1. A Diagnostic Dilemma

2. Patient History
A 28-year old male presents to the Emergency Department (ED) with a one-day history of sore throat and fever.
Upon examination, tonsillar swelling is noted without pus.
A rapid antigen detection test (RADT) for Group A Streptococcus (GAS) performed in the ED is negative.
A throat swab is collected for culture.
The patient is sent home with instructions for symptomatic relief of a presumed viral pharyngitis.
The next day the patient returns to the ED with worsening sore throat and difficulty swallowing. Pus is now seen on the tonsils and the uvula is deviated towards one side of his throat.

3. Microbial Causes of Acute Pharyngitis
Viral (60%)
Rhinovirus
Adenovirus
Coronavirus
Epstein-Barr virus
Other upper respiratory pathogens
Bacterial (10-15%)
Group A Streptococcus
Group C and G Streptococcus
Arcanobacterium haemolyticum
Fusobacterium nucleatum
Corynebacterium diptheriae
Neisseria gonorrhea
Others
Non-infectious or Unknown (25%)
Photo Credits:

4. Processing of Throat Cultures for Bacterial Pathogens
Throat swab is sent to the microbiology laboratory and plated to blood agar
Plates incubated aerobically at 35°C for 24 to 48 hours
A trained microbiologist examines the plates for common bacterial causes of pharyngitis
Add a picture of rapid method.
Photo Credits:

5. Laboratory Results
The clinical microbiologist notes β-hemolytic colonies on the blood agar plate after 24 hours incubation
A Gram stain of a colony reveals Gram-positive cocci growing in long chains
The organism is catalase negative and susceptible to the antibiotic/biochemical bacitracin
Photo Credits:

6. Diagnosis
Peritonsillar abscess caused by Group A Streptococcus (Streptococcus pyogenes)
The final diagnosis could only be made by performing throat culture in the microbiology laboratory since the rapid antigen test result was falsely negative

7. Potential Complications of Group A Streptococcal Pharyngitis
Suppurative (pus)
Non-Suppurative
Peritonsillar abscess
Acute rheumatic fever
Lymphadenitis
Acute glomerulonephritis
Sinusitis
Otitis Media
Mastoiditis
Invasive infections (e.g. toxic shock syndrome, necrotizing fasciitis)

8. Sensitivity of Diagnostic Tests for GAS Pharyngitis
RADT
55-85% sensitive1,2
Throat Culture
95% sensitive3
Lower sensitivity of RADT indicates false negative results are not uncommon
Reflexive culture of specimens with negative RADT results is recommended for diagnosing GAS
Since RADT is highly specific for GAS, specimens with positive results do not need to be cultured
1 Uhl, JR., et al J Clin Microbiol 41:
2 Ruiz-Aragon, J., et al Anales de Pediatria 72:
3 Bisno, AL NEJM 344:

9. Patient Outcome
Due to the false negative RADT, the patient was initially sent home without antimicrobial treatment
A throat culture performed by the microbiology laboratory led to the diagnosis of Group A streptococcal pharyngitis
Upon receipt of the culture results, the physician contacted the patient for follow up
The patient’s peritonsillar abscess was drained and the patient was treated for 10 days with penicillin
The patients symptoms completely resolved with this course of treatment

10. Tanis C. Dingle, Ph.D, D(ABMM)
Dr. Dingle is an Associate Professor in the Department of Pathology at the Icahn School of Medicine at Mount Sinai and Co-Assistant Director of Microbiology for the Mount Sinai Health System in New York City. Dr. Dingle is a Diplomate of the American Board of Medical Microbiology and trained in the CPEP program at the University of Washington in Seattle. Her research interests include antimicrobial resistance and the application of MALDI-TOF mass spectrometry in the clinical microbiology laboratory.