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- DFA, direct fluorescent antibody
- EIA, enzyme immunoassay
- LPS, lipopolysaccharide
- MOMP, major outer membrane protein
- NAATs, nucleic acid amplification tests
- PCR, polymerase chain reaction
- SDA, strand displacement amplification
- TC, tissue culture
- TMA, transcription mediated amplification
Nucleic acid amplification tests
The role of the nucleic acid amplification technology in the routine diagnosis of Chlamydia trachomatis infections is evolving rapidly. Three commercial assays are now available for routine use:
Polymerase chain reaction (PCR; Roche Diagnostics)
Strand displacement amplification (SDA; Becton Dickinson)
Transcription mediated amplification (TMA; GenProbe).
Although these commercial assays differ in their target sequence and their method of amplification, it is their ability to produce a positive signal from theoretically a single copy of the target DNA or RNA (see pack inserts from the kit manufacturers) that has led to the reported increased sensitivity of nucleic acid amplification tests (NAATs).1 Similar to other non-culture tests, NAATs do not require viable organisms.
With the advent of molecular diagnostic technology, it is now appreciated that no single test provides 100% sensitivity and specificity. Currently, NAATs are proving to be the best tests on the market. There is no room for complacency, however, as further work is required to eliminate test problems, such as inhibitors, contamination,2 reproducibility,3 and hormonal factors4 that have played a part in lowering sensitivity.
Confirming positive NAATs by another technique
Only another NAAT is sensitive enough to confirm a positive result.5 This approach needs further evaluation, as it is rare that individual laboratories will be able to offer more than one NAAT platform.
Retest the original sample (according to manufacturer’s instructions).
Inhibitors can be identified from all sites, in particular first void urine. An internal amplification control to identify inhibition should be used and is available using some of the commercial kits. The Gen-Probe TMA test has a stage in the extraction process that the manufacturer claims removes the majority of inhibitors and therefore no inhibitory control is needed (see individual manufacturer’s instructions).
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