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UBC Theses and Dissertations

The value of the tuberculin skin test size in predicting the development of tuberculosis in contacts of active cases Morán Mendoza, A. Onofre


Objectives: A) to identify the risk factors for tuberculosis (TB) development among contacts of TB cases. B) To determine the tuberculin skin test (TST) size cut-offs that best predict the development of TB for various groups. Methods: A retrospective cohort study of the contacts of active TB cases recorded in the Division of TB control (DTB) of BC from 1990 to 2000 was carried out to identify the development of TB. The prognostic factors for TB development were obtained from the DTB and from several provincial databases. Contacts with HIV or previous TB were excluded. The prognostic factors evaluated were: TST size, age, gender, ethnicity, latent TB infection (LTBI) treatment, type of contact, previous BCG, immunosuppressive conditions and others. Results: Among the 33,146 contacts identified in the DTB, 228 developed TB during the 12- year follow-up period (TB rate 688/100,000) and 82% of TB cases occurred within 2 yrs after exposure. The main prognostic factors for TB development were: malnutrition, no LTBI treatment, TST size ≥5 mm, being a household contact, and young age (all hazard ratios >5; p-values <0.0001). Other significant factors were: malignancy, use of corticosteroids, injection drug use, previous BCG status, alcoholism, exposure to a smear positive TB case, lower socioeconomic status and being an Aboriginal person. The TST sizes that best predicted the development of TB varied among different groups: household contacts with TST sizes 0-4 mm (reference category) had a TB rate of 1,014/100,000; those with 5-9 mm a TB rate of 2,162/100,000 (adjusted hazard ratio: 10.0; 95% CI: 3.3 to 28.9). Children 0-10 yrs old with 0- 4 mm had a TB rate of 806/100,000; those with 5-9 mm a TB rate of 5,556/100,000 (adjusted hazard ratio: 6.0; 95% CI: 0.8 to 47.4). Immunosuppressed contacts with TST sizes of 0-4 mm had a TB rate of 686/100,000; those with 5-9 mm a TB rate of 2,000/100,000 (adjusted hazard ratio: 2.6; 95% CI: 0.3 to 22.9). The risk of TB increased progressively with larger TST sizes. Conclusions: Household contacts, close contacts 0-10 yrs old and immunosuppressed contacts may benefit from preventive treatment regardless of their TST size.

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