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Literature reviews > Articles for review > Chalker et al. External quality assessment for detection... |
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External quality assessment panels enabled the comparison of C. trachomatis detection by different types and manufacturers of assays. External quality assessment for detection of Chlamydia trachomatis. Question How accurate was the detection of C. trachomatis in external quality assessment specimens by participants, different methods (enzyme immunoassay (EIA) and nucleic acid amplification (NAAT)), and commercial assays? Design This article describes a retrospective study to analyze the detection of C. trachomatis in simulated endocervical swab specimens by clinical laboratories participating in the EIA and molecular external quality assessment (EQA) schemes distributed internationally by the United Kingdom National External Quality Assessment Scheme for Microbiology. Participants Ten panels containing four dilutions of either of two strains of C. trachomatis, a laboratory-adapted L2 strain and a primary clinical isolate of unknown serotype, were tested by 91 to 295 participating laboratories predominantly in the United Kingdom but also in 13 other European countries and 4 non-European countries over a 2.5 year period. Description of Tests and Diagnostic Standard The two C. trachomatis strains were grown in McCoy cells, harvested, diluted in Earle's balanced salt solution (L2 strain) or in 5% sucrose (clinical isolate), and 0.5 ml was dispensed into vials.The L2 strain was shipped in liquid form and the clinical isolate was shipped as a freeze-dried preparation, which was reconstituted in 0.5 ml RNase-free water prior to analysis at the testing laboratories.One-tenth ml of reconstituted specimen was added to each laboratory's transport medium and extracted by the usual laboratory routine.One-tenth ml of liquid specimen was examined for C. trachomatis by each laboratory's routine method. The methods included in the category of molecular methods were LCx (Abbott), BDProbeTecET (Beckton-Dickinson), PACE 2 (Gen-Probe), AMP-CT (Gen-Probe), AMPLICOR CT/NG or COBAS AMPLICOR CT/NG (Roche), and an in-house PCR (cryptic plasmid).The methods included in the EIA category were 10 commercial products from 9 manufacturers.Only molecular tests, excluding the LCx, which was withdrawn from the market, were performed on the clinical isolate. Main Outcome Measures The number of participants detecting C. trachomatis was calculated by method per specimen for 22 specimens from 10 panels including 14 positive and 8 negative. Main Results The number of C. trachomatis positive specimens among the total number tested by all EIA methods, all molecular methods, and each of the three most common NAAT for 14 positive and 8 negative specimens is shown in the table.The frequency of accurate detection of C. trachomatis ranged from 32% to 100%.The rate of C. trachomatis detection was significantly higher with molecular methods than with EIA methods (P < 0.0001).All the methods were significantly less likely to detect C. trachomatis at lower concentrations (<500 EB/ml).For detection of the L2 strain, the BDProbeTecET method was less likely to give a positive result at concentrations of 833 EB/ml or less and the LCx was less likely to give a positive result at concentrations of 400 EB/ml or less compared to the AMPLICOR method.For detection of the clinical strain, the BDProbeTecET and the AMPLICOR methods were equally likely to give a positive result. Number of C. trachomatis positive specimens reported by laboratories participating in the EQA by method used
1L2 strain 2clinical isolate 3Negative specimen 4Not tested Authors' Conclusions Laboratories using EIA were significantly less likely to report detection of C. trachomatis than those using the molecular methods.The BDProbeTecET method was less likely to detect lower concentrations of the L2 strain than the AMPLICOR method.The rate of false positive results by all methods was low. Source of funding:None given For correspondence:V. J. Chalker, UK NEQAS for Microbiology, Quality Assurance Laboratory, Health Protection Agency Centre for Infections, 61 Colindale Ave., London, NW9 5HT, United Kingdom.E-mail address: Vicki.chalker@hpa.org.uk |
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