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Repeat testing of
urine samples with positive or near-cutoff results by the ligase
chain reaction assay for the detection of Chlamydia trachomatis
DNA does not ensure accuracy of results.
Accuracy of results
obtained by performing a second ligase chain reaction assay and PCR
analysis on urine samples with positive or near-cutoff results in
the LCx test for Chlamydia trachomatis.
Castriciano S, Luinstra K, Jang D,
Patel J, Mahony J, Kapala J, Chernesky M.
Journal of Clinical
Microbiology 2002;40:2632-2634.
Summary:
Question
Are the results obtained using the repeat testing algorithm developed by
the manufacturer of the LCx Chlamydia assay similar to the results
obtained by extracting new aliquots from the original urine sample and
testing with the LCx assay and with the AMPLICOR PCR test?
Design
This study describes a comparison of the LCx Chlamydia ligase chain
reaction assay, performed by following the manufacturer's repeat testing
algorithm, with the same assay run one time on a new aliquot of the same
sample, and with a PCR assay for the detection of C. trachomatis
DNA in urine samples.
Participants
One hundred twenty (59 men and 61 women) out of 1040 urine specimens
tested by the LCx Chlamydia assay for which the results fulfilled the
criteria for retesting described by the manufacturer were included in the
analyses for this study.
Description of Tests and Diagnostic
Standard
Urine samples were tested by the LCx Chlamydia ligase chain reaction assay
(Abbott Laboratories, Abbott Park, IL) and, according to the
manufacturer's instructions, were selected for repeat testing using the
same extracted DNA sample if the signal-to-cutoff ratio was greater than
or equal to 0.80. If the ratio by the repeat test was greater than or
equal to 1.00, the sample was considered positive. For comparison, a new
aliquot of each of the urine samples selected for repeat testing was
extracted and tested by the LCx assay. If the signal-to-cutoff ratio was
greater than or equal to 1.00, the sample was considered positive. An
second new aliquot of each of the urine samples selected for repeat
testing was extracted and tested by the AMPLICOR PCR Chlamydia test (Roche
Diagnostic Systems, Branchburg, NJ) according to the manufacturer's
instructions.
Main Outcome Measures
A comparison of the results of testing 120 urine samples by 3 different
protocols was determined.
Main Results
The results of the 120 urine samples that were selected for repeat testing
by ligase chain reaction and PCR amplification are presented in the table.
Eighteen (15%) and 72 (60%) were negative and positive, respectively, by
all the repeat tests, leaving 30 (25%) specimens with discordant results
among the tests. Twenty of the 30 discordant samples had discordant
results between the two repeat LCx assays. All of the 72 samples positive
by all three repeat tests had initial signal-to-cutoff ratios that were
>=1.00.
| Results
of repeat LCx and PCR testing of 120 urine samples having
signal-to-cutoff ratios greater than or equal to 0.80 when
originally tested by LCx |
| Repeat
Assay Result |
Number
of samples with indicated results |
| LCx
on original sample extraction |
LCx
on newly extracted aliquot |
PCR
on newly extracted aliquot |
| - |
- |
- |
18 |
| + |
+ |
+ |
72 |
| + |
+ |
- |
9 |
| - |
- |
+ |
1 |
| - |
+ |
+ |
3 |
| + |
- |
- |
12 |
| + |
- |
+ |
2 |
| - |
+ |
- |
3 |
| Total
positive=95 |
Total
positive=87 |
Total
positive=78 |
Total
samples=120 |
Authors' Conclusions
Repeat testing of the initially extracted
samples by the LCx Chlamydia assay would have enabled reporting of
positive results for 95 specimens, of which some may have been
false-positive results based on the results of testing new aliquots by LCx
(87 positive) and PCR (78 positive).
Source of funding: Not given.
For correspondence:M. Chernesky,
Father Sean O'Sullivan Research Centre, St. Joseph's Healthcare, McMaster
University, 50 Charlton Ave. East, Hamilton, Ontario L8N 4A6, Canada.
E-mail address: chernesk@mcmaster.ca.
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