Evaluation of the tuberculin skin test and the interferon-γ release assay for TB screening in French healthcare workers

Abstract

IntroductionUsing French cut-offs for the Tuberculin Skin Test (TST), results of the TST were compared with the results of an Interferon-γ Release Assay (IGRA) in Healthcare Workers (HCW) after contact to AFB-positive TB patients.MethodsBetween May 2006 and May 2007, a total of 148 HCWs of the University Hospital in Nantes, France were tested simultaneously with IGRA und TST. A TST was considered to indicate recent latent TB infection (LTBI) if an increase of >10 mm or if TST ≥ 15 mm for those with no previous TST result was observed. For those with a positive TST, chest X-ray was performed and preventive chemotherapy was offered.ResultsAll HCWs were BCG-vaccinated. The IGRA was positive in 18.9% and TST ≥ 10 mm was observed in 65.5%. A recent LTBI was believed to be highly probable in 30.4% following TST. Agreement between IGRA and TST was low (kappa 0.041). In 10 (16.7%) out of 60 HCWs who needed chest X-ray following TST the IGRA was positive. In 9 (20%) out of 45 HCWs to whom preventive chemotherapy was offered following TST the IGRA was positive. Of those considered TST-negative following the French guidelines, 20.5% were IGRA-positive. In a two-step strategy - positive TST verified by IGRA - 18 out of 28 (64.3%) IGRA-positive HCWs would not have been detected using French guidelines for TST interpretation.ConclusionThe introduction of IGRA in contact tracings of BCG-vaccinated HCWs reduces X-rays and preventive chemotherapies. Increasing the cut-off for a positive TST does not seem to be helpful to overcome the effect of BCG vaccination on TST.

IFN-γ release assay versus tuberculin skin test for monitoring TB infection in healthcare workers

“…Routine TB screening for HCW is considered as a cornerstone for preventing TB in hospitals and it is performed in many highincome countries, for example, USA [9], Japan [21], France [22], Spain [23] and Switzerland [24]. In the UK, TB screening of HCW is mainly performed as a pre-employment screening [25].…”

“…Following ATS guidelines, a transversal induration diameter of 5+ mm is considered a positive test result [28]. In most countries, a diameter of 10+ mm is considered as a positive Review result, for example, Portugal [13], France [22] and Vietnam [29]. In France, recent infection is considered likely if the diameter is larger than 15 mm [22].…”

“…In most countries, a diameter of 10+ mm is considered as a positive Review result, for example, Portugal [13], France [22] and Vietnam [29]. In France, recent infection is considered likely if the diameter is larger than 15 mm [22]. In addition, an increase of 6+ or 10+ mm compared with an earlier negative TST (<10 mm) is considered indicative of a recent LTBI [26,30].…”

“…In BCG-vaccinated HCW, the prevalence of positive TST is two-to five-times as high as in unvaccinated HCW [13,22,27,46,47]. No clear relationship between time of last BCG vaccination and TST is observed [13].…”

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“…20 The situation is worse in countries such as Portugal and France where occurrence rates are decidedly higher at 33% and 19% compared to Germany's 10%. 21,22 HCW, especially physicians and nurses are at a higher risk of acquiring active and latent TB infection (LTBI) because of prolonged and frequent contact with infected patients. In contrast, this group has lower transmission rates to contacts than other groups of infected individuals as a result of frequent health checks.…”