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ABSTRACT |
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The risk of occupational tuberculosis (TB) infection and associated factors was estimated among all microbiology and pathology technicians and compared with a sample of nonclinical personnel in 17 Canadian acute care hospitals. Participants underwent tuberculin skin testing and completed questionnaires. Prior skin tests and vaccinations and all patients with TB hospitalized in the preceding 3 years were reviewed. Of the work areas where direction of air flow and air changes per hour were measured, only 51% were adequately ventilated. Among participating lab workers the average annual risk of tuberculin conversion was 1.0%. This was associated with lower hourly air exchange rates (16.7 versus 32.5 in workers with no conversion, p < 0.001) work in pathology (adjusted odds ratio [OR]: 5.4; [95% confidence interval: 1.3, 22], higher proportion of patients with missed diagnosis in the first 24 hours (per 20% increase—OR: 2.0; [1.3, 3.2], treatment delayed 1 week or more (per 20% increase—OR: 2.0; [3.2, 3.2]), and higher mortality (per 20% increase—OR: 2.5; [1.1, 5.6]). We conclude that laboratory workers, with no direct patient contact, have increased risk of tuberculin conversion in hospitals where a greater proportion of patients with TB die, or have delayed, or missed diagnosis, although this may be modified by workplace ventilation.
Key Words: tuberculosis • nosocomial transmission • occupational tuberculosis infection • autopsy • pathology and microbiology
The risk of occupational tuberculosis (TB) among microbiology technicians was recognized (1, 2) and resulted in development of engineering controls and infection control procedures. These measures resulted in substantial reduction in morbidity (3–6), although transmission continues to occur (7). However, case reports (8, 9) and population-based studies (10–12) have demonstrated that pathology workers, particularly those involved in autopsies (8, 9), continue to have significantly increased risk (13). An illustrative case report is described in the online supplement. Is this because of greater exposure or less effective infection control procedures for pathology workers?
Among all pathology and microbiology technicians and a sample of nonclinical personnel in 17 acute care hospitals in 4 Canadian cities we have estimated the association of tuberculin skin test conversion with work-site ventilation, and occupational TB exposure.
METHODS |
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A total of 17 acute care hospitals in 4 Canadian cities were selected: 15 classified as moderate to high risk for nosocomial TB transmission because they had at least six patients with TB admitted annually (14, 15), and 2 low-risk hospitals with fewer than two TB admissions annually. Within these hospitals, nursing personnel, respiratory therapists, physiotherapists, microbiology, and pathology technicians were studied. A sample of nonclinical workers served as a reference group for clinical and laboratory personnel. Because the laboratory technicians had very different working conditions and potential exposures, their results were analyzed separately from the clinical personnel with direct patient contact as reported elsewhere (16, 17). Signed informed consent was obtained from the participants. This study was approved by ethics committees in all the participating institutions.
In microbiology laboratories, ventilation was assessed where specimens were initially processed and where TB specimens were handled. In pathology departments, the cutting rooms and autopsy suites were studied. Each room was inspected and temperature, humidity, air movement, area, and volume were measured. Smoke tubes were used to measure direction of airflow, and air changes per hour were estimated using pure carbon dioxide as a tracer gas (18), with doors and windows closed and ventilation systems in operation.
Workers completed self-administered questionnaires regarding demographic information, training, work, and recognized episodes of TB exposure. Prior tuberculin testing and bacillus Calmette-Guérin (BCG) vaccination was verified from employee health records. For Quebec-born participants, information regarding BCG vaccinations was obtained from a previously validated (19) central registry. Charts were reviewed of all patients with newly diagnosed confirmed active pulmonary TB admitted to the study hospitals during the 3 years preceding the study.
Workers with prior documented positive tuberculin tests and/or history of prior anti-TB therapy were not retested. Five tuberculin units of PPD-T, bioequivalent to PPD-S (Tubersol; Connaught Laboratories, Toronto, ON, Canada) was administered intradermally to the volar aspect of the forearm using the Mantoux technique to all other participants. The transverse diameter of induration was demarcated using the ballpoint technique 48 to 72 hours later, measured using machinists calipers and recorded in millimeters. Participants with initial tuberculin reactions less than 10 mm underwent repeat testing 1 to 4 weeks later.
Statistical Analysis
Tuberculin conversion was defined according to Canadian standards (20) as a reaction of 10+ mm, with an increase of at least 6 mm from a baseline negative tuberculin skin testing (TST) from at least 1 year earlier and after at least 1 year of work. All tuberculin testing for this study was completed within 1 month at each center; to define conversion, the baseline negative TST of 1 or more years earlier had to be taken from employee records.
