Jpn.J.Infect.Dis., 52, 1999

Laboratory and Epidemiology Communications

High Frequency of Individuals with Strong Reaction to Tuberculin among Newly Employed Clinical Trainees

Mieko Toyota*, Koichiro Kudo, Morito Sumiya and Oichiro Kobori

Respiratory Department, International Medical Center of Japan, Toyama 1-21-1, Shinjuku-ku, Tokyo

Communicated by Hiroshi Yoshikura

(Accepted July 5, 1999)

In December 1997, the Japanese Society for Tuberculosis recommended tuberculin tests for all new employees under forty years old working in clinical services. For all individuals except those who were strongly positive in the first test, the second step tuberculin test (1) was recommended.

In April 1999, the 34 new employees for clinical training in our center (24 males and 10 females; the age distribution 24-29 years) received the tuberculin test. Twenty-two had already received BCG immunization and four had not. The BCG immunization history for the remaining eight individuals was obscure. The first tuberculin test was conducted before they started to work. The second tuberculin test was conducted 1-3 weeks (15.4 +/-4.6 days) later in 29 individuals who did not show strong reactions (reddening larger than 30 mm diameter) in the first test. The tuberculin used was the diagnostic tuberculin reagent for general diagnostic use (BCG Institute, Tokyo, lot PHV214).

The reactions were classified into strongly positive (+++), positive (++), weakly positive (+), and negative (-). There were four and eight strongly positive individuals in the first and the second tests, respectively (Table 1). Among the four new employees who were strongly positive in the first test, two had once received BCG immunization and one had not; the BCG vaccination history of the one remaining was obscure.

Though evaluation of the data provided by the tuberculin test is difficult in Japan on account of the widely practiced BCG immunization, the strongly positive individuals in the first test could be counted as those who were actually infected with Mycobacterium tuberculosis (2). In that case, the estimated frequency of individuals with a history of the infection was 11.8%. As the average infection rate among those in their twenties was estimated as 2% in Japan (3), the figure was much higher than the average. Sato et al. conducted tuberculin tests on 707 undergraduate and graduate students in Tohoku University Medical School; they found 24% strongly positive, while only 12% were negative (4). They argued that the students in medical schools belong to a high risk group owing to frequent contact with M. tuberculosis-infected patients.

We frequently observe a reduced tuberculin response in individuals who were BCG-vaccinated or infected with M. tuberculosis some time ago. However, a second tuberculin test performed several days or weeks later often shows the original positive response in such individuals, which is known as a "booster effect" (5). It is also known that tuberculin response peaks at two months and then declines (6). As diagnosis of M. tuberculosis infection with a single tuberculin test is difficult, it will be of value to conduct the two-step tuberculin test on the new employees before they begin clinical work so as to evaluate their basal level of immunological responses to M. tuberculosis.

REFERENCES

1. The Japanese Society for Tuberculosis, Committee on Prevention (1998): Preventing hospital infection of tuberculosis. Kekkaku (Tuberculosis), 73, 95-100 (in Japanese).
2. Aoki, M. (1998): Guidelines for preventing the transmission of Mycobacterium tuberculosis in hospitals. Visual Note. p. 60-61. The Japanese Anti-Tuberculosis Association, Tokyo (in Japanese).
3. Omori, M. (1994): Estimating the year of eradication of tuberculosis in Japan. Kekkaku (Tuberculosis), 69, 575-579 (in Japanese).
4. Sato, K, Sato, H. and Miura, Y. (1999): Screening for tuberculosis infection in undergraduate and graduate students in a medical school. Kekkaku (Tuberculosis), 74, 107 (in Japanese).
5. Thompson, N.J., Glassroth, J.L., Snider, D.E. and Farer, L.S. (1979): The booster phenomenon in serial tuberculosis testing. Rev. Respir. Dis., 119, 587-597.
6. Huebner, R.E. (1996): BCG vaccination in the control of tuberculosis. Curr. Top. Microbiol. Immunol., 215, 263-282.

*Corresponding author: Fax: +81-3-3207-1038

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