Throat swab

Throat (pharyngeal) swabs are primarily submitted for the diagnosis of Group A streptococcal pharyngitis. If you are interested in therapy of streptococcal pharyngitis look here Tonsilo-pharyngitis therapy.

Occasionally, specific requests may be received for the following:

  • Gonococcal pharyngitis
  • Diphtheria pharyngitis
  • Vincent’s angina
  • Candida pharyngitis (thrush)
  • Viral pharyngitis
  • Mycoplasma pharyngitis
  • Enterovirus culture

If no specific organism or infection is suggested, it should be assumed that the specimen is for the diagnosis of streptococcal pharyngitis only.

Setup

Direct Examination

Not indicated for Group A streptococcus or Neisseria gonorrhoeae

If Vincent’s angina or yeast / thrush is suspected or requested perform Gram stain.

Culture

Media should be stabbed with the inoculation loop in the “well” to enhance the detection of hemolytic reactions.

Media Incubation
Selective Streptococcus Agar (SSA) CO2, 35°C x 24 hours

If GC is requested:

Vancomycin-Colistin-Nystatin Agar (VCN) CO2, 35°C x 72 hours

Interpretation

Gram stain

Examine for presence of pseudohyphae and/or budding yeast in the case of yeast or thrush requests and spirochetes and/or fusiform bacilli and pus cells in the case of Vincent’s angina.

Culture

Examine the SSA plate after 18-24 hours incubation and identify all morphologically distinct beta hemolytic colonies by performing:

  • Gram stain: Gram positive cocci in pairs and chains
  • Strep grouping: Identify only group A, C and G.

For all specimens processed after 1600 hours, re-incubate SSA for a further 24 hours.

If applicable examine the VCN plate after 24, 48 and 72 hours incubation.

Reporting

Gram Stain

Negative
“No yeast seen on direct examination. Yeast culture not performed”
“No organisms suggestive of Vincent’s angina seen”.

Positive
“Yeast seen on direct examination. Yeast culture not performed”.
“Organisms suggestive of Vincent’s angina seen”

Culture

Negative
“No Group A streptococcus isolated”.
“No Neisseria gonorrhoeae isolated” if requested.

Positive
“Group A streptococcus isolated”.

“Group C streptococcus isolated” or “Group G streptococcus isolated”. Include “No Group A streptococcus isolated” and add “Group C and G beta hemolytic Streptococcus can cause pharyngitis but are not associated with rheumatic fever. The disease is usually self limited and needs no antibiotic therapy. If your patient is still symptomatic at the time you receive this report, consideration could be given to antibiotic treatment. Penicillin is the drug of choice.”

Neisseria gonorrhoeae isolated”, beta-lactamase negative or positive.

Telephone all positive N. gonorrhoeae isolates to ward / ordering physician and record in significant isolates log.

References P.R. Murray, E.J. Baron, M.A. Pfaller, R.H. Yolken. 2003. Manual of Clinical Microbiology, 8thed. ASM Press, Washington, D.C.

H.D. Izenberg. 2003. Respiratory Tract Cultures, 3.11.1.1 in Clinical Microbiology Procedures Handbook, 2nd ed. Vol.1 ASM Press, Washington, D.C.

H.D. Izenberg. 2003. Group A Streptococcus Culture, 3.11.8.1 in Clinical Microbiology Procedures Handbook, 2nd ed. Vol.1 ASM Press, Washington, D.C.