Bethesda (MD): Eli Lilly & Company and the National Center for Advancing Translational Sciences 2004–. In: Markossian S, Grossman A, Brimacombe K, Arkin M, Auld D, Austin C, Baell J, Chung TDY, Coussens NP, Dahlin JL, Devanarayan V, Foley TL, Glicksman M, Gorshkov K, Haas JV, Hall MD, Hoare S, Inglese J, Iversen PW, Kales SC, Lal-Nag M, Li Z, McGee J, McManus O, Riss T, Saradjian P, Sittampalam GS, Tarselli M, Trask OJ Jr, Wang Y, Weidner JR, Wildey MJ, Wilson K, Xia M, Xu X, editors. Haas JV, Eastwood BJ, Iversen PW, Devanarayan V, Weidner JR. A wide range of typical assay conditions are considered in the development of the guidelines. The absolute EC50/IC50 should only be used if there are at least two assay concentrations whose predicted response is less than 50% and two whose predicted response is greater than 50%. The relative EC50/IC50 should only be used if there are at least two assay concentrations beyond the lower and upper bend points. Next, the guidelines provide rules for deciding when the EC50/IC50 estimates are reportable. Assays that can be demonstrated to produce an accurate and stable 100% control and less than 5% error in the estimate of the 50% control mean may gain efficiency as well as accuracy by using the absolute EC50/IC50. Assays having a stable 100% control but for which there may be more than 5% error in the estimate of the 50% control mean should use the relative EC50/IC50. Assays for which there is no stable 100% control must use the relative EC50/IC50. The guidelines first describe how to decide whether to use the relative EC50/IC50 or the absolute EC50/IC50. The absolute EC50/IC50 is the response corresponding to the 50% control (the mean of the 0% and 100% assay controls). The relative EC50/IC50 is the parameter c in the 4-parameter logistic model and is the concentration corresponding to a response midway between the estimates of the lower and upper plateaus. Two definitions of EC50/IC50s are considered: relative and absolute. This article provides minimum requirements for having confidence in the accuracy of EC50/IC50 estimates.
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