~ By Dr Vincent Chia Wei Meng ~
With reference to the Straits Times on 28th April 2012, "tuberculosis (TB) is making a resurgence after some years in decline [in Singapore]." (1)
I am increasingly concerned with the way foreign workers are screened for tuberculosis in Singapore, which could have contributed to this resurgence.
According to Khin Mar Kyi Win et al, due to the “liberalization of the immigration policy in Singapore in 2005, there was an influx of migrant workers and immigrants from countries with high incidences of TB and a corresponding increase in TB notifications among this population. Of these persons with TB, >75% came from 5 of the 7 countries (India, China, Indonesia, Bangladesh, and the Philippines) with highest incidences of TB.” (2)
The work permit health screening for foreign workers uses the Chest X-ray as the screening tool for pulmonary tuberculosis. It does not include a Mantoux test or any other test for the detection of TB in these foreign workers.
However, the Chest X-ray is notoriously known for its inability to detect early disease or recurrences. (3) Although the Chest X-ray is commonly used for the diagnosis and screening of active TB, it has poor sensitivity, specificity, and reproducibility. (4)
One likely reason for the delay in diagnosis of TB is an over-reliance on X-rays. “In Hong Kong, general practitioners' practice was reviewed, and it became clear that they depend too much on X-rays rather than sputum examinations, and that they were slow in referring tuberculosis patients to the government tuberculosis service.” (5)
Some cases of active pulmonary tuberculosis could be missed in early disease as X-ray changes have no begun to show. To compound the diagnostic problem further, scarring from previously cured disease seen on X-rays might sometimes be difficult to distinguish from current active tuberculosis.
Furthermore, pregnant foreign workers are exempted from Chest X-rays, which really is the only – albeit inadequate – means of detecting active TB in the work permit health screen.
In conjunction with radiological evidence, the Centres for Disease Control and Prevention (CDC) recommend at least a TB Skin test (Mantoux test) or a TB blood test (interferon-gamma release assays or IGRAs) for the diagnosis of TB. (6) Suspicious cases are also sent for further laboratory tests such as sputum smears and cultures. This is reasonable since “[sputum] culture-positive pulmonary TB with a normal CXR is not uncommon, and the incidence of this presentation is increasing.” (7) In other words, a patient with a normal Chest X-ray could actually have active TB, and this would go undiagnosed if no other tests were done.
If the foreign worker does not declare his symptoms accordingly, clinical suspicion would hence be lowered in clinicians performing the work permit health screening.
Combining a simple Mantoux test with the Chest X-ray would enhance the quality and effectiveness of TB screening in foreign workers. Suspected cases would then be referred for further diagnostic tests (e.g. sputum smears and cultures) to delineate those with latent TB infection from those with active TB disease.
Shouldn't the Ministry of Manpower change her current practice of using merely a Chest X-ray for the screening of TB in foreign workers?
References
(1) “Health Ministry working to dispel myths about TB.” http://www.straitstimes.com/BreakingNews/Singapore/Story/STIStory_793514.html
(2) Kyi Win KM, et al., “Tuberculosis among foreign-born persons, Singapore, 2000–2009,” Emerg Infect Dis 17, no. 3 (2011), http://dx.doi.org/10.3201/eid1703.101615 (accessed April 28, 2012).
(3) Ito K, “Limits of chest X-ray investigation in the diagnosis of recurrent pulmonary tuberculosis,” Kekkaku 80 (2005): 521-6. http://www.ncbi.nlm.nih.gov/pubmed/16167778;.
(4) Toman K. 1979. Tuberculosis—Case-Finding and Chemotherapy: Questions and Answers. World Health Organization, Geneva.
(5) Christoph Lange and Toru Mori, “Advances in the diagnosis of tuberculosis,” Respirology 15 (2010): 220-40
(6) http://www.cdc.gov/tb/topic/testing/default.htm; also see www.cdc.gov/mmwr/pdf/rr/rr5905.pdf
(7) Darcy D. Marciniuk et al, “Detection of Pulmonary Tuberculosis in Patients With a Normal Chest Radiograph,” CHEST 115 (1999): 445-52. http://chestjournal.chestpubs.org/content/115/2/445.full
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