Expression of results of histamine challenge

Different aspects of standardization of histamine challenges have been extensively investigated. However, several methods have been used for the actual derivation of the provocation concentration causing a 20% fall in lung function (PC20) from the results of the serial lung function tests (e.g. forced expiratory volume in 1 second [FEV1] ). We have compared these methods on a series of 21 histamine challenges to see if any were systematically inconsistent with the others. The simplest method, with least potential for further error, is interpolation of two points, on either side of the line for 20% fall in lung function, calculated by computer. We conclude that, as this method was consistent with the majority of others, it is the method of choice for deriving the PC20.