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physicians have been recorded in other studies from low-incidence countries (12) (13) (14) (15). As a result, many studies have been undertaken to identify factors associated with non-acceptance (16) (17) (18). For instance, Dobler and colleagues have shown that physicians’ decisions on treatment depended on the estimated risk of developing TB, which depends on the probability that a positive test result is indicative of true infection with M. tuberculosis (18). Thus, as a result of the increased specificity of the interferon-gamma (IFN-γ) release assay (IGRA) for detection of LTBI compared to the tuberculosis skin test (TST), it is expected that both screening coverage and initiation of preventive treatment will increase further when the IGRA is implemented as a standard diagnostic test in the Netherlands. Indeed, chapter 3 showed that, already following the introduction of the IGRA at the PHS Amsterdam in 2008, coverage of screening among contacts of PTB patients increased each year.
The impact of expanding preventive treatment depends, in addition to the acceptance,
completion and efficacy of treatment, on the number of TB cases that would have
been prevented if they had received treatment. Chapter 4 shows that, using 10 years
of follow-up data from the electronic surveillance system on contact investigations
at the PHS Amsterdam, the 5-year risk of incident TB among contacts with LTBI who
did not receive preventive treatment was low at 2.4%. This chapter also showed that,
even if LTBI screening and preventive treatment would have been restricted to a more
selected group of contacts with increased risk of progression to TB, the 5-year risk
of incident TB among contacts with LTBI who did not receive preventive treatment
remained low at 3.5%. Assuming total treatment efficacy, the number of cases that
would have been prevented if all would complete treatment would be low compared
to the overall disease burden of 610 TB cases observed over the ten year study period
in the catchment area of the PHS Amsterdam. Thus, expanding preventive treatment
among TB contacts who are regarded as high risk individuals, or among a subgroup
of contacts with increased risk, is unlikely to dramatically improve the population 7 impact of preventive treatment.
In order to enhance the impact of preventive treatment in contact investigations, efforts should be taken to improve current contact tracing strategies and diagnostics tests for adequate identification and diagnosis of recently infected contacts at risk for progression to TB. As shown in chapter 4, if based on current screening strategies, targeted testing for LTBI, aimed to identify persons at high risk for TB who would benefit from treatment of LTBI, is likely to result in overtreatment. LTBI diagnosis in
General Discussion
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