Pharyngitis

Accurate diagnosis of pharyngitis (sore throat) is crucial for appropriate treatment and the prevention of complications. The laboratory plays a vital role in identifying the causative pathogens, especially those requiring specific therapies, like antibiotics

General Principles

• Key Agents: Pharyngitis is often caused by viral infections, but bacterial infections require specific therapies. Streptococcus pyogenes (Group A Streptococcus, GAS) is the most significant
• Specimen Source: Throat swabs are the standard
• Rapid Testing and Culture: Often combines rapid testing (e.g., rapid antigen tests) with culture for enhanced sensitivity
• Contamination: The URT contains normal flora, and thus specimens need to be interpreted with care
• Rapid Reporting: Prompt communication of results is essential to improve patient outcomes

Major Pathogens: Colony Morphology, Gram Stain, and Identification

Streptococcus pyogenes (Group A Streptococcus, GAS)

• Source: Throat swab
• Colony Morphology
  • BAP: Small, translucent colonies with a large zone of beta-hemolysis (complete clearing of red blood cells)
• Gram Stain: Gram-positive cocci in chains
• Identification
  • Catalase: Negative
  • Rapid Antigen Test: For GAS
  • Bacitracin Susceptibility: Bacitracin-susceptible. Zone of inhibition around the bacitracin disk
  • PYR Test: Positive
  • Latex Agglutination: (Lancefield Group A test). Confirmatory testing
  • Serologic testing

Neisseria gonorrhoeae

• Source: Throat swab (less common)
• Colony Morphology
  • CHOC: Small, gray, non-hemolytic, translucent colonies
  • Appearance: Can be mucoid
• Gram Stain: Gram-negative diplococci (kidney bean-shaped)
• Identification
  • Oxidase: Positive
  • Carbohydrate Utilization: Glucose positive
  • Rapid Tests: Can be used
  • Molecular Testing: Nucleic acid amplification tests (NAATs) on the swab

Arcanobacterium haemolyticum

• Source: Throat swab
• Colony Morphology
  • BAP: Small, gray, beta-hemolytic colonies
  • Appearance: Can resemble GAS
• Gram Stain: Gram-positive, pleomorphic rods
• Identification
  • Catalase: Negative
  • Reverse CAMP Test: Positive. Enhances hemolysis when streaked perpendicular to Staphylococcus aureus
  • Other biochemicals

Corynebacterium diphtheriae

• Source: Throat swab
• Colony Morphology
  • Tinsdale Agar: Gray to black colonies
  • Loeffler’s medium: Granular appearance
• Gram Stain: Gram-positive rods
• Identification
  • Catalase: Positive
  • Toxigenicity Test: To confirm toxin production. Important, because only toxin-producing strains cause diphtheria
  • Other biochemicals

Bordetella pertussis

• Source: Nasopharyngeal swab
• Colony Morphology
  • Bordet-Gengou Agar (BGA): Small, smooth, gray-white colonies
  • Regan-Lowe Agar: Small, gray-white, “mercury drop” appearance
• Gram Stain: Gram-negative coccobacilli, very small
• Identification
  • Culture: Slow growth
  • Rapid tests: Direct fluorescent antibody (DFA)
  • PCR: Molecular testing

Additional Considerations

• Testing Strategy: In many labs, a rapid antigen test for GAS is the primary method
  • Culture: If the rapid test is negative, culture is needed to help identify a patient with GAS
• Viral Etiology: Most sore throats are caused by viruses, which the laboratory does not routinely test for. Treat symptomatically
• Report Findings: A clear report is needed
• Test limitations
  • Many pathogens are difficult to culture. PCR can be helpful

Key Terms

• Pharyngitis: Inflammation of the pharynx (sore throat)
• Streptococcus pyogenes (Group A Streptococcus (GAS)): The main cause of strep throat
• Throat Swab: A specimen from the back of the throat
• BAP: Blood agar plate
• CHOC: Chocolate agar plate
• Gram Stain: Staining technique to differentiate bacteria
• Beta-Hemolysis: Complete lysis of red blood cells
• Rapid Antigen Test: Test to detect the presence of an antigen
• Bacitracin Susceptibility: Antibiotic test to determine the bacterial susceptibility
• PYR Test: Rapid test for detecting the enzyme pyrrolidonyl arylamidase
• Lancefield Grouping: Test for the capsular polysaccharides
• Neisseria gonorrhoeae: Gram-negative bacteria that can infect the throat
• NAATs: Nucleic acid amplification tests (PCR)
• Arcanobacterium haemolyticum: Gram-positive bacteria, can cause sore throat
• Reverse CAMP Test: A test to determine hemolysis when streaked next to S. aureus
• Corynebacterium diphtheriae: Gram-positive bacteria that causes diphtheria
• Tinsdale Agar: Selective media to culture C. diphtheriae
• Bordet-Gengou Agar (BGA): Selective medium for Bordetella pertussis
• Regan-Lowe Agar: Another medium for Bordetella pertussis
• Bordetella pertussis: Gram-negative bacteria that causes whooping cough
• DFA: Direct fluorescent antibody testing
• Incubation: Temperature and environment of bacterial growth