Abstract
BackgroundIn Norway, screening for tuberculosis infection by tuberculin skin test (TST) has been offered for several decades to all children in 9th grade of school, prior to BCG-vaccination. The incidence of tuberculosis in Norway is low and infection with M. tuberculosis is considered rare. QuantiFERON®TB Gold (QFT) is a new and specific blood test for tuberculosis infection. So far, there have been few reports of QFT used in screening of predominantly unexposed, healthy, TST-positive children, including first and second generation immigrants. In order to evaluate the current TST screening and BCG-vaccination programme we aimed to (1) measure the prevalence of QFT positivity among TST positive children identified in the school based screening, and (2) measure the association between demographic and clinical risk factors for tuberculosis infection and QFT positivity.MethodsThis cross-sectional multi-centre study was conducted during the school year 2005–6 and the TST positive children were recruited from seven public hospitals covering rural and urban areas in Norway. Participation included a QFT test and a questionnaire regarding demographic and clinical risk factors for latent infection. All positive QFT results were confirmed by re-analysis of the same plasma sample. If the confirmatory test was negative the result was reported as non-conclusive and the participant was offered a new test.ResultsAmong 511 TST positive children only 9% (44) had a confirmed positive QFT result. QFT positivity was associated with larger TST induration, origin outside Western countries and known exposure to tuberculosis. Most children (79%) had TST reactions in the range of 6–14 mm; 5% of these were QFT positive. Discrepant results between the tests were common even for TST reactions above 15 mm, as only 22 % had a positive QFT.ConclusionThe results support the assumption that factors other than tuberculosis infection are widely contributing to positive TST results in this group and indicate the improved specificity of QFT for latent tuberculosis. Our study suggests a very low prevalence of latent tuberculosis infection among 9th grade school children in Norway. The result will inform the discussion in Norway of the usefulness of the current TST screening and BCG-policy.
Highlights
In Norway, screening for tuberculosis infection by tuberculin skin test (TST) has been offered for several decades to all children in 9th grade of school, prior to Bacillus Calmette-Guérin (BCG)-vaccination
We suspect that previous BCG-vaccination or infection with non-tuberculosis mycobacteria (NTM) are the causes of most TST reactions as few of the children have any known exposure to tuberculosis and the transmission rate in Norway, based on molecular epidemiology, is known to be very low [3,4]
In order to evaluate the current TST screening and BCG-vaccination programme and inform the discussion on any future targeted screening approach, we aimed to (1) measure the prevalence of QuantiFERON®TB Gold (QFT) positivity among TST positive children identified in the school based screening and (2) measure the association between demographic and clinical risk factors for tuberculosis infection and QFT positivity
Summary
In Norway, screening for tuberculosis infection by tuberculin skin test (TST) has been offered for several decades to all children in 9th grade of school, prior to BCG-vaccination. There have been three objectives of this screening: (1) to measure the transmission rate of tuberculosis infection in the population, (2) to identify cases with latent tuberculosis infection for preventive treatment, and (3) to ensure that only tuberculin negative children receive Bacillus Calmette-Guérin (BCG) vaccine, which is offered in the national vaccination programme to all previously unvaccinated, TST negative children at the age of 14 years. Children born in Norway of parents from high prevalence countries and a limited number of other children assumed to be at risk are offered BCG-vaccination at birth or on other occasions [2] Each year, this screening results in several hundred children with a positive TST (defined as => 6 mm) being referred to hospitals for medical evaluation, chest X-ray and a three year follow up procedure. We suspect that previous BCG-vaccination or infection with non-tuberculosis mycobacteria (NTM) are the causes of most TST reactions as few of the children have any known exposure to tuberculosis and the transmission rate in Norway, based on molecular epidemiology, is known to be very low [3,4]
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