Question
How well does a new, rapid, point-of-care test perform when compared to wet preparation and culture for the detection of T. vaginalis infections in women?
Design
The performance of a new rapid test for detection of T. vaginalis proteins was compared to T. vaginalis culture on vaginal swab specimens from women attending STD clinics in Seattle, WA and Birmingham, AL.
Participants
Nine hundred thirty-six women attending STD clinics in Seattle, WA (n = 497) and Birmingham, AL (n = 439) who were at least 14 years old and were scheduled to receive a pelvic examination were tested. The mean ages of the women were 29 and 28 years in Seattle and Birmingham, respectively; 51.3% of Seattle women compared to 11.4% of Birmingham women identified themselves as white; and 47.3% of Seattle women compared to 72.5% of Birmingham women reported vaginal discharge, itching, or dysuria.
Description of Tests and Diagnostic Standard
After collection of a vaginal swab for wet preparation microscopy, two additional swabs were collected in random order from vaginal fluid pooling in the lateral fornices for the rapid assay and for culture (InPouch, Biomed, San Jose, CA). The swab for wet preparation was mixed with saline. A drop was placed on a slide and examined under a microscope at X200 magnification. The cultures were processed according to the manufacturer‚s instructions and examined for trichomonad morphology and motility after approximately 24 h and then every other day after inoculation for up to 7 days.
The swab for the rapid test, XenoStrip (Xenotope Diagnostics, Inc., San Antonio, TX), which uses color immunochromatographic capillary flow dipstick technology to detect the presence of T. vaginalis antigens, was stored at 4oC and processed within 24 h of collection. The swab was placed into buffer, mixed for 1 min, and the liquid expressed into the tube. The XenoStrip test strip was placed into the specimen buffer and read after 10 min. Initially negative specimens were read again after 20 min. The strip utilizes immunoglobulin G1 monoclonal antibodies XDTv1 and XDTv2 to bind intracellular and surface secretory proteins. Antigen-antibody binding results in a red line on the test strip; a second red line serves as an internal positive control for test performance.
In Seattle, two technicians who were blinded to the results of the other tests, read the rapid tests and cultures. In Birmingham, three nurses who were not blinded to the results of the other tests, read the rapid tests and wet preparations, while laboratory staff, who were blinded, read the cultures.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the XenoStrip test for detection of T. vaginalis in vaginal swabs from women in Seattle and Birmingham as determined by culture were calculated.
Main Results
The prevalence of T. vaginalis by culture was 8.7% (43 of 497) and 21.0% (92 of 439) in Seattle and Birmingham, respectively. The performances of the XenoStrip and wet preparation compared to culture, by city and in total, are shown in the table. The XenoStrip assay sensitivity varied by the number of days until T. vaginalis was first detected by culture. Of 94 specimens positive by culture at day 1, 88 (93.7%) were positive by XenoStrip, while 2 (22.2%) of 9 specimens positive by culture at 4 or more days were positive by XenoStrip (P<0.001). The performance of the XenoStrip did not vary by the presence of symptoms, other syndromes or pathogens, or by swab order. Of 135 women with positive cultures, 38 had a negative or missing wet preparation. Of these 38, 11 had a positive XenoStrip result. In contrast, XenoStrip incorrectly identified as positive 11 culture-negative women (1 in Seattle and 10 in Birmingham).
Performance of XenoStrip rapid test and wet preparation for the detection of T. vaginalis infections in 497 women in Seattle and 439 women in Birmingham as determined by culture
|
T. vaginalis assay
|
Specimen collection site
|
Test performance (%)
|
|
Sensitivity
|
Specificity
|
PPV
|
NPV
|
|
XenoStrip
|
Seattle
|
76.7
|
99.8
|
97.1
|
97.8
|
|
Birmingham
|
79.4
|
97.1
|
87.9
|
94.7
|
|
Total
|
78.5
|
98.6
|
90.6
|
96.5
|
|
Wet preparation
|
Seattle
|
66.7
|
100
|
ND*
|
ND
|
|
Birmingham
|
75.0
|
100
|
ND
|
ND
|
|
Total
|
72.4
|
100
|
ND
|
ND
|
*not determined
Authors‚ Conclusions
The XenoStrip assay performed as well or better than wet preparation microscopy and makes rapid T. vaginalis identification available in settings without microscopy. In low-prevalence populations, further evaluation of the specificity should be considered before widespread use can be recommended.
Source of funding: Xenotope Diagnostics, Inc., San Antonio, TX
For correspondence: Ann Kurth, Center for AIDS and STDs, Box 359931, 325 Ninth Ave., Seattle, WA 98104. E-mail address:
akurth@u.washington.edu
.gif){kind=small}