Until 1985, admissions were increasing in all ages but from 1988, the trends were all downward. The gap in 1986 and 1987 corresponds to the change from the HAA to the HES system (see factsheet 96/2), but there was no change in coding practice.

Changes in coding and information systems are unlikely to explain these trends. Hospital admissions can vary due to changes in medical practice or patient behaviour. Other studies have shown that the rising trend in admissions was not due to an increase in re-admission rates or diagnostic transfer from related conditions. Admission rates could also reflect changes in underlying morbidity. This can relate to the prevalence of asthma itself or the prevalence or severity of asthma exacerbations. There is evidence from general practice of fewer asthma attacks (factsheet 2000/1) and also evidence that suggests patients are being prescribed more medication, both of which would correspond to this explanation (factsheet 99/1). The underlying prevalence of asthma may have also increased, but little is known about trends in UK prevalence through the 1990s.