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Interleukin-13 (IL-13) has been difficult to quantify within human serum due to low abundance. Available assays have not been sensitive enough to detect IL-13 at the femtogram level. Thus, there are inconsistencies within the published literature as to IL-13 concentrations in normal or disease populations. To better understand IL-13 serum concentrations, a highly sensitive immunoassay was developed and used to determine concentrations from asthmatics with varying clinical severity.

METHODS: A single molecule counting (SMC) method was used to analyse serum samples from a total of 145 individuals (60 severe asthma, 60 moderate asthma, 60 mild asthma and 23 healthy donors).

RESULTS: IL-13 concentrations correlated with severity of asthma, with overlapping ranges. Mean IL-13 levels were highest in severe asthma. Mean IL-13 levels in moderate asthma population were second highest followed by mild asthma with the lowest IL-13 concentration. IL-13 concentrations in healthy donors were similar to the mild asthmatic population. The average concentrations of IL-13 in severe, moderate, mild and healthy donors were 1.286pg/mL, 0.672pg/mL, 0.508pg/mL and 0.155pg/mL respectively.

CONCLUSION: Severe asthma patients have elevated levels of IL-13.