A study comparing latent tuberculosis (TB) infection rates among different groups of healthcare workers in the Russian city of Samara has found a high rate of latent TB infection. The study, published today in the February 2007 edition on-line journal PLoS Medicine, also validated a new interferon-gamma-based test for detecting latent TB infection in people who have previously received the BCG vaccine.
It is estimated that a third of the world’s population is latently infected with Mycobacterium tuberculosis, of whom around 10% will go on to develop active TB, a potentially fatal infection. A recent systematic review found high reported rates of latent TB infection in low- and middle-income countries in Africa, Asia, South and Central America. However, a major limitation of the studies that were reviewed was the lack of data on latent TB in these countries at large, making it difficult to compare healthcare workers to the general population.
Russia is one of 22 countries that account for around 80% of the world's active TB. Although healthcare workers are particularly at risk of TB, the increased risks of latent infection compared with the general population is not known. In 2004, researchers established that Samara City in southeastern Russia had a ten-fold higher rate of active TB disease among healthcare staff compared with the general population: 741.6 per 100,000 versus 69.3/ 100,000.
Since there were no data on the relative risks of latent TB, investigators from the UK and Russia sought to identify the proportion of healthcare workers in Samara City with latent TB infection, and compare groups considered to be at different levels of risk (medical and non-medical students, hospital workers outside of TB wards, and hospital workers on TB wards).
One of the limitations of previous studies of latent TB is the unreliability of the tuberculin skin test – the traditional test for detecting TB infection – among individuals who have received BCG vaccination earlier in life, which is the case in Russia. The investigators, therefore, used a new interferon-gamma-based test (QuantiFERON–TB Gold) which measures the immune response to two proteins produced by M. tuberculosis that are not present in the BCG vaccine.
A total of 630 individuals were tested for latent TB via blood samples, and possible risk factors for TB were assessed by questionnaire. This included all 140 healthcare workers (including doctors, nurses and laboratory workers) from all the TB clinics in Samara City, as well as 368 medical and other students, and 122 general hospital staff.
The highest rate of latent TB infection was seen amongst the healthcare workers in the TB wards (46.9% in TB healthcare workers vs. 29.3% on non-TB healthcare worker; 95% confidence interval [CI] 1.1–2.3). However, a high rate of latent TB infection was seen in all healthcare workers – no matter where they worked – in comparison with the student population (40.8% vs. 8.7%; 95% CI 3.1–6.5).
The investigators note that the differences in latent TB infection rate between students, non-TB ward healthcare workers, and TB clinic staff reflected progressively increasing exposure to TB: the highest rates of latent TB infection were seen in TB ward doctors (55%) and TB laboratory workers (61.1%).
When discussing the relevance of their findings, the investigators say that healthcare workers in this region of Russia “are likely to be representative of Russia as a whole, since the organisation of the healthcare system in Samara is almost identical to that of Russia and is similar to all countries of the former Soviet Union. Similarly, TB incidence and prevalence is close to the national average for Russia.”
The investigators believe that the results of this study suggest “that appropriate preventive strategies and/or staff health monitoring should be undertaken.”
They go on to suggest that their findings are strong arguments for the modification of “institutional infection control and occupational health policies, which have traditionally been weak.” They note that “negative pressure facilities are not usually available in Russian hospitals, including those centres dealing exclusively with TB...the main feature of TB control is the use of ultraviolet germicidal radiation and daily surface disinfection. A similar practice operates in the laboratories. Except in winter, windows are opened to provide ventilation.”
They add that the study also shows that the new latent TB test offers “rapid in vitro testing requiring a single screening visit, which is less subjective than skin testing and can be repeated as an annual occupational screening.”
“Nevertheless”, they conclude, “larger cohort studies are needed to assess the value of these tests results, risks of active TB development in positive individuals, and effectiveness of preventive therapy based on [interferon-gamma] test results.
Reference Drobniewski F et al. Rates of latent tuberculosis in health care staff in Russia. PLoS Med 4(2): e55, 2007. doi:10. 1371/journal.pmed.0040055
CST Price at posting:
0.0¢ Sentiment: Buy Disclosure: Held
