Background: The study of contacts of patients with tuberculosis is an important preventive measure which helps to identify the risk factors for contagion, to detect new cases of the disease early and to break the epidemiological chain of transmission.
Material and method: Three thousand and seventy-one contacts of 635 patients with tuberculosis were studied in our department over a period of 6 years. With the aid of uni- and multivariate analyses we established the importance of different factors in the appearance of tuberculous disease and infection in contacts.
Results: There were 1,341 tuberculin positive contacts (44%). The proportion increased in relation to the degree of the relationship and the bacillary density of the index case. A hundred and seventy-six new cases of tuberculosis were diagnosed among the contacts (5.7%). To study the risk factors for contagion, we analyzed the characteristics of 322 patients with positive sputum smears and their 1,623 contacts; 124 new cases were diagnosed among them (124/176; 70%). The most important factors for contagion in the multivariate analysis were the closer relationship (odds ratio [OR] = 8.32; confidence interval [Cl] 95% = 3.9-17.6), greater TST reaction of contacts (OR = 4.43; Cl 95% = 2.5-7.7), presence of more than 10 bacilli per microscopic field in the sputum samples of the index case (OR = 1.97; Cl 95% = 1.1-3.3), male contacts (OR = 1.86; Cl 95% = 1.2-2.7) and those younger than 15 years (OR = 1.58; Cl 95% = 1.01-2.45). Conversely, the sex of the index case and the history of previous tuberculosis in contacts did not influence the contagion.
Conclusions: The study of contacts offers a high yield in the diagnosis of new tuberculosis cases, because the incidence of the disease is much higher than that of the general population. Moreover, it allows the detection of newly infected subjects in whom the application of chemoprophylaxis prevents the development of the disease, and thus the epidemiologic chain of transmission is broken.