Radioimmunoassay of interleukin-6 in plasma

We present a double-antibody radioimmunoassay for determining human interleukin-6 (IL-6) in biological fluids. The detection limit of the assay is 20 ng/L (B0 - 2 SD). Bound radioactivity in the range of 30% to 90% of the B0 counts corresponds to IL-6 concentrations of 100 to 14,000 ng/L. Analytical recovery of IL-6 added to EDTA-treated plasma averaged 25% more than that added to serum. The plasma concentration of IL-6 was therefore approximately 85 ng/L more than the concentration in simultaneously drawn serum. The mean serum concentration of IL-6 in 45 healthy subjects was 83 ng/L (range 20-290 ng/L), in 20 patients with multiple myeloma 303 ng/L, in 20 patients with rheumatoid arthritis 234 ng/L, and in 13 patients with systemic lupus erythematosus 183 ng/L. Markedly increased (greater than 3000 ng/L) concentrations of IL-6 were found in sera of patients with meningococcus meningitis and infectious peritonitis.