Medical Surveillance for
Occupational asthma (OA)
Susan M Tarlo MB BS FRCP(C)
Toronto Western Hospital,
and Gage Occupational and
Environmental Health Unit
Prevention
Primary – avoidance
Secondary – early detection
(best medical outcome for sensitizer-induced
OA is with early diagnosis and removal from
exposure when asthma is relatively mild)
Medical Surveillance - early identification by
worker/ physician)
Tertiary – optimal treatment
No clinical studies have looked at
medical surveillance for OA as an
isolated intervention
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All include exposure controls, education
Usually a resp questionnaire, spirometry and
when feasible skin tests/specific serum IgE
The combination of interventions has shown
benefit
Examples of effective interventions that
include medical surveillance for OA
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Natural rubber latex (NRL): -also included primary
prevention with changes in gloves to low-protein lowpowder or non-latex gloves, and also included worker
education
Incident reports for NRL allergy in the largest Ontario
hospital fell from a peak of 45 per year (with
surveillance) in ’94 to 1 in ’99 (after glove changes).
The onset of symptoms had been relatively steady
between ’89 and ’94 (4-9 cases per year) but were not
reported until surveillance began in ‘94
Tarlo et al, JACI 2001
Ontario MOL Surveillance requirements for
diisocyanate-induced OA (from 1983)
1. Workplace air monitoring for TWA
0.005ppm diisocyanates, TLV 0.02 ppm.
2. Medical surveillance
 Pre-employment
respiratory questionnaire,
physical exam and spirometry.
 6 monthly questionnaires, at least 2- yearly
spirometry.
 Physician referral: symptoms or 15% fall in
FEV1 or FVC.
Diisocyanate OA and medical
surveillance in Ontario
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Workers’ compensation (WSIB) claims for
diisocyanate-induced OA ’80-’87 comprised
~50% of all accepted OA claims, rose from
30/year to 58/year by 1988 (likely effective case
finding), then progressively fell to 21 (vs 39
other OA) in 1993 and 10 in 2002
An earlier diagnosis of diisocyanate-induced OA
occurred after the surveillance program was
introduced
Tarlo et al OEM ’97, OEM ’02, Buyantseva et al PATS (abst) ’08
If medical surveillance is medically
effective, how cost-effective is it?
Wild, Redlich, Paltiel, OEM ‘05
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A simulation model of yearly diisocyanate
asthma surveillance (Markov model)
Input data from several published sources, but
some estimates from “expert opinion”
Ranges used for sensitivity analyses
Model input parameters
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Sensitization rate
Time to diagnosis
 no surv
 with surveillance
Chance of removal
 If diagnosed
 If not diagnosed
Likelihood of disability
 If removed
 If still exposed
2.8% per year (0.7-5.3%)
2.7y (1.5-5.9y)
1.7y (0.5-2.7y)
71% at 6 mo (<15-99%)
24% at 1y (0-71%)
30% disabled at 4y (0-69%)
60% disabled at 4y (expert panel)
Cost estimates
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Lost productivity
 Employer
 society
Diagnosis costs
 Screening
 confirmation
Medical absenteeism
costs
 Symptomatic
 Chronic
 Disabled
$0 (?? Could not estimate)
$28% loss x 10y at 50K/y (28-56%)
$104 (50-200%)
$454
$42.60/mo (50-200%)
$85.24/mo
$1513.60/mo
Model results
For 100,000 workers over 10y vs passive case
finding, surveillance would result in
 683 fewer disabled workers
 3.3 million more symptom-free days
 1831 additional QALYs
 Additional costs of $44 million
(~ 50% of the additional costs for surveillance and
confirmation were offset by reductions in meds,
absenteeism and disability costs)
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Cost-effectiveness estimate
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From the employer perspective
$24K per QALY (a cited threshold is $50K/QALY)
 $13.3 per symptom-free day
 $64K per case of disability prevented
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From the society perspective surveillance was cost
saving (including lost wages)
Sensitivity analyses
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Benefits increase with:
 High incidence of OA
 High likelihood of becoming disabled
 Rapid removal after diagnosis
 Faster diagnosis with surveillance vs passive
case finding
Variations in above can change the difference in
disability cases vs passive finding, from 0-2500
If sensitization rate is 0.7%/y then surveillance has a
CER >$50K per QALY from both employer and
society perspective, i.e. not cost effective
Other factors
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Benefits also relate to predictive values,
sensitivity and specificity of instruments used
(questionnaires, spirometry, immunologic
tests) – will determine the number of false
positive and false negative results
Frequency of surveillance can affect costeffectiveness
Job-security may affect self-reporting of
symptoms on questionnaire
Extent of participation can affect results
Costs can vary widely by country/system
Other examples of medical
surveillance for OA
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Complex platinum workers – skin tests are highly
predictive for OA
Detergent enzymes: routine skin prick testing used to
indicate inadequate exposure
Bakers: 4% asthma sx after 1 y, 9% after 2y
 - good results of surveillance in UK research study
 poor results in actual practice (poor participation,
and not admitting to sx)
 - estimated that asthma was 7x under-recognized in
practice in bakers and 4x in all bakery workers (Brant
’05)
When should medical surveillance
for OA be introduced?
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? All workplaces with sensitizer exposures and reliable
tests available or all higher risk companies?
- medical surveillance program feasibility and limitations
should be considered
- need valid tests: questionnaires, well-performed
spirometry, specific IgE
- include worker education in the program
- address group analyses as well as individual results
- need to initially establish reporting mechanisms, action
levels, action plans
Conclusion
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Medical surveillance as a component of
preventive strategies can be effective for medical
outcome but should be carefully implemented
and evaluated prospectively
For many current “higher risk” occupations,
such as cleaners and nurses where the specific
sensitizer is often unclear, and work settings are
widespread, this would be difficult to implement