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Background
Isoniazid (INH) resistant Mycobacterium tuberculosis (Hr-TB) is the most common type of drug resistant TB, and is defined as M tuberculosis complex (MTBC) strains resistant to INH but susceptible to rifampicin (RIF). Resistance to INH precedes RIF resistance in almost all multidrug resistant TB (MDR-TB) cases, across all MTBC lineages and in all settings. Therefore, early detection of Hr-TB is critical to ensure rapid initiation of appropriate treatment, and to prevent progression to MDR-TB. We assessed the performance of the GenoType MTBDRplus VER 2.0 line probe assay (LPA) in detecting isoniazid resistance among MTBC clinical isolates.
Methods
A retrospective study was conducted among M. tuberculosis complex (MTBC) clinical isolates obtained from the third-round Ethiopian national drug resistance survey (DRS) conducted between August 2017 and December 2019. The sensitivity, specificity, positive predictive value, and negative predictive value of the GenoType MTBDRplus VER 2.0 LPA in detecting INH resistance were assessed and compared to phenotypic drug susceptibility testing (DST) using the Mycobacteria Growth Indicator Tube (MGIT) system. Fisher’s exact test was performed to compare the performance of LPA between Hr-TB and MDR-TB isolates.
Results
A total of 137 MTBC isolates were included, of those 62 were Hr-TB, 35 were MDR-TB and 40 were INH susceptible. The sensitivity of the GenoType MTBDRplus VER 2.0 for detecting INH resistance was 77.4% (95% CI: 65.5–86.2) among Hr-TB isolates and 94.3% (95% CI: 80.4–99.4) among MDR-TB isolates (P = 0.04). The specificity of the GenoType MTBDRplus VER 2.0 for detecting INH resistance was 100% (95% CI: 89.6–100). The katG 315 mutation was observed in 71% (n = 44) of Hr-TB phenotypes and 94.3% (n = 33) of MDR-TB phenotypes. Mutation at position-15 of the inhA promoter region alone was detected in four (6.5%) Hr-TB isolates, and concomitantly with katG 315 mutation in one (2.9%) MDR-TB isolate.
Conclusions
GenoType MTBDRplus VER 2.0 LPA demonstrated improved performance in detecting INH resistance among MDR-TB cases compared to Hr-TB cases. The katG315 mutation is the most common INH resistance conferring gene among Hr-TB and MDR-TB isolates. Additional INH resistance conferring mutations should be evaluated to improve the sensitivity of the GenoType MTBDRplus VER 2.0 for the detection of INH resistance among Hr-TB cases.

背景：耐异烟肼（Isoniazid, INH）结核分枝杆菌（Hr-TB）是最常见的耐药结核病类型，特指对异烟肼耐药但对利福平（rifampicin, RIF）敏感的结核分枝杆菌复合群（Mycobacterium tuberculosis complex, MTBC）菌株。几乎所有耐多药结核病（multidrug resistant TB, MDR-TB）病例中，对异烟肼的耐药均先于利福平耐药出现，且这一规律在所有MTBC谱系及所有流行场景中均成立。因此，早期检出Hr-TB对于及时启动合理治疗、防止病情进展为MDR-TB至关重要。本研究评估了GenoType MTBDRplus VER 2.0线性探针测定法（line probe assay, LPA）在MTBC临床分离株中检测异烟肼耐药的性能。
方法：本研究为回顾性研究，研究对象为2017年8月至2019年12月开展的埃塞俄比亚第三次全国耐药性调查（drug resistance survey, DRS）中获得的MTBC临床分离株。以分枝杆菌生长指示管（Mycobacteria Growth Indicator Tube, MGIT）系统的表型药物敏感性试验（drug susceptibility testing, DST）为参照标准，评估GenoType MTBDRplus VER 2.0 LPA检测异烟肼耐药的灵敏度、特异度、阳性预测值及阴性预测值，并采用Fisher精确检验比较LPA对Hr-TB与MDR-TB分离株的检测性能。
结果：本研究共纳入137株MTBC分离株，其中62株为Hr-TB、35株为MDR-TB、40株为异烟肼敏感株。GenoType MTBDRplus VER 2.0检测异烟肼耐药的灵敏度在Hr-TB分离株中为77.4%（95%置信区间：65.5~86.2），在MDR-TB分离株中为94.3%（95%置信区间：80.4~99.4），组间比较差异具有统计学意义（P=0.04）。该方法检测异烟肼耐药的特异度为100%（95%置信区间：89.6~100）。在表型鉴定为异烟肼耐药的分离株中，71%（n=44）的Hr-TB株及94.3%（n=33）的MDR-TB株存在katG 315位点突变。仅inhA启动子区域-15位点突变的情况在4株（6.5%）Hr-TB分离株中检出，另有1株（2.9%）MDR-TB分离株同时存在该突变与katG 315突变。
结论：相较于Hr-TB病例，GenoType MTBDRplus VER 2.0 LPA在MDR-TB病例中检测异烟肼耐药的性能更优。katG315突变是Hr-TB及MDR-TB分离株中最常见的异烟肼耐药相关基因变异。为提升GenoType MTBDRplus VER 2.0 LPA对Hr-TB病例异烟肼耐药的检测灵敏度，需进一步评估其他可介导异烟肼耐药的基因突变。