Smoker, ex-smoker or non-smoker? The validity of routinely recorded smoking status in UK primary care: a cross-sectional study

Louise Marston reports here on a recent paper published with BMJ Open.

When is a non-smoker an ex-smoker?  You may wonder this, and as we found out, the answer is often in primary care records.  These come from the computerised records that general practitioners make during consultations with their patients.  These include symptom, diagnoses and drugs prescribed.  They also include other information which may be relevant to healthcare such as weight, blood pressure and smoking status.

Following a previous paper, where we discovered that of the smoking status information present in the year following registration, there was a greater percentage of patients recorded as smokers than in UK population surveys we sought to look at smoking status in greater detail.

In our new paper we looked deeper into smoking status recording in the first year of registration in those who registered during 2008 and 2009 with a general practice in England that were part of The Health Improvement Network (THIN).  We found that smoking status recording was good, with 84% of those aged 16 or over had a smoking status record within a year of registering.  However, compared with the 2008 Health Survey for England (HSE), the percentage of ex-smokers was substantially lower in THIN (26% versus 14% respectively) and there was a lower percentage of current smokers in the HSE compared with THIN (21% versus 24% respectively).

Firstly we imputed assumed missing smoking status data could be any of current smoker, ex-smoker or non-smoker.  After age standardising to account for the differing age structures between datasets, there were still notable differences between THIN and the HSE, for ex-smoking.  Further analysis assuming data on all smokers in THIN had been collected and missing data were ex or non-smokers resulted in a lower percentage of ex-smokers in THIN compared with the HSE (23% versus 26% respectively).  Using time since quitting in the HSE, we estimated that those who quit smoking before the age of 30 are unlikely to be recorded as ex-smokers in general practice records and instead recorded as non-smokers.

Differences in smoking status between datasets may be due to the way smoking status is ascertained; in THIN it is self-report without strict protocol.  Some people who quit smoking a long time ago or only smoked for a short period may not consider themselves to be ex-smokers, and describe themselves as non-smokers when asked by their GP or other practice staff. In the HSE, smoking status is collected using a strict algorithm, categorising those who smoked at any time even for a short period as ex-smokers.  The HSE data are collected in the same way from all participants.

Smoking status matters for epidemiological research as ex-smokers carry a higher risk of many diseases than never smokers, especially soon after quitting.  This has been shown in the 50 year follow up of British doctors (Doll et al, 2004).  However, it is reassuring to know that soon after registration, most current smokers will have been recorded as such in general practice and misclassification is most likely in those who quit smoking a long time ago.

This study was funded by a UK Medical Research Council grant [G0900701].

 

 

Marston L, Carpenter JR, Walters KR, Morris RW, Nazareth I, White IR, Petersen I Smoker, ex-smoker or non-smoker? The validity of routinely recorded smoking status in UK primary care: a cross-sectional study. BMJ Open 2014;4:e004958. doi:10.1136/bmjopen-2014-004958

 

Doll R, Peto R, Boreham J, et al Mortality in relation to smoking: 50 years’ observations on male British doctors. BMJ 2004;328:1519.