Incidence of Latent Tuberculosis Infection in a Study of School Contacts Comparing Two Cut-off Points of the Tuberculin Test / Incidencia de infección tuberculosa latente en un estudio de contactos escolares comparando dos puntos de corte de la prueba de tuberculina

ABSTRACT

Introduction: The risk of tuberculosis infection is associated with close and prolonged contact. Daily contact, poorly ventilated classrooms or delay in diagnosing the index case in a school setting could contribute to contagion. Objective: To know the incidence of latent tuberculosis infection (LTBI) in school contacts comparing two cut-off points of the tuberculin test (PPD, Purified Protein Derivative): ≥ 10mm and ≥ 5mm. To determine the degree of compliance with the performance of control and chemoprophylaxis (ChP) studies. Materials and Methods: We carried out a retrospective analysis of bacillary TB school contacts in teenagers between 12 and 19 years old of the Programmatic Area of the Hospital Parmenio Piñero between February 2016 and December 2017. The evaluation included chest x-ray, lab tests and PPD. We analyzed 2 cut-off points of the PPD: ≥ 10mm and ≥ 5mm, and repeated the test after 3 months in cases with negative results. Primary ChP with isoniazid was indicated for all the contacts, and secondary ChP for those with basal PPD+ or conversion at the third month. We considered ChP compliance in cases where patients followed > 80% of the indicated regime. Results: 331 (89%) of 373 students to be evaluated participated in the study. TB was diagnosed in 4 students (1.2%) through chest x-ray, and they were excluded. Within the remaining 327, the mean age was 15 ± 1.6 years, and 132 (40%) were male. The basal PPD was ≥ 10mm in 20 cases (6.1%), being more frequent in the group of 16-19 years versus the 12-15 years: 10.7% vs. 3.1%; p = 0.004. There wasn't any significant difference regarding sex. 135 contacts (45%) did the second PPD, and 3 conversions were found (2.2%). Primary ChP was indicated for everyone, and secondary ChP for 23 contacts, with 75% and 26% compliance respectively, and without any significant association with age or sex. 4 adverse events were reported (1.2%): polineuritis (n=2), skin allergy and food intolerance. 63 contacts (19.3%) were positive PPD, with PPD ≥ 5mm, being more frequent in males: 26.5% vs. 14.4%, p = 0.006; and in 16-19 vs. 12-15 years: 28.2% vs. 13.2%; p < 0.001. With the second PPD we observed 3 conversions (2.2%). Conclusion: We found 6.1% of LTBI considering a PPD+ ≥ 10mm and 19.3% with PPD ≥ 5mm. Compliance with the second PPD and the secondary ChP scheme was low.