Operational definition of asthma by peak flow rate (PEF)
Dear Sir,
I am a medical doctor planning to do research on occupational asthma due to chrome dust. I am planning to evaluate asthma by history and peak expiratory flow rate (one time measurement). Could you please tell me an operational defintion for such screening of asthma.
Thank you
Dr khurram shahzad, MBBs
|
If you intend to screen a group of exposed workers for occupational asthma then you must do two things. First determine whether they have asthma and then whether it is made worse by work.
The venables questionnaire has been validated for the epidemiological screening of asthma (Thorax 1993;48:214-218). It is a sensitive instrument for screening but is not specific. The addition of the questions asking whether symptoms improve on days away from work and on holidays start to address the question of work relationship. A single measure of peak flow will probably add little as a normal peak flow on a single occasion cannot exclude asthma. The same applies to spirometry. I would suggest recording 2 hourly peak flows over 4 weeks both at work and at rest in order to allow comparison. These records are best interpreted by an expert or an expert system such as OASYS (please see resources on website). For more info on the diagnosis of Occupational asthma see the PDF document on this website.
|
This is not such an easy question as it first appears. Asthma can be estimated from questionnaires; most questionnaires identify more cases than can be confirmed physiologically. The first stage is to use a questionnaire validated in the language that you are using. Asthma can be defined as either a prior doctor diagnosis (have you ever been told by a doctor that you have asthma?), or from symptoms. The most general symptom probably relates to chest tightness. Questions on wheeze are understood differently by different English speakers, and translate badly into a number of other languages.
Having identified either an asthmatic symptom or a doctor asthma diagnosis there are several qualifications that can be added. The most important is to ask “is your chest tightness (or asthma) better on days away from work”, and “is it better on holidays”. Either of these two qualifications define a work-related symptom, which should be the minimum criteria for occupational asthma.
Work related symptoms can be further separated by asking about frequency. This is inappropriate when exposures are intermittent, but for workers exposed daily for instance to an isocyanate it would be more specific to count only symptoms that occurred at least weekly. Asthma can be further separated into asthma occurring for the first time after the current job started, and those with pre-existing asthma. Pre-existing asthma does not exclude a diagnosis of occupational asthma with a new sensitization, and in practice needs management in the same way as new onset occupational asthma. It can be very difficult in practice to be sure whether an adult had asthma as a small child or not.
Single measurements of lung function add little to the diagnosis. Measurements within the normal range are common in workers with occupational asthma, and reduced values can be due to other causes, such as cigarette smoking. The best method for physiological validation is with serial measurements of peak expiratory flow, as described on the website. Adequate records have a sensitivity of about 70% and a specificity of about 94%, much higher than other reported tests. If the putative cause is a biological agent, or one of the few small molecular weight chemicals which cause specific IgE production, skin prick testing or specific IgE measurements can be added.
Some advocate the measurement of non-specific reactivity. A >3.2 fold change between periods with and without exposure can be used to confirm a diagnosis of occupational asthma. In my experience there are about 30% of workers with occupational asthma who have measurements within the normal range when tested within 24 hours of exposure, and so a normal result cannot be used to exclude the diagnosis.
|
Please sign in or register to add your thoughts.