[Comparative study of bactec MGIT 960 AST and conventional proportion method using Ogawa medium for the drug susceptibility testing of Mycobacterium tuberculosis to isoniazid].

OBJECTIVE: To evaluate the accuracy of drug susceptibility testing to isoniazid with BACTEC MGIT 960 (MGIT AST) comparing with the standard proportion method using Ogawa medium. METHOD: A total of 1109 M. tuberculosis strains, which were selected from the collection of RYOKEN drug resistance survey in 2002, were selected and subjected to the susceptibility testing to isoniazid using MGIT AST and 1% Ogawa standard methods. The results from MGIT AST were compared with the judicial diagnosis by Ogawa. The sensitivity to detect drug resistance, the specificity for susceptible strain, the efficiency of overall agreement, and kappa coefficient were calculated to evaluate the performance. The treatment process, outcome and prognosis were analysed for the patients on whom the tests showed discrepant results. RESULTS: Compared with the judicial results, the sensitivity, specificity, efficiency, and kappa coefficient of MGIT AST were 100%, 97.1%, 97.3%, and 0.798, respectively. The strains, which showed discrepant results between MGIT AST and Ogawa, were all susceptible by Ogawa and resistant by MGIT AST. A total of 11 out of 30 discrepant cases were followed clinically and no relapse cases were identified, irrespective of the modification of the treatment regimen. As for the proportion of primary INH drug resistance in the present study, it was 5.3% with MGIT AST but was 2.7% with Ogawa, and the difference was statistically significant (p = 0.005). DISCUSSION: The discrepancies on the results of drug susceptibility testing of M. tuberculosis strains to isoniazid between MGIT AST and 1% Ogawa proportion method have been reported. In the present study, the sensitivity, specificity, and overall efficiency of MGIT AST on the prevalent strains in Japan were all beyond 95%, and considered sufficient as the anti-tuberculosis drug susceptibility testing (AST), though 2.7% of discrepancy was observed. Even for the discrepant cases, there was no difference in the treatment outcome and prognosis. Thus, MGIT AST was confirmed as a reliable AST method comparable to Ogawa standard. However, MGIT AST might increase the proportion of INH resistance if it was used as a major AST method, compared with Ogawa.

[Repeated tuberculin skin tests in nurse students--observation for 3 years].

In Japan, two-step tuberculin skin test (two-step TST) is recommended for health care workers (HCWs) if the diameter of erythema in the first test is less than 30 mm, and to detect new infection if there is 10 mm or more increase from the base-line diameter. We observed TST in nurse students from the entrance to the graduation for 3 years, and analyzed the change of TST reaction to discuss the usefulness of two-step TST and the criteria for detecting new tuberculous infection among BCG vaccinated HCWs. Mean +/- S.D. (mm) in erythema and induration in each occasion were: in a group with single TST at entrance (T1) and before graduation (TG) (n = 99, group I); T1 = 19.8 +/- 11.3, TG = 23.6 +/- 14.3 for erythema and T1 = 12.5 +/- 5.3, TG = 13.8 +/- 5.0 for induration: in a group with two-step TST at entrance (T1 and T2) (n = 40, group II); T1 = 11.9 +/- 7.8, T2 = 20.4 +/- 10.6, TG = 22.1 +/- 13.5 for erythema and T1 = 8.6 +/- 6.1, T2 = 12.0 +/- 5.1, TG = 13.4 +/- 5.4 for induration: in a group BCG vaccinated after single negative TST (less than 10 mm of erythema and/or less than 5 mm of induration) (n = 12, group I-B); T1 = 4.0 +/- 3.5, TB = 19.8 +/- 5.8, TG = 16.3 +/- 7.6 for erythema and T1 = 0.5 +/- 1.5, TB = 14.1 +/- 3.7, TG = 11.9 +/- 4.4 for induration: in a group BCG vaccinated after two negative TST (n = 10, group II-B); T1 = 3.6 +/- 3.1, T2 = 6.2 +/- 2.4, TB = 23.9 +/- 8.5, TG = 14.1 +/- 6.9 for erythema and T1 = 1.7 +/- 2.7, T2 = 3.2 +/- 1.8, TB = 16.0 +/- 3.2, TG = 7.5 +/- 7.8 for induration. One student in the group II was diagnosed as tuberculosis before the graduation. If we exclude this case form the group II, mean +/- S.D. (mm) of TG in group II were 20.7 +/- 11.2 for erythema and 13.1 +/- 5.0 for induration. Booster phenomenon was significant on two-step TST. There was moderate booster phenomenon even after 34 months from the previous single TST. There was also significant waning of reaction 27 to 31 months after BCG vaccination. But there were no significant waning nor booster after two-step TST. Concerning the difference between the reaction before graduation and at entrance, mean +/- S.D. (mm) in erythema and induration, [TG-T1] in the group I were +3.8 +/- 11.4 and +1.6 +/- 5.8, respectively, [TG-T2 or 1] in the group II were +0.7 +/- 11.4 and +0.4 +/- 5.6, respectively, [TG-TB] in the group I-B and II-B were -6.4 +/- 9.9 and -5.0 +/- 6.5. If we exclude one case who got tuberculosis from the group II, mean +/- S. D. (mm) in erythema and induration of [TG-T2] in group II were -0.6 +/- 8.1 and +0.2 +/- 5.4, respectively. According to the criteria in Japan of more than 10 mm increase in erythema and more than 6 mm increase in induration, recommended by Menzies, significant values of [TG--(T1, T2 or TB)] was observed in 24 (24%) by erythema and 22 (22%) by induration in the group I, while in the group II only in 4 (10%) by erythema and 5 (12.5%) by induration, which included case diagnosed as active tuberculosis. The criterion of 10 mm increase in erythema seems to correspond to that of 6 mm increase in induration. 95% confidential limit in the differences between two tests were 15.6 mm (mean plus 2 standard deviation) in erythema and 11.0 mm in induration in the group II. Considering that these data may include some newly infected persons, appropriate criteria for detecting new tuberculosis infection is estimated to be between 10 to 15 mm increase in erythema and 6 to 11 mm increase in induration from the baseline by two-step TST. Among BCG vaccinated, TST two months after vaccination is useful as the base line. As there is moderate booster phenomenon even after three years from the previous single test and variation is more common, the detection of new tuberculous infection can be made more accurately with the two-step TST in HCWs.