In 2010, 8.8 million new cases of tuberculosis (TB) and 1.4 million TB-related deaths were reported worldwide 
. While the global incidence of TB appears to be decreasing, the emergence of multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB) has significantly complicated TB eradication efforts. MDR-TB strains, as defined by World Health Organization (WHO), are Mycobacterium tuberculosis
) strains resistant to the most effective “first-line” TB medications: isoniazid (INH) and rifampin (RIF). XDR-TB strains are characterized by resistance to INH, RIF (ie MDR-TB) plus any fluoroquinolone (e.g., moxifloxacin), and at least one of the three “injectable” anti-TB drugs: amikacin (AMK), kanamycin (KAN), and capreomycin (CAP) 
. At an estimated 5% of all tuberculosis cases globally, the incidence of MDR-TB recently reported by the WHO is the highest ever recorded 
, and XDR-TB cases have been confirmed in 58 countries to date 
Appropriate use of the “second-line” injectable drugs, AMK, KAN and/or CAP is critical to the effective treatment of MDR-TB and to the prevention of XDR-TB. It is therefore imperative that the MDR-TB strains in patients starting second-line drug treatment are first tested for sensitivity to these drugs to ensure appropriate treatment choices are made and that resistance is not further amplified 
. Conventional diagnosis of drug-resistance in Mtb
strains relies heavily upon mycobacterial culture and drug susceptibility testing in liquid or solid media. While this method is effective for detecting INH and RIF resistance, detecting resistance to AMK, KAN and CAP is more complicated and less reliable 
. Additionally, results are only obtained after weeks to months of incubation and many developing countries lack the resources to establish the stringent laboratory conditions needed for these growth-based methods. From a clinical perspective, the existing growth-based diagnostics also reveal too little, too late, as patients undergoing treatment with drugs to which they are resistant, remain contagious, and those with XDR-TB and HIV often die before they are even diagnosed 
. Rapid molecular diagnostic tests that do not rely upon the culture of this slow-growing pathogen are essential to the future management of M/XDR-TB.
Molecular diagnostics, which focus on the detection of mutations in pathogen genes that have been associated with drug resistance, have shown the most promise for rapid diagnosis of M/XDR-TB 
. Their accuracy, however, is dependent largely on the strength of the association between a specific Mtb
gene mutation and the phenotypic resistance of the isolate with that mutation, which is not always 100%. This relationship between mutation and phenotypic resistance is well established and consistent for the first-line drug RIF 
, and to a lesser extent INH 
, but it is less well-studied and understood for the injectable drugs.
To date, the primary genes associated with injectable drug resistance in Mtb
have been identified based on an understanding of the mechanisms of action of the aminoglycosides (AMK, KAN) and cyclic peptides (CAP) against Mtb
. Both AMK and KAN bind to the 16S rRNA in the 30S ribosomal subunit and inhibit protein synthesis 
. The mechanism of action of CAP in Mtb
is not well understood, but it appears to interfere with translation and inhibit phenylalanine synthesis in mycobacterial ribosomes 
. Mutations in Mtb
, mostly non-synonymous, single nucleotide polymorphisms (SNPs), that prevent the binding of the injectable drugs to their pathogen gene targets or that change the mechanism of action of the drugs, have been associated with drug resistance to AMK, KAN and CAP 
. The most well-studied gene mutations believed to confer resistance to the injectable drugs lie in the rrs
genes. The rrs
gene encodes 16S rRNA 
encodes a 2′-O-methyltransferase that modifies nucleotide C1409 in helix 44 of 16S rRNA and nucleotide C1920 in helix 69 of 23S rRNA 
. The mutations most commonly reported to cause resistance to the injectable drugs include: A1401G, C1402T, and G1484T in the rrs
. More recently, the eis
promoter, which has been demonstrated to enhance the intracellular survival of a related bacterium—Mycobacterium smegmatis
—has also been considered for its utility as a marker for resistance to KAN when mutated 
. Other resistance-associated mutations in these genes and other genes have been proposed 
, but their utility as accurate predictors of resistance has yet to be demonstrated. Consequently, current molecular detection tests for XDR-TB are considered inferior to those used for MDR-TB due to insufficient knowledge about the key mutations responsible for conferring resistance to AMK, KAN and CAP.
We therefore conducted a systematic review of all published studies evaluating Mtb mutations associated with resistance to AMK, KAN, CAP in order to characterize the diversity and frequency of mutations as well as describe the strength of the association between specific mutations and phenotypic resistance in global populations. The objective of our study was to determine the potential utility and reliability of these mutations as diagnostic markers for detecting AMK, KAN and CAP resistance in Mtb.