We would like to thank Cooper and colleagues for their letter regarding our recent article (1). In their letter, Cooper and colleagues raised a concern regarding our method of measuring plasma interleukin-6 (IL-6) and the sensitivity of the chosen assay. The authors are correct to point out that the ELISA kit from ENDOGEN suggests a standard curve between 10.24 and 400 pg/mL. We too were aware that plasma levels of IL-6 in humans tend to be low and, therefore, included a dilution to 5.12 pg/mL and excluded the highest 400 pg/mL concentration from each standard curve. With this approach our correlation coefficients for the standard curve regression equations ranged from 0.9954 to 0.9971 demonstrating excellent linearity and, therefore, providing confidence in extrapolating values between 0 and 5.12 pg/mL.

However, based on the concern of Cooper and colleagues and their assertion that exercise should increase circulating levels of IL-6 in children, we returned to our raw data and analyzed the absorbance units derived from the plate reader, before submitting to the standard curve equations. With this approach, our conclusions remained the same insofar as the absorbances after exercise (p = 1.0) and during recovery (p = 0.9) were not different than at rest for the boys, but were significantly higher after exercise (p = 0.001) and during recovery (p = 0.0009) than at rest in the men.

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