Bivariate associations were tested for statistical significance, with unpaired t tests for continuous variables and 2 tests for categoric variables (21). Multivariate logistic regression (22) provided adjusted estimates of the effect of nonoccupational and occupational factors on TST conversion. The resultant parameter estimates and standard errors were used to calculate odds of tuberculin conversion and 95% confidence intervals (22).
RESULTS |
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The study of the 429 hospitalized patients with TB in the period reviewed is reported in detail elsewhere (18). In brief, the diagnosis was missed (and the patient was not isolated) in the first 24 hours in 45%, treatment was delayed by 1 week or more in 30%, and 12% died. In 20 microbiology laboratories air change rates averaged 31.1 per hour, although only 10 (50%) had more than 15 air changes per hour and negative pressure (inward airflow). In the 27 pathology work areas measured there was an average of 16.7 air exchanges per hour; only 14 (52%) had more than 15 air exchanges per hour and negative pressure.
As shown in , workers' tuberculin conversions were associated with BCG vaccination, work in pathology, lower ventilation, indicators of delayed diagnosis, and overall mortality of patients with TB admitted to the same hospital.
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DISCUSSION |
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Strengths of the study include the involvement of 17 Canadian hospitals in which the number of TB admissions ranged from 1 to 135 in the period reviewed. Potentially confounding workers' characteristics were measured and standardized protocols for tuberculin testing, exposure definition, and ventilation measurement were used in all sites. This made it possible to estimate the effect of workplace ventilation and delayed TB diagnosis on tuberculin conversion, adjusted for potentially confounding occupational and nonoccupational factors.
Nevertheless there were a number of important limitations. A large number of laboratory workers participated but relatively few had documented prior negative TST, potentially limiting power. Despite this, a number of significant relationships were detected. Indirect indicators of exposure were used rather than actual handling of specimens from patients with recognized or (at the time) unrecognized TB. Tuberculin conversion was inversely associated with the number of known TB admissions but strongly correlated with higher proportion of missed/delayed diagnosis, a more frequent phenomenon in hospitals with fewer TB admissions (17). This suggests that indices of exposure based on the number of known patients may not accurately reflect exposure to the unknown patients—the most important sources of transmission in case reports and outbreak investigations (13).
Ventilation conditions were measured once and may have been different at other times or in the past. To minimize this problem, information on prior renovations was collected, and at the time of measurement, ventilation was controlled to be representative of normal working conditions. Indicators of exposure to patients with TB were based on the preceding 3 years only. Exposure could have been misclassified if incidence in that hospital had changed. However, this seems unlikely because overall incidence in the four Canadian cities did not change appreciably between 1980–1995 (24, 25).
The most important interpretation of these results is that delayed or missed diagnosis of patients with TB, a common phenomenon in the hospitals studied (17) and elsewhere (26, 27), results in similar level of risk for laboratory workers as it does for clinical personnel. This is presumably because laboratory workers handle specimens from patients with unrecognized TB. Involvement in autopsies of undiagnosed patients may play an important role (28). Nine of the 52 patients who died of TB in this study were only diagnosed at autopsy, and the elevated risk of this procedure has been recognized in other studies (8, 9) (10–12) due to massive aerosolization of TB bacilli (9). Because delayed diagnosis is associated with fewer TB admissions (18, 27), delayed diagnosis and death from unrecognized TB will continue to occur despite the best efforts of health care providers (29). In view of this, laboratory technicians must use universal precautions (30) for handling microbiologic and pathologic specimens, particularly for autopsies (28).
The introduction of (1) much stricter infection control procedures and engineering controls resulted in dramatic reduction in rates (2, 6). The effectiveness of the engineering controls may be inferred from recent reports of high rates of TB infection and disease in laboratory workers in resource-poor countries (31) where these measures are too costly to be implemented (32). The present findings demonstrate that risk for pathology workers remains elevated, consistent with several other studies (8–10, 12). It is unclear whether this is due to greater exposure or deficiencies of infection control procedures. The finding that the air exchange rates in pathology work areas were about half those in microbiology areas and that lower ventilation was associated with TST conversion suggests that re-evaluation of the current recommended (14, 15) minimum air exchange rates may be warranted, although inferences based on these findings are limited.
In conclusion, risk of tuberculin conversion was elevated in laboratory workers, particularly in pathology, and was associated with lower levels of ventilation and indicators of delayed TB diagnosis. Implementation of strict universal precautions could reduce the risk of TB infection and subsequent disease among these laboratory workers.
Acknowledgments |
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