Læknablaðið - 15.07.1995, Blaðsíða 25

LÆKNABLAÐIÐ 1995; 81
539
Table I. Results of the three different tests from 203 patients.
Positive True positive False positive False negative True negative
Culture 34 34 0 5 164
PCR cx 38 36 2 3 162
PCR urine 37 37 0 2 164
All 3 tests 32 39 164
Table II. Calculations on the performance of three different tests.
Culture PCR cx swabs PCR urine
Sensitivity 87 92 95
Specificity 100 98 100
Predictive value of positive 100 95 100
Predictive value of negative 97 98 98
both PCR tests positive but in that case a con-
firmatory test with MOMP primers had to be
positive. Percent infected was 19.2% but if
Chlamydia culture was used as a definition of
infection only 16.7% were positive. The results
and calculations on the performance of the
three different tests are shown in tables I and
II. Fifteen of the 34 culture positive patients
had no symptoms and 16 of 34 PCR cx/urine
positive had no symptoms. Twenty three of
those positive had white, purulent or transpar-
ent cervical discharge but 12 were normal on
examination. Only one Chlamydia positive pa-
tient had always used a condom, five used it
often 13 used it sometimes and 15 never. No
difference in the use of condoms, was found
between those positive in the PCR only and
those positive in culture. Forty percent of
those infected but only 28% of those not in-
fected had two or more sexual partnerns dur-
ing the three months prior to the study.
Discussion
Because collection of samples for the diag-
nosis of Chlamydia infections has often caused
discomfort and sometimes humiliation for the
female patients, new methods, where exam-
ination is not required, have definate advan-
tages. Collecting a urine sample is in most
instances considered acceptable to the patient.
The PCR urinary diagnostics have been used
for males in our clinic for over one year with
very satisfying results and have been well ac-
cepted by our patients. It can be argued that
omitting examination in the clinic may cause
other diseases, like genital warts, to be missed.
This is of some concern and examination of
men and women is strongly recommended,
whenever possible, regardless of which meth-
od is used for diagnosing Chlamydial infec-
tions.
Amplicor® PCR has been shown to be more
sensitive than the old gold standard, cell cul-
ture (7,8). It is difficult to resolve the issue of
“false positive" Amplicor® PCR tests because
of the lack of a standard sensitive enough.
Using PCR for the MOMP gene is not quite
satisfactory because it is slightly less sensitive
than the Amplicor® PCR. The reason for this
is that the Amplicor® PCR detects plasmid
genes, of which there may be multible copies in
each Chlamydia cell. The MOMP gene, on the
other hand, resides on the chromosome of
which there is only one copy per cell. The two
“false positive" tests from the cervix may
therefore indeed have been true positives.
The results show that Amplicor® PCR per-
formed on female urine is more sensitive and
as specific as cell culture. The prevalence of
isolated urethral asymptomatic C. trachomatis
in the absence of cervical infection has been
shown to be up to 24% (11,12). This could
partly explain the higher sensitivity of urinary
PCR. This urinary test can be used to test and
screen asymptomatic females where other
methods like cervical swabs for culture and
PCR can be difficult to apply. Obviously,
screening of asymptomatic populations like
pupils of schools was difficult or impossible
due to reluctance to undergo an examination
where an urinary test is more acceptable. This
opens new venues, in screening asymptomatic
populations for C. trachomatis, that are now
being explored (13).