Tanis C. Dingle, Ph.D, D(ABMM) - PowerPoint Presentation

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Tanis C. Dingle, Ph.D, D(ABMM) Icahn School of Medicine at Mount Sinai

A Diagnostic Dilemma

Patient HistoryA 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.

Microbial Causes of Acute PharyngitisViral (60%)Rhinovirus

Adenovirus

Coronavirus

Epstein-Barr virus

Other upper respiratory pathogens

Bacterial (10-15%)Group A StreptococcusGroup C and G StreptococcusArcanobacterium haemolyticumFusobacterium nucleatumCorynebacterium diptheriaeNeisseria gonorrheaOthersNon-infectious or Unknown (25%)

Photo Credits: Centers for Disease Control and Prevention and Flickr (NIAID)

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

Photo Credits:

WIkimedia

,

Wikimedia

,

Wikimedia

Throat is swabbed in the area of the

tonsils

Laboratory ResultsThe 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 Credit: Centers for Disease Control and Prevention

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.

Photo Credit:

Wikimedia

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)

Sensitivity of Diagnostic Tests for GAS Pharyngitis

RADT

55-85% sensitive

1,2

Throat Culture

95% sensitive

3

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

Patient OutcomeDue 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.

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.

Photo Provided by Tanis Dingle, Ph.D., D(ABMM)