Recently reported declines in asthma morbidity¹ may be difficult to interpret as they could reflect not only changes in incidence, but also changing disease severity, patient expectations, healthcare provision, and efficacy of pharmacological management. Indeed, it has been suggested that general practitioners may choose differing diagnostic labels for respiratory disease to justify prescribing medication.²

In view of these apparent inconsistencies, we used 37 practices taking part in the Scottish Continuous Morbidity Recording project (CMR) to determine possible changes in diagnostic fashion. Changes in the yearly age specific incidence (per 1000 population) were ascertained for the recording of diagnoses and symptoms including asthma, wheeze, and other respiratory illnesses including acute bronchitis, bronchiolitis, lower respiratory tract infection (LRTI), croup, chest infection, and acute respiratory infections combined (Read codes (version 2) used listed in table 1⇓). The CMR project’s data collection processes have been described previously.³ Two child age groups were defined, namely those aged under 5 years of age (n = 12 693 children) and those aged 5–14 years (n = 30 165 children). Trends of disease incidence for six 12-month periods starting 31 March 1996 and ending 31 March 2002 were tested for linear association using the Mantel-Haenszel χ² test, giving p for trend using Epi Info version 6.0 (Centers for Disease Control and Prevention, Atlanta, Georgia, USA). The study protocol was approved by the Scientific Advisory Group of the Primary Care Clinical Informatics Unit–Research, which is the registered guardian for these anonymised data.

In the youngest age group, there was a declining trend in the incidence of asthma (p < 0.001), with the rate of wheeze incidence more than doubling over the study period. There was a small increase over the study period in the number of patients presenting with both asthma and wheeze (n = 16). Increases in the six year study period were observed for all other diseases considered as an alternative diagnosis for asthma (diagnostic transfer) (p < 0.001). Overall, there was an increase in incidence rates for those recorded as having any one study disease or symptom (p < 0.001). Similar trends for asthma and wheeze were found for children aged 5–14 years.

In the present study, physician diagnostic labelling has been shown to change with time. There was a clear reduction in the labelling of the incident cases of asthma and evidence was also found for an increase in other diseases and symptoms that could be used as alternative diagnostic labels for asthma. Although these changes may have been influenced by British asthma guidelines published in 1997,⁴ which reiterated the importance of a correct diagnosis, changes in computer coding procedures should not have occurred, as a standard Read code dictionary was used by trained CMR practice data operators throughout the study period. These trends may have implications for large scale population surveys or studies that utilise data collected from routine clinical activity, leading to the accidental reporting of artefact.