inclusion of pza enables considerable shortening of the treatment period from the previously 912 months to 6 months , thus the drug plays a pivotal role in the current short - course chemotherapy for drug susceptible tb . the powerful sterilizing activity of pza is due to its ability to kill a population of persister tubercle bacilli that are not killed by other tb drugs . studies in the mouse model of tb showed that substitution of pza , but not inh and rif , invariably led to poorer treatment outcomes . furthermore , the synergistic activity of pza with newly developed agents such as the diarylquinoline bedaquiline suggests that the use of pza in regimens including novel agents could improve efficacy substantially , if the organism retains susceptibility to pza . there is fairly good evidence from animal and human studies that the treatment duration of z - mdr - tb can be shortened to a minimum of 9 months with a regimen comprising pza accompanied by two bactericidal drugs . mccune et al . demonstrated in the mouse model that murine tb could be better sterilized with pza plus a companion drug , especially a bactericidal one . in the treatment of drug - susceptible tb , the 2-year relapse rates of 9-month regimens comprising streptomycin , inh and pza given daily or intermittently were only 5%6% . a small retrospective study suggested that inclusion of pza in the treatment regimen was associated with a favorable outcome . a recent observational study among second - line treatment - naive mdr - tb patients suggested that the treatment duration of mdr - tb could be shortened to a minimum of 9 months with a gatifloxacin - based regimen that contained pza and clofazimine throughout with kanamycin , high - dose inh and prothionamide given for at least 4 months in the initial phase . although the impressive treatment outcome was partially attributed to clofazimine by conjecture , interpretation of findings could have been confounded by pza susceptibility , which was not checked and might be present in at least 31% of the study sample according to a systematic review . the feasibility of shortening tb treatment for selected mdr - tb patients was further corroborated by a recent report . subsequent retrospective analysis with the same updated dataset suggests that pza use with in vitro activity alongside later - generation fqs and slid may considerably increase the proportion with three - month sputum culture conversion , and marginally increase that with two - year treatment success . however , the above results are preliminary and future prospective studies are required to assess the possibility that pza , alongside two or three bactericidal agents might improve the treatment of z - mdr - tb . despite the potential importance of pza resistance in mdr - tb treatment outcome , standard phenotypic pza susceptibility testing is seldom performed owing to technical challenges . this also explains why pza resistance data are generally unavailable in tb drug resistance surveys . however , a number of studies have demonstrated a high prevalence of pza resistance among mdr - tb patients in different localities , ranging from 10% in papua new guinea and 25% in turkey , to 49% in thailand , 50% in central africa , 52% in south africa , 53% in japan , 55% in taiwan , 77% in pakistan and 85% in south korea and in india . bacillary resistance to pza is generally higher in xdr - tb than mdr - tb cases , ranging from 72% in chongqing , china ( zhang wh , unpublished ) , 86% in south korea , to 93% in fq - resistant pre - xdr - tb in cambodia . the reason for the high pza resistance rates in many high - burden areas may be partly related to widespread use of pza in retreatment regimens without drug susceptibility guidance or maybe false resistance . it is also possible that some of the above studies that reported very high pza resistance among mdr - tb overestimated the pza resistance frequency due to false resistance . however , the xdr - tb studies that reported very high pza resistance were all based on molecular test of pnca sequencing rather than conventional pza susceptibility testing . jensen medium and 7h10/11 agar at ph 5.5 , bactec 460 and mgit 960 or bact / alert systems at ph 6.0 . however , pza susceptibility testing is prone to errors , which arise from : ( i ) acidity of the medium required for pza activity inhibits the growth of mycobacterium tuberculosis about 20%25% of clinical isolates do not grow on acidic 7h10 plates ( ph 5.5 ) , and even with ph 6.0 in bactec 460 liquid medium , 3.5% of the strains did not grow ; and ( ii ) use of too large an inoculum ( over 10  bacilli / ml ) leads to increase in medium ph , which then inactivates pza . in a recent study , the mgit 960 pza susceptibility testing method was found to be even less reliable than the radioactive bactec 460 method giving rise to more false resistant results , presumably due to the larger inoculum used in the mgit method . the authors suggested retesting of pza - resistant strains by the  gold standard ' bactec 460 method and pnca sequencing of pza - resistant strains identified by the mgit method . automated pza susceptibility testing methods , including the bactec 460 method , are not exempt from false resistance owing to the use of either a lower resistance breakpoint ( 100  g / ml ) or an inadvertently large inoculum . hasselbalch equation , the minimum inhibitory concentration ( mic ) cutoff for pza resistance should be at least 156  g / ml , rather than 100  g / ml , which is the currently used breakpoint for pza resistance in mgit 960 or bactec 460 . to circumvent the above problems , use of nicotinamide at high concentrations ( 0.52  mg / ml ) at neutral ph has been proposed as a surrogate method for pza susceptibility testing in acidic lowenstein  jensen medium , with promising results . the nicotinamide test can be used potentially as an inexpensive alternative for pza susceptibility testing in clinical microbiology laboratories , but it has a long ( several weeks ) turnaround time . the pzase enzyme test ( the wayne test ) , using pzase as a surrogate of pza susceptibility may also give rise to false resistance , due to the need for a sufficiently large inoculum that inevitably increases its turnaround time . mutation in the pnca gene encoding pzase is the major mechanism for pza resistance in m. tuberculosis . although a lower percentage of pnca mutations in pza - resistant strains , i.e. 64% and 72% , has been reported , these studies did not retest pza - resistant strains without pnca mutations to rule out false resistance . because of the problem of pza susceptibility testing discussed above , the lower percentage of pza - resistant strains , i.e. , 64% and 72% , with pnca mutations is most likely due to false resistance , lack of vigorous retesting to rule out false resistance , the low resistance breakpoint ( 100  g / ml pza in bactec or mgit ) used , or the small number of strains analyzed in the study . a recent systematic review with meta - analysis showed no significant difference between pnca sequencing and the wayne pzase test by sensitivity and specificity in detecting pza resistance , indicating good correlation between pnca mutations and lack of pzase activity and pza resistance . in analysis of pza - resistant strains , pnca mutations were found in an average of 87% of pza - resistant strains and sometimes in as high as 99% of pza - resistant strains . however , some studies suggest that a few pza susceptible strains have pnca mutations that do not appear to alter the pzase enzyme activity , indicating that false resistance can potentially occur by the sequencing approach . in addition , a few pz - resistant strains with no pzase activity did not have pnca mutations , indicating a potential regulatory gene of pnca that may have acquired a mutation . a few genuine low level pza - resistant strains do not have pnca or rpsa mutations . however , the above three situations are rare and do not pose a significant problem for use of pnca sequencing for rapid detection of pza susceptibility or resistance . nevertheless , it would be of interest to develop a database of rare mutations that are not associated with pza resistance to guide clinical treatment . in view of the good correlation of pnca mutations and pza resistance . the extremely diverse pnca mutations that are impossible to be included in current molecular tests such as mtbdrplus ( hain lifescience ) and genexpert ( cepheid ) and new advances in sequencing technology and increasing affordability of dna sequencing , we propose pnca sequencing as the best available molecular test for rapid pza susceptibility testing . although various molecular tests such as pcr single stranded conformation polymorphism ( pcr - sscp ) , microarray , expression of pnca protein followed by pzase activity testing , and line - probe assay have been used to detect pnca mutations in pza - resistant strains , these tests are generally more onerous and expensive than pnca sequencing . as phenotypic pza susceptibility testing is prone to false resistance , pnca sequencing can be more sensitive and specific than the bactec 460 or mgit 960 method . indeed , a recent study showed a disturbingly low sensitivity of mgit 960 pza susceptibility testing in comparison with the molecular test owing to a high false resistance rate of 68% . clinical studies comparing these two tests and the molecular test with treatment outcome are needed . in the area of drug - resistant tb , emphasis has previously been focused on inh and rif resistance as in mdr - tb . in addition , in the management of mdr - tb , attention has been focused on the use of fqs and slid . however , in view of the potentially important role of pza in treatment outcome of mdr - tb , its unique sterilizing activity , and a considerable proportion of mdr - tb strains that are susceptible to pza ( about 50% resistance on average ) , we propose to classify mdr - tb based on pza susceptibility into z - mdr - tb and pza - resistant mdr - tb ( z - mdr - tb ) ( figure 1 ) . this classification may allow z - mdr - tb treatment to be shortened without compromising cure rates and also will improve evaluation of treatment outcomes of novel regimens in observational studies . because of the good correlation between pnca mutations and pza resistance , we further propose to use molecular tests such as sequencing of pnca ( and fq and slid mutations , e.g. , gyra , rrs ) to rapidly identify z - mdr - tb and z - mdr - tb with backup phenotypic tests to guide therapy . moreover , we propose to sequence the pnca gene for all drug - resistant tb , including mdr / xdr - tb , even inh- or rif - resistant tb . as pza may cause hepatotoxicity , it may be prudent to omit pza in the treatment of z - mdr - tb . finally , with the implication of pza susceptibility on treatment outcome of mdr - tb in human studies and its superior sterilizing activity , we suggest actively exploring a simple and shortened treatment regimen for z - mdr - tb ( possibly 915 months ) comprising pza plus at least two bactericidal agents including new agents like tmc207 or pa-824 or delamanid as companion drugs , with one or two other agents . the above measures may potentially help to shorten therapy , protect against development of resistance to pza , reduce costs and ameliorate side effects in mdr - tb treatment . future clinical studies are needed to validate these propositions for better mdr - tb treatment .
indispensable for shortening treatment of drug - susceptible tuberculosis ( tb ) , pyrazinamide ( pza , z ) is also essential in the treatment of multidrug - resistant ( mdr)-tb . 
 while resistance to pza in mdr - tb is associated with poor treatment outcome , bacillary susceptibility to pza along with the use of fluoroquinolone ( fq ) and second - line injectable drugs ( slids ) may predict improved treatment success in mdr - tb . despite a high prevalence of pza resistance among mdr - tb patients ( 10%85% ) , pza susceptibility testing 
 is seldom performed because of technical challenges . to improve treatment of mdr - tb 
 , we propose to : ( i ) classify mdr - tb into pza - susceptible mdr - tb ( zs - mdr - tb ) and pza - resistant mdr - tb ( zr - mdr - tb ) ; ( ii ) use molecular tests such as dna sequencing ( pnca , gyra , rrs , etc . ) to rapidly identify zs - mdr - tb versus zr - mdr - tb and susceptibility profile for fq and slid ; ( iii ) refrain from using pza in zr - mdr - tb ; and ( iv ) explore the feasibility of shortening the treatment duration of zs - mdr - tb with a regimen comprising pza plus at least two bactericidal agents especially new agents like tmc207 or pa-824 or delamanid which the bacilli are susceptible to , with one or two other agents . these measures 
 may potentially shorten therapy , save costs , and reduce side effects of mdr - tb treatment .