EDITOR, - The study by Karl G Nicholson and colleagues showing that upper respiratory tract viral infections were associated with 44% of exacerbations of asthma1 gives a surprisingly low result in view of recent evidence. Beasley et al showed viruses to be associated with 36% of severe exacerbations using cell culture methods and serology, which as Nicholson and colleagues point out, are five times less sensitive than the polymerase chain reaction they used. Johnston et al showed upper respiratory tract viruses to be associated with 78% of asthma exacerbations in children3 and also a time course correlation between viral isolation rates in children and hospital admission rates for asthma in all ages, thus suggesting viruses as the major precipitating factor for attacks in adults.4

The low isolation rate for viruses in asthma in the study by Nicholson and colleagues can perhaps be explained by their wide definition of an asthma exacerbation. Symptoms are notoriously unreliable as indicators of asthma severity.5 Variations in peak flow are the most reliable indicators of …