RESUMO
BACKGROUND: Healthcare workers (HCW) are a risk group for tuberculosis (TB). That is why interferon-gamma release assay (IGRA) serial testing is performed on HCWs repeatedly exposed to infectious patients or materials. However, the variability of IGRA in serial testing is not yet well understood. We therefore analysed the prevalence of positive IGRA results as well as conversion and reversion rates in the serial testing of healthcare trainees in a low-incidence country. METHODS: In a prospective cohort study, all trainees (n=194) who began training as a nurse or healthcare worker at the Vivantes Healthcare Training Institute in Berlin on 1 October 2008 or 1 April 2009 were IGRA-tested at three different times during the three years of training. Socio-demographic data and possible risk factors (e.g., TB contacts, time spent abroad, area of work) were recorded by means of a standardised questionnaire. The QuantiFERON Gold In-Tube (QFT) was used as an IGRA. RESULTS: At the beginning of the training the cohort comprised 194 trainees. 70% were female. Their average age was 23. The prevalence of positive QFT was 2.1% (4/194). In the first follow-up test, 2 out of 154 (1.3%) tested IGRA-positive, 151 (98%) had constantly negative results. One IGRA was constantly positive (0.6%) and there was one conversion and one reversion (0.6% respectively). In the second follow-up (n=142) there was again one conversion (0.7%), one reversion and the one constantly positive test result in all three QFT. This trainee had active TB in 2002. All other test results were constantly negative (n=139; 98%). No case of active tuberculosis was diagnosed over the three-year observation period. Contact with TB patients was reported by 42 (29.6%) trainees during the follow-up. The two trainees with a conversion in QFT had no known contact with TB patients. Discordant results in the three consecutive QFT were observed in three trainees (2.1%). Using a borderline zone from 0.2-0.7 IU/mL reduced the number of trainees with discordant results from three to one - a reversion. CONCLUSION: The prevalence rate of latent TB infection is low in healthcare trainees without known risk factors for TB infection in their history. The infection risk seems to be low in this population even though contacts with TB patients during the training were reported. Introducing a borderline zone for the interpretation of reversions and conversions in this cohort appears to be safe and reduces the number of discordant results and helps to avoid unnecessary chest X-rays and preventive treatment.
RESUMO
BACKGROUND: Interferon-gamma release assays (IGRA) are used for tuberculosis (TB) screening in healthcare workers (HCWs). However, data on specificity of IGRA in serial testing of HCWs is sparse. Therefore the specificity and the negative predictive value of the IGRA - QuantiFERON-TB Gold In-Tube (QFT) - in German nursing students was investigated. METHODS: 194 nursing students at the start of their professional career were tested with the QFT. 14 nursing students were excluded from the specificity analysis, due to exposure to mycobacterium tuberculosis. Two of these subjects were QFT- positive. None of them developed disease during the year of follow-up. A study group of 180 students, all with very low risk of prior TB infection, remained in the specificity analysis. Subjects were monitored for at least two years with respect to the development of active TB disease. IGRA was performed at the start of the training and after one year. RESULTS: The mean age of the study group (n = 180) was 23 years (range 18-53) with 70.9% female and 99.4% German born. The specificity of QFT was 98.9% (178/180; 95% CI 0.96-0.99); lowering the cut-off from 0.35 IU/ml to 0.1 IU/ml would have decreased specificity only slightly to 97.8% (176/180; 95% CI 0.94-0.99). Of the 154 nursing students available for re-testing, one student who initially scored positive reverted to negative, and one student initially negative converted to positive. None of the monitored group with initially negative QFT results developed TB disease, indicating a high negative predictive value of the IGRA in this population. CONCLUSIONS: Following our data, QFT can serve as an effective tool in pre-employment TB screenings for HCWs. As its negative results were stable over time, specificity of the QFT in serial testing of HCWs is high. As the risk of acquiring TB infection in the German healthcare system appears to be low, our data supports the recommendation of performing TB screening only in those HCWs with known contact to TB patients or infectious materials.
