Comparison of interferon-γ release assay and tuberculin test for screening in healthcare workers

Torres Costa, José; Silva, Rui; Sá, Raul; João Cardoso, Maria; Ribeiro, Carla; Nienhaus, Albert

doi:10.1016/S2173-5115(10)70031-2

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Abstract

Healthcare workers (HCWs) have an increased risk of tuberculosis (TB). Screening for latent tuberculosis infection and active TB is therefore essential in infection control programs. Tuberculin skin test (TST) and Interferon-γ Release Assay (IGRA) were used simultaneously in 1686 HCWs between May 2007 and April 2009. A chest X-ray was performed in order to exclude active TB when TST was≥10mm or IGRA was positive and in HCWs with TB contact or symptoms. IGRA was positive in 33.1% and TST was>10mm in 78.3% of the HCWs. The proportion of positive IGRA results increased with the TST diameter. In those with a TST>15mm, 49.2% were IGRA positive. TST was more than twice as often positive than the IGRA. Therefore, TST+/IGRA-results were more often observed than concordant negative or positive results. In none of the HCWs with a TST+/IGRA-result active TB was diagnosed during the study period. Repeated BCG vaccination increased the number of TST+/IGRA-discordance. The smaller the interval after BCG vaccination, the higher was the TST+/IGRA-discordance.

In the screened HCWs population, active TB was diagnosed in 9. At the time of diagnosis TST and IGRA were positive in all active TB cases. The study period covers 24 months, therefore the average annual incidence rate was 268/100 000.

TB burden in HCWs in Portugal is high. Considering the limitations that TST and IGRA present, the best solution seems to be the use of both, using the IGRA higher specificity for confirming a positive TST, taking advantage of the best characteristics of each test.

Key-words:

Tuberculosis

healthcare workers

tuberculin skin test

interferon-γ release assay

Portugal

Resumo

Os profissionais de saúde (PS) têm um risco aumentado de tuberculose. O rastreio da tuberculose latente e da tuberculose activa é portanto essencial nos programas de controlo de infecção. Entre Maio de 2007 e Abril de 2009, foram utilizados simultaneamente a prova de tuberculina (PT) e o teste de libertação do interferão-γ (IGRA) em 1686 PS. Quando PT≥10mm ou IGRA positivo, e em PS com contacto com tuberculose ou sintomáticos, foi realizada uma radiografia torácica para excluir tuberculose activa. O IGRA foi positivo em 33,1% e a PT foi>10mm em 78.3% dos PS. A proporção de resultados IGRA positivos aumentou com o diâmetro da PT. Nos PS com PT>15mm, 49.2% são IGRA positivos. A PT foi positiva em mais do dobro dos casos do IGRA. Assim, foram observados mais frequentemente resultados PT+/IGRA-do que resultados concordantes negativos ou positivos. Em nenhum PS com resultado PT+/IGRA-foi diagnosticada tuberculose activa durante o período do estudo. A vacinação repetida pelo BCG aumentou o número de casos discordantes PT+/IGRA-. Quanto menor o intervalo após a vacinação pelo BCG, maior a discordância PT+/IGRA-.

Na população de PS rastreada, foram diagnosticados 9 casos de tuberculose activa, sendo todos PT e IGRA positivos na altura do diagnóstico. Este estudo durou 24 meses, pelo que a taxa de incidência anual média foi de 268/100 000.

A tuberculose é um problema importante nos PS em Portugal. Considerando as limitações que a PT e IGRA apresentam, a melhor solução parece ser o uso de ambos, utilizando a maior especificidade do IGRA para confirmar uma PT positiva, aproveitando as melhores características de cada teste.

Palavras-chave:

Tuberculose

profissionais de saúde

prova de tuberculina

teste de libertação do interferão-γ

Portugal

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