Limitations of First-line Line Probe Assay [FL-LPA]

First Line - Line Probe Assay (FL - LPA) can detect the mutations that are most frequently identified in resistant strains. 

However, there are certain limitations of the test as follows:

1. Some mutations that confer resistance are outside the regions covered by the test. Therefore, resistance cannot be completely excluded even in the presence of all wild type (WT) probes. Thus, in some cases, additional phenotypic Drug Sensitivity Testing (DST) may be necessary to provide a full assessment.
2. Some mutations are identified specifically by mutation (MUT) probes, whereas others are only inferred by the absence of binding of the amplicons to WT probes. This lack of binding of a WT probe, without simultaneous binding of a MUT probe, is likely caused by the presence of a resistance mutation​.
3. Systematic errors are possible due to synonymous and non-synonymous mutations (e.g., phylogenetic mutations). Globally, this is rare (<1% of isolates), but these isolates can be frequent locally​.
4. LPA is less efficient than conventional culture-based methods in finding resistance in samples harbouring both drug-susceptible and resistant bacteria (i.e., hetero-resistance).
   
   1. More specifically, with LPA, it is possible to detect resistant bacteria with mutations detected by the MUT probes, if resistant bacteria represent at least 5% of the total population​.
   2. However, resistant bacteria with mutations inferred by the absence of WT probes would probably be missed, if the resistant population is less than 95% of the total bacterial population​.

Resources

• GenoType MTBDR plus ver 2.0 kit, Instructions for Use.

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