GasDirect test

The GasDirect test for group A Streptococcus is a DNA probe assay that uses nucleic acid hybridization for the qualitative detection of group A streptococcal RNA. This direct throat swab test eliminates the time and difficulty of culturing group A Streptococcus, while delivering high sensitivity and specificity.

About

American Academy of Pediatrics guidelines recommend that all negative rapid antigen assays be backed up by culture or a test that is equivalent to culture. The Hologic GasDirect test can be used as both backup to negative rapid antigen assays or as a standalone test.

Because the signs and symptoms of viral and bacterial infections are broadly similar, accurately diagnosing these upper respiratory tract illnesses is the key that allows physicians to properly treat their patients without contributing to the further development of antibiotic resistant bacteria. The GasDirect test is documented as being comparable to culture in terms of cost and assay performance, yet it offers an improved turnaround time of 24 hours versus 72 hours for culture.

The GasDirect test from Hologic provides the following:

  • Fast, accurate diagnosis same day results vs. 2-3 days for culture.
  • Higher sensitivity than rapid direct antigen tests.
  • Accuracy equivalent to standard culture.
  • User-friendly non-isotopic test format, helping reduce technician training.

The benefits to users include:

  • Streamlined workflow with a simple 3-step procedure.
  • Batch-testing enabled.
  • Decreased technician time.
  • Automation options available for higher-volume labs.

 

 

Performance Data

The within-run precision of the GasDirect group A Streptococcus direct test was calculated by assaying two concentrations of ribosomal RNA isolated from Streptococcus pyogenes and a single concentration of S. pyogenes cells using 10 replicates in a single assay.
 

1. Within-run Precision*      
  High Level RNA Low Level RNA Cells
Number of Replicates 10 10 10
Mean Response (RLU) 28,412 12,387 5,851
Standard Deviation 1,201 411 103
Coefficient of Variation 4.2% 3.3% 1.8%

*Please see package insert for detailed information.

 

The between-run precision was calculated by assaying one concentration of Streptococcus pyogenes ribosomal RNA and two suspensions of Streptococcus pyogenes per assay. Each of three operators ran one replicate in each of four separate runs.

2. Between-run Precision*    
  High Level RNA Low Level RNA
+ Swab
Cells
+ Swab
Number of Replicates 12 12 12
Mean Response (RLU) 22,830 6,937 87,076
Standard Deviation 1,900 517 11,157
Coefficient of Variation 8.3% 7.5% 12.8%

*Please see package insert for detailed information.

 

A clinical reproducibility study was conducted at three sites using coded materials for inoculation onto swabs. All sites tested two coded swabs for each of the variables listed below. Two replicate hybridization assays were run on each swab.

3. Clinical Reproducibility Study    
Swab Inoculum Intersite Mean RLU
(N=12)
S.D. % CV
Lysis Reagent 1,882 406 21.6
Throat Swab Material Only 731 416 24.0
Low Group A Cells Level #1 14,253 2,785 19.5
Medium Group A Cells Level #2 49,457 12,934 26.2
Medium Group A Cells Level #1 97,045 21,236 21.9
High Group A Cells Level #2 182,806 38,006 20.8
High Group A Cells Level #1 608,513 172,252 28.3
Negative Control 1,146 313 27.4
Positive Control 38,581 2,392 6.2

*Please see package insert for detailed information.

 

Regulatory

The GasDirect test was FDA-cleared for in vitro diagnostic use in 1994.

For additional availability in other countries beyond the U.S., please contact your local sales representative or distributor.

References
  1. Chapin, K.C., P. Blake, and C.D. Wilson. 2002. Performance characteristics and utilization of rapid antigen test, DNA probe and culture for detection of group A streptococcus in an acute care clinic.Clin Microbiol. 40:4207-4210.