Histamine dose-response curves in asthma: relevance of the distinction between PC20 and reactivity in characterising clinical state.

The aim of the present study was to determine if the measurement of the slope of teh histamine dose-response curve (bronchial reactivity) could provide useful information on the clinical state of asthma. Fifteen adult asthmatic subjects were studied twice at an interval of one year. Their clinical state was assessed by comparing their respiratory symptoms, need for medication, and FEV1 on both visits. Histamine inhalation challenges were carried out in a similar manner both times using a standardised procedure. PC20FEV1 (the histamine concentration causing a 20% fall of FEV1) and reactivity were obtained from the dose-response curves. AS others have shown, we found that PC20FEV1 reflects clinical state. Indeed, changes of PC20FEV1 greater than a single two-fold dilution of histamine were shown by five of six patients who were not in a steady clinical state, and by none of the nine patients who were. Changes of PC20FEV1 were significantly (p less than 0.005) more important in those who were not in a steady state in comparison with those who were. The reproducibility of reactivity was slightly better in those individuals who were in a steady clinical state as compared with those who were not. Nevertheless, changes in reactivity did not allow significant differentiation between the two groups of subjects. We conclude that PC20FEV1 is a more helpful index than reactivity in characterising the clinical state of asthmatics.