Molecular Methods
Antigen detection and molecular methods have revolutionized the diagnosis of upper respiratory tract (URT) infections, offering significant advantages over traditional culture-based techniques, particularly for specific pathogens such as Streptococcus pyogenes (GAS) and Bordetella pertussis. These methods provide rapid results, which is critical for treatment and preventing the spread of contagious diseases
General Principles
- Rapid Diagnosis: Fast results are important. These tests give results in a matter of minutes or hours
- Enhanced Sensitivity and Specificity: PCR is very good
- Utility: Helpful when culture may be slow
- Antimicrobial Stewardship: Guides appropriate antibiotic use
- Specimen Types: Throat swabs, NP swabs
- Limitations: Cost, possible false negatives
- Quality Control: Necessary procedures
Antigen Detection Methods
0.8.1 **Streptococcus pyogenes (Group A Streptococcus, GAS)} {-}
- Purpose: Rapid detection of GAS antigens from throat swabs
- Principle: Antibodies react with GAS antigens
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Types of Tests
- Lateral Flow Immunoassays: Simple, results in minutes. (e.g., immunochromatographic assays)
- Enzyme Immunoassays (EIAs): More sensitive and can be automated
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Procedure
- Specimen Preparation: Throat swab is collected
- Testing: Follow the manufacturer’s instructions
- Result Interpretation: A positive result indicates the presence of GAS antigens, and that treatment is needed
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Advantages
- Rapid, easy to perform
- Relatively inexpensive
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Disadvantages
- Lower sensitivity than culture
- False negatives
- Clinical Application: Use to rule in GAS pharyngitis, and help with treatment
Molecular Methods
0.8.2 **Streptococcus pyogenes (Group A Streptococcus, GAS)} {-}
- Purpose: Rapid and sensitive detection of Streptococcus pyogenes DNA
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Method
- PCR: Amplifies specific regions of the S. pyogenes genome
- Real-time PCR: Quantifies the amount of DNA
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Procedure
- Specimen Preparation: DNA is extracted from the throat swab
- Amplification: PCR is performed
- Detection and result interpretation
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Advantages
- High sensitivity and specificity
- Rapid results
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Disadvantages
- More expensive
- Requires specialized equipment
- Clinical Application: Used in difficult cases. Confirm the diagnosis
Bordetella pertussis
- Purpose: Detection of B. pertussis DNA
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Method
- PCR: The specific genes are amplified
- Real-time PCR: Quantifies the amount of DNA
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Procedure
- Specimen Preparation: DNA is extracted from the NP swab
- Amplification: PCR is performed
- Detection and result interpretation
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Advantages
- High sensitivity and specificity
- Rapid results
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Disadvantages
- Cost, expertise
- Clinical Application: Used in difficult cases. Confirm the diagnosis
Additional Considerations
- Specimen Collection: Collection is critical. Proper collection helps with an accurate diagnosis
- Interpretation of Results: The result must be looked at in the clinical context
- Reporting: Prompt reporting is needed
- Limitations: These tests have specific limitations
Key Terms
- Antigen: A substance that triggers an immune response
- Antibody: A protein produced by the immune system that recognizes and binds to a specific antigen
- Antigen Detection Test: A test that detects the presence of a specific antigen
- Lateral Flow Immunoassay: A simple test
- Enzyme Immunoassay (EIA): The use of enzymes
- PCR (Polymerase Chain Reaction): Amplifies a specific DNA region
- Real-Time PCR (qPCR): Used to quantify PCR products
- Nucleic Acid Extraction: The process of isolating DNA or RNA from a sample
- Sensitivity: Ability to correctly identify a pathogen
- Specificity: Ability to correctly identify the absence of a pathogen
- Streptococcus pyogenes (Group A Streptococcus (GAS)): The main cause of strep throat
- PCR: Polymerase chain reaction
- NP swab: Nasopharyngeal swab
- Bordetella pertussis: The bacteria that causes whooping cough
- Antimicrobial Therapy: The use of antibiotics