Question
How accurate is the rapid immunochromatographic antigen detection assay, NOW Gonorrhea Test, for the diagnosis of gonococcal infection using urine samples compared to standard urethral culture performed on specimens from men with urethritis?
Design
The results of the NOW Gonorrhea Test, a rapid immunochromatographic antigen detection assay, performed on urine samples were compared to those of standard culture of urethral specimens from men with urethritis.
Participants
Fifty-eight male outpatients enrolled in routine screening for STIs at the Department of Urology, Toho University Omori Hospital in Tokyo were tested.
Description of Tests and Diagnostic Standard
Two urethral swab specimens and a urine specimen (30 mL) were collected from each patient. The first swab was immediately streaked onto a Thayer-Martin agar plate, the second swab was used to make a Gram stain. The urine was centrifuged and the pellet streaked onto Thayer-Martin plates. The cultures were incubated at 35oC in 5% CO2 for 24 h and colonies were identified using the Haemophilus-Neisseria identification kit (Modified ID-Test HN-20, Nissui Pharmaceutical, Tokyo, Japan). An aliquot of urine was stored at =80oC and used later for the NOW Gonorrhea Test (Binax Inc., Portland, ME) performed according to the manual provided with the kit. The procedure included suspending the pellet from 2 mL of urine in the kit Reagent A, inserting the test strip, and observing the reading zone after a 10 min incubation at room temperature. A sample was positive if a sample line appeared below the control line. The limit of detection of the NOW assay was evaluated by testing suspensions containing concentrations of N. gonorrhoeae ranging from 103 to 108 cfu/mL. The influence of sample storage conditions on test results was evaluated by testing duplicate sets of 10-fold dilutions of a positive urine specimen that were stored at either room temperature or 4oC by culture and the NOW Test at 0, 3, 6, 9, 21, 33, and 45 h after collection.
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the NOW Gonorrhea Test compared to urethral culture performed on specimens from 58 men with urethritis were determined.
Main Results
The results of the NOW Gonorrhea Test performed on urine and of culture of urethral swabs for 58 men with urethritis are shown in the table. Of the 34 patients positive for N. gonorrhoeae by urethral culture, 3 were positive by urine culture. The sensitivity, specificity, PPV, and NPV of the NOW Test compared with urethral culture were 94.1%, 95.8%, 96.9%, and 92.0%, respectively. The level of sensitivity of the NOW Test was 5 X 104 cfu/mL. N. gonorrhoeae was detected by NOW in all dilutions (up to 10-5) of the urine specimen incubated at either 4oC or room temperature for up to 45 h. In contrast, N. gonorrhoeae was detected by culture only in urine diluted up to 10-3 and held at room temperature for 6 h and in urine diluted 10-2 and held at 4oC for 3 h.
Results of NOW Gonorrhea Test on urine and culture on urethral swab specimens obtained from 58 men with urethritis
|
NOW Gonorrhea Test
|
Urethral culture
|
|
Positive
|
Negative
|
Total
|
|
Positive
|
32
|
1
|
33
|
|
Negative
|
2
|
23
|
25
|
|
Total
|
34
|
24
|
58
|
Authors' Conclusions
The NOW Test performed on urine specimens from men with urethritis was highly sensitive and specific compared to urethral culture. The procedure is simple and the results can be available within 25 min. Another advantage in that the specimen can be stored at room temperature for up to 45 h without affecting detection of gonococcal antigens.
Source of funding: Binax Inc., Portland, ME, provided test kits.
For correspondence: Tetsuya Matsumoto, Department of Microbiology, Toho University School of Medicine, 5-21-16 Omori-nishi, Ota-ku, Tokyo 143-8540, Japan. E-mail address:
tetsu@med.toho-u.ac.jp.
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