PATHCHAT Edition No. 74: March 2021
Dr. Marieke Brauer (Clinical Virologist)
Introduction
Overview of COVID-19 Testing
- Since the emergence of SARS-CoV-2 in December 2019, the repertoire of laboratory tests for COVID-19 has expanded.
- Ampath offers multiple COVID-19 testing options for diagnosis, immune response assessment, and travel requirements.
Testing methods are classified into direct virus detection and indirect immune response detection.
Direct Methods: Detecting the Virus Itself
1. SARS-CoV-2 RT-PCR (Reverse Transcription Polymerase Chain Reaction)
- Detects viral RNA (genetic material).
- Gold standard for diagnosing suspected acute COVID-19 infection.
- Required for international travel testing.
- Used for pre-admission and return-to-work screening.
2. SARS-CoV-2 Antigen Test
- Detects viral proteins (antigens).
- Used when RT-PCR is unavailable or turnaround time precludes clinical utility.
- May be used for triaging suspected cases.
RT-PCR is the most sensitive test, while antigen testing is faster but less sensitive.
Indirect Methods: Detecting the Immune Response to the Virus
1. SARS-CoV-2 Nucleocapsid (N) Protein IgG Antibody Test
- Qualitative test to determine prior COVID-19 exposure.
- A small percentage of infected individuals may not develop detectable antibodies.
- Uncertain whether nucleocapsid antibodies correlate with protective immunity.
2. SARS-CoV-2 Spike (S) Protein IgG Antibody Test
- Used to assess antibody response following COVID-19 vaccination.
- Current COVID-19 vaccines contain only spike proteins, so vaccine recipients test negative on N-protein IgG assays.
- Recommended testing at least 14–21 days post-vaccination.
3. SARS-CoV-2 IgM Antibody Test
- Required for travel purposes to China.
- Can only be ordered together with the COVID-19 PCR test for China travel.
Antibody tests do not diagnose current infections but indicate past exposure or vaccination response.
Specimen Types for COVID-19 Tests
1. RT-PCR Test Specimen Collection:
- Nasopharyngeal swab.
- Other respiratory samples (e.g., oropharyngeal swab, nasal swab, sputum, bronchoalveolar lavage).
2. Antigen Test Specimen Collection:
- Nasopharyngeal swab.
3. Antibody Test Specimen Collection:
- Blood sample (SST tube).
Proper specimen collection ensures accurate test results.
Advantages and Disadvantages of COVID-19 Tests
RT-PCR Test
- Advantages:
- Highly accurate and sensitive.
- Gold standard for COVID-19 diagnosis.
- Disadvantages:
- Unable to detect past infections.
- More expensive than antigen tests.
- Takes longer to perform than antigen tests.
- Requires laboratory facilities.
Antigen Test
- Advantages:
- Less expensive than RT-PCR.
- Faster turnaround time.
- Potential for use in point-of-care settings.
- Disadvantages:
- Lower sensitivity than RT-PCR.
- Negative results in symptomatic patients require confirmation via RT-PCR.
- Manual processing limits suitability for mass testing.
Antibody Test
- Advantages:
- Indicates previous exposure to SARS-CoV-2.
- Can help assess immune response after infection or vaccination.
- Disadvantages:
- Not useful for diagnosing current infection.
- Less reliable in the first 14 days of illness.
- Protective immunity from antibodies remains uncertain.
Each test has specific use cases, and no single test is suitable for all situations.
Key Takeaways for Clinicians and Employers
✅ RT-PCR remains the gold standard for diagnosing active COVID-19 infections.
✅ Antigen tests are faster and more affordable but less sensitive than RT-PCR.
✅ Antibody tests assess prior infection and vaccination response but are not used for active infection diagnosis.
✅ Specimen collection and timing are crucial for accurate test results.
✅ Test selection should be based on clinical presentation, travel requirements, or epidemiological studies.
For the latest testing protocols and availability, consult your local Ampath laboratory.