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British Guideline on the MAnAGeMent of AsthMA
7.9.4 SENSISTIvITy AND SPECIFICITy OF SERIAL PEAK FLOW MEASUREMENTS
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Direct and blinded comparisons of serial peak flow measurement with either specific bronchial
provocation testing, or an expert diagnosis based on a combination of other types of evidence,
reported consistently high sensitivities and specificities, averaging 80% and 90% respectively. 575-
578,580,588,589
Just one computed method of analysis has been reported, with a sensitivity of 75% and a
specificity of 94%. 97,590
Computed analysis of peak flow records has good diagnostic performance, but statistical analysis
of serial peak flow measurements appears to be of limited diagnostic value compared to expert
interpretation. 578,588,589
Serial measurements of peak expiratory flow
Measurements should be made every two hours from waking to sleeping for four weeks, keeping
treatment constant and documenting times at work.
Minimum standards for diagnostic sensitivity >70% and specificity >85% are:
At least three days in each consecutive work period
At least three series of consecutive days at work with three periods away from work (usually
about three weeks)
At least four evenly spaced readings per day. 580
The analysis is best done with the aid of a criterion-based expert system. Suitable record forms
and support are available from www.occupationalasthma.com
d Objective diagnosis of occupational asthma should be made using serial peak flow
measurements, with at least four readings per day.
7.9.5 NON-SPECIFIC REACTIvITy
Studies of non-specific reactivity are confounded by different methods used, different cutoffs
for normality and the interval between last occupational exposure and the performance
of the test (increasing time may allow recovery of initial hyper-reactors). Such studies show
that non-specific bronchial hyper-reactivity may be normal in 5-40% of specific challenge
positive workers. Testing with higher concentrations of methacholine or histamine, at which
some people without asthma would react, reduces the number of non-reacting people with
occupational asthma, but still leaves some non-reactors. One study showed no additional benefit
of non-specific bronchial reactivity measurement over and above a history and specific IgE to
inhaled antigens. A normal test of non-specific reactivity is not sufficiently specific to exclude
occupational asthma in clinical practice. 576,581,583,586,587,589,591-602
Changes in non-specific reactivity at and away from work alone have been found to have only
moderate sensitivity and specificity for diagnosis. Three studies were identified where at and
away from work exposure measurements were attempted. One did not investigate workers
further when at work reactivity was normal, limiting its interpretation. Using a 3.2 fold change
in reactivity, one study found a sensitivity of 48% and a specificity of 64%. Reducing the
required change to twofold increased the sensitivity to 67%, reducing specificity to 54%. A
smaller study with 14 workers with occupational asthma showed a sensitivity of 62% and
specificity of 78%. 577,589,601
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