing spondylitis, and inflammatory bowel disease), and cancers.(4-8) Thus, IL-6 serum levels have evaluated, and reported to predict severity and response to IL-6 targeted therapies in PJI,(4) sepsis,(5) COVID-19,(6) and RA.(7-8) Overall, IL-6 cytokine is nonspecific biomarker of systemic inflammation that may have relevance in clinical decision making in the appropriate context.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

< or =1.8 pg/mL

Interpretation Provides information to assist in interpretation of the test results

Elevated concentrations of interleukin-6 (IL-6) may indicate an ongoing inflammatory response.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

Interleukin-6 (IL-6) is a nonspecific marker associated with an inflammatory response and is not diagnostic for any specific disease or disease process. Elevated concentrations of IL-6 must be interpreted within the clinical context of the patient.


Normal concentrations of IL-6 do not exclude the possibility of an ongoing inflammatory process.


IL-6 has limited stability. Following centrifugation, plasma must be either immediately frozen or refrigerated. Specimens can be stored at refrigerated temperatures for only 24 hours, after which time they must be frozen. Storage of plasma for any length of time at ambient temperature is not acceptable.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1.Hilligan KL, Ronchese F: Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses. Cell Mol Immunol. 2020;17:587-99

2.Rose-John S: Interleukin-6 signalling in health and disease. F1000Res. 2020 Aug 20;9:F1000 Faculty Rev-1013.

3.Jones SA, Jenkins BJ: Recent insights into targeting the IL-6 cytokine family in inflammatory diseases and cancer. Nat Rev Immunol. 2018;18:773-89

4.Rose-John S, Winthrop K, Calabrese L: The role of IL-6 in host defence against infections: immunobiology and clinical implications. Nat Rev Rheumatol. 2017 Jul;13(7):399-409

5.Gallo J, Svoboda M, Zapletalova J, et al: Serum IL-6 in combination with synovial IL-6/CRP shows excellent diagnostic power to detect hip and knee prosthetic joint infection. PLoS One. 2018;13:e0199226

6.Song J, Park DW, Moon S, et al: Diagnostic and prognostic value of interleukin-6, pentraxin 3, and procalcitonin levels among sepsis and septic shock patients: a prospective controlled study according to the Sepsis-3 definitions. BMC Infect Dis. 2019;19:968

7.Galvan-Roman JM, Rodriguez-Garcia SC, Roy-Vallejo, E, et al: IL-6 serum levels predict severity and response to tocilizumab in COVID-19: An observational study. J Allergy Clin Immunol. 2021;147:72-80

8.Udomsinprasert W, Jittikoon J, Sangroongruangsri S, et al: Circulating levels of interleukin-6 and interleukin-10, but not tumor necrosis factor-alpha, as potential biomarkers of severity and mortality for COVID-19: Systematic review with meta-analysis. J Clin Immunol. 2021;41:11-22

9.Boyapati A, Schwartzman S, Msihid J, et al: Association of high serum interleukin-6 levels with severe progression of rheumatoid arthritis and increased treatment response differentiating sarilumab from adalimumab or methotrexate in a post hoc analysis. Arthritis Rheumatol. 2020;72:1456-66