Laboratory and Testing
This section contains discussion and/or specific guidelines for
the following topics:
Laboratory Testing Methods
There are a number of
laboratory methodologies available to test for Chlamydia trachomatis
(Ct). Cell culture of inoculate prepared from urogenital specimens was
considered the gold standard until recently. Culture test should still be used
for cases of suspected sexual abuse or assault, but newer technologies are now
available and preferable for large scale screening activities. Antigen and
nucleic acid detection technologies are now widely used in public health
settings. Newer nucleic acid amplification tests are rapidly gaining favor as
evidence is accumulating that these tests can be even more sensitive than
culture. Screening programs using nucleic acid amplification tests have found
Ct prevalence rates to be higher than previously
believed.
Compared to culture,
antigen and nucleic acid detection tests are less expensive, faster to process,
and have fewer specimen transport restrictions. Although these are distinct
advantages to these tests, distinct limitations have also been noted. These
tests can be less specific than culture and may produce false-positive results.
The accuracy of these tests is highly dependent on correct specimen collection.
All of the Region VI public
health laboratories are currently using the Gen-Probe® PACE 2, nucleic acid
detection test. However, as the nucleic acid amplification tests become more
affordable and some of the transport issues are resolved, it is possible that
these newer technologies will supplement or supplant existing technologies.
Essentially, the
amplification technologies are designed to extract DNA from client specimens and
to copy the DNA sequence enough times to render the DNA easily detectable by a
variety of methods.
Sensitivity and specificity
are both extremely high due to the nature of the amplification. Currently, the
costs associated with amplification technologies are quite high, but industry
competition is rapidly lowering prices available to public health laboratories.
Amplification tests can have useful applications in situations where urine can
be collected but physical exams may be difficult to undertake, for example, in a
school based screening program.
A laboratory test must be
proven to possess the necessary sensitivity and specificity to give valid,
usable results. Sensitivity is defined as
the ability of a test to detect infection if it is present, i.e., to
correctly classify infected individuals as positive. A highly sensitive
test gives few false-negatives. Specificity is
defined as the ability of a tests to detect absence of infection if it is NOT
present, i.e., to correctly identify uninfected individuals as negative. A
test with high specificity gives few false-positives.
Interfering substances may
give false negative reactions to a small percentage of amplification tests.
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Specimen Rejection Criteria - Gen-Probe
An essential component of quality assurance is the
establishment of criteria for rejection of Ct specimens of poor or substandard
quality. The following rejection criteria are utilized by the laboratory staff
to determine when and why specimens must be rejected for testing. Rejection
criteria also give guidance to submitters to help them improve collection and
handling at the clinic site.
- No Client Identification on Specimen or
Mismatched Identification on Specimen and Laboratory Requisition Form.
Collection directions and federal regulations require writing the client’s
name and other unique identifier, on the specimen collection tube as well as
on the test requisition form. The names or unique identifiers must match
exactly between the specimen tube and the requisition form.
-
Expired Transport Medium.
The expiration date of each collection kit is printed on the outside of
the kit wrapper and on each collection tube. Specimens will not be tested if
the collection kit has expired.
-
No Swab in Tube.
The
male collection kit contains one cotton-tipped swab. The female kit contains
two swabs. The first female swab is used to remove excess mucous and is
discarded. The second is used to collect the specimen and is broken off at
the scored area in the collection tube. The swabs, from both male and female
kits, must be left in the collection tube for transport to the laboratory.
-
Wrong
Swab Used. The system is only approved for use with swabs supplied
in the collection kits. These swabs have been tested and will not alter the
biochemical reactions of the test. Wooden swabs or other non-Gen-Probe swabs
are not acceptable.
-
Wrong Kit Used.
The
collection kits are different for males and females. The male kit contains
0.5 ml of fluid. The female kit contains 1.0 ml of fluid to make up for the
fluid that is soaked up by the larger female swab. Using the wrong tube or
swab changes the proportion of fluid to specimen and may give erroneous
results.
-
Improper Collection Site.
The Gen-Probe PACE 2 system is only designed to test conjunctival, female
endocervical and male urethral specimens. It is not recommended for
medico-legal cases and is not acceptable for throat or rectal swabs or for
antimicrobial susceptibility testing. Additionally, vaginal specimens
including those collected from hysterectomized women are not suitable for
testing.
-
Quantity Not Sufficient (QNS).
There is not enough specimen to perform the testing. It may have leaked
out in transit or the tube may have broken. Sometimes the specimen is very
mucoid, an indication that the collection area was not swabbed clean prior to
collection, or the swab has disintegrated and the specimen cannot be pipetted.
-
Specimens Greater than 7 Days Old.
The system is approved for use on specimens up to seven days after
collection. Testing after seven days may result in invalid results. It is
therefore imperative that the date of collection be provided on the
requisition form.
-
Specimens Too Bloody for
Testing. Grossly bloody specimens may interfere with the
performance of the PACE 2 or PACE 2C systems, therefore, specimens containing
greater than 80 Fl per
ml of blood are not accepted for testing.
-
Specimens received on the following individuals should be
flagged for inclusion of a disclaimer
if
tested for C. trachomatis using non-culture methods:
According to CDC guidelines, only cell
culture methods that employ C. trachomatis-specific antibodies should
be used to detect chlamydial infections in the investigation of a possible
sexual abuse or assault case.
Non-culture tests like the Gen-Probe PACE 2
are not sufficiently specific to be used in the investigation of sexual
abuse or assault for legal purposes.
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Turnaround Times
Laboratory
results should be reported to the clinic as rapidly as possible so that
immediate follow-up and client management can be initiated. Turnaround time
should be monitored periodically to determine if there are delays in specimen
transport to the laboratory, testing and reporting of laboratory results, or
delays in reports reaching the clinics. A reasonable turnaround time within the
laboratory is three working days after the date of receipt of specimen in the
laboratory.
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Summary
A culture test should be used for cases of
suspected sexual abuse or assault.
The accuracy of all laboratory tests depends on
correct specimen collection and handling.
Laboratories are required to reject specimens
that do not meet essential elements of laboratory quality assurance standards.
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Case Study
Case 5: A 19 year old female presents with lower
abdominal pain and fever. She is currently menstruating.
Question:
Can this client be tested? Will blood affect the accuracy of the Gen-Probe
test?
Case 6:
You receive a phone call from the laboratory telling you that the swab used for
the specimen was not from the Gen-Probe collection kit.
Question:
Will this affect the test result? Why or Why not?
Case 7:
Your lab person tells you that the doctor collected a chlamydia swab late Friday
and put it in the freezer to be mailed out Monday morning.
Question:
Can this specimen be tested?
(Case Study answers found in Appendix)
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NM Department of Health Family Planning Program
