Use of Swabs without Transport Media for the Gen-Probe Group A Strep Direct Test
For several years we used rayon or Dacron swabs with liquid transport media for collection and transport of throat swab specimens for testing with the Gen-Probe Group A Strep Direct Test (GASDT). A report of favorable results with a Dacron swab without any transport media for GASDT by another laboratory prompted us to compare detection of group A streptococci (GAS) with and without transport media (referred to as “wet” and “dry” swabs, respectively). Phase one of this study used swabs seeded with GAS. Initially, six recent clinical isolates of GAS were inoculated onto wet and dry swabs and stored at room temperature (RT). After 1, 2, and 3 days of storage, colony counting and GASDT were performed with the swabs. The results, expressed as the mean percentage of the results at zero time, were as follows: for GASDT with wet swabs at 1, 2, and 3 days, 62, 51, and 56%, respectively; for GASDT with dry swabs at 1, 2, and 3 days, 105, 80, and 85%, respectively; for colony counts with wet swabs at 1, 2, and 3 days, 52, 26, and 13%, respectively; for colony counts with dry swabs at 1, 2, and 3 days, 10, 0, and 0%, respectively. An additional six strains of GAS were tested in a similar manner, except that extracts of pharyngeal flora (PF) were added to the inocula. The results obtained with extracts of PF were comparable to those obtained with GAS alone. We also compared the performance of GASDT with wet and dry swabs stored at RT and 4°C. Ten strains of GAS were inoculated onto wet and dry swabs, and GASDT was performed each day for 9 days. The GASDT results for swabs on day 9, expressed as the mean percentage of the results obtained at zero time, were as follows: dry swab and 4°C, 59%; wet swab and 4°C, 31%; dry swab and RT, 33%; and wet swab and RT, 19%. In phase two of this study we conducted a clinical evaluation to determine whether the differences observed with seeded specimens would also be evident with patient specimens.
We used a single dry Dacron swab paired with a single rayon Bacti-Swab with liquid Stuart transport medium for the clinical evaluation. Specimens were collected from 1,005 outpatients, plated onto a Strep Selective Agar plate, and then tested within 30 min by GASDT. If culture of GAS from the same swab is used to define a true-positive test result, the sensitivities and specificities were as follows: GASDT with wet swabs, 86.2 and 98.5%, respectively; GASDT with dry swabs, 90.7 and 98.1%, respectively. However, the use of culture as the “gold standard” may understate the actual performance characteristics of GASDT, particularly for the dry swabs. In conclusion, for GASDT the use of swabs without transport media may be preferable to the use of swabs with transport media.