Pre-analytical and biological variability in circulating interleukin 6 in healthy subjects and patients with rheumatoid arthritis

Abstract

Interleukin (IL)-6, a key player in the inflammatory response, may be a useful biomarker in rheumatoid arthritis (RA). The aim was to determine analytical variability, a reference interval in healthy subjects, and long- and short-term variation in serum and plasma IL-6 in healthy subjects and RA patients. An enzyme-linked immunosorbent assay from R&D was used for determination of serum and plasma IL-6. The IL-6 concentration did not depend on the type of anticoagulant used or the 3-h time delay between sampling and processing or repeated freeze-thaw cycles. The median plasma and serum IL-6 in 318 healthy subjects were 1.3 pg ml(-1) (range 0.33-26) and 1.4 pg ml(-1) (range 0.25-23), respectively. The median coefficient of variation in plasma IL-6 in 27 healthy subjects during 1 month, and repeated after 6 and 12 months were 27%, 31% and 26%, respectively. No significant long-term changes were observed in serum IL-6 over a 3-year period (14%, p = 0.33). Exercise (cycling) increased serum IL-6 in healthy subjects but not in RA patients. In conclusion, circulating IL-6 is stable regarding sample handling and shows little variation over time. Changes in IL-6 concentrations > 60% (2 times the biological variation) are likely to reflect changes in disease activity and not only pre-analytical or normal biological variability.

Evaluation of Serum Interleukin‐6 Level as a Surrogate Marker of Synovial Inflammation and as a Factor of Structural Progression in Early Rheumatoid Arthritis: Results From a French National Multicenter Cohort

“…For instance, variations in IL-6 concentration have been reported according to subjects' age (38) and food intake (39). Physical exercise also increases serum IL-6 in healthy subjects but not in RA patients (40). Stress and emotional problems have been suggested to influence IL-6 levels (41).…”

Molecular Effects of Exercise in Rheumatoid Arthritis

“…This suggests that the anti-inflammatory effect of exercise can be attributed, at least in part, to the induction of IL-6 and the creation of an anti-inflammatory environment (Table 2). Of note, no changes in serum IL-6 were detected in RA patients after exercise, but this might be attributable to a less strenuous exercise regimen when compared to healthy individuals (Knudsen et al 2008). It is also possible that the anti-inflammatory effect of exercise is blunted in patients with RA, but this requires further study.…”

Non-invasive, ultra-sensitive, high-throughput assays to quantify rare biomarkers in the blood

“…All proteins have been detected with a sensitivity about 10 3 to 10 4 -fold higher than by ELISA [45], confirming that FACTTbased assays have broad applications. Blood tests are currently being used to detect biomarkers for cardiovascular disease [46][47][48], rheumatoid arthritis [49][50][51], Alzheimer's disease [52,53], and hepatitis [54,55]. Detecting and quantitatively monitoring the serum concentrations of cytokine inhibitors or soluble cytokine receptors, as well as the clinical response of patients to treatment with cytokine antagonists, might generate important information for monitoring autoinflammatory diseases [56].…”