PATHCHAT Edition No. 76
Published: 5 November 2021
Please contact your local Ampath pathologist for more information.
Author:
- Dr. Marieke Brauer (Clinical Virologist)
Introduction
✅ The Role of Diagnostic Testing in COVID-19
- COVID-19 diagnostic testing is crucial in preventing viral spread.
- RT-PCR remains the gold standard for SARS-CoV-2 detection.
- However, RT-PCR is expensive, requires laboratory facilities, and has long turnaround times.
- There is an increasing demand for faster, cost-effective, and near-patient diagnostic tests.
✅ COVID-19 Antigen Testing:
- Detects SARS-CoV-2 proteins in respiratory secretions.
- Mostly formatted as rapid diagnostic test (RDT) kits for near-patient use.
- Less sensitive than RT-PCR, meaning false negatives are possible.
- Most effective from two days before symptoms onset to 5–7 days after onset, when viral load is highest.
📌 Over 200 different SARS-CoV-2 antigen tests are available, with varying accuracy. WHO recommends a minimum of 80% sensitivity and 97% specificity compared to RT-PCR.
COVID-19 Antigen Testing Algorithms
✅ Importance of Pre-Test Probability in Interpretation
- Pre-test probability affects the reliability of antigen test results.
- Recommendations for confirmatory RT-PCR testing depend on the setting.
✅ High Pre-Test Probability (High Prevalence Settings)
- Examples: Symptomatic individuals in areas with established community transmission.
- Interpretation:
- Positive antigen test: Considered a true positive, no further confirmation required.
- Negative antigen test: Requires RT-PCR confirmation due to the risk of false negatives.
✅ Low Pre-Test Probability (Low Prevalence Settings)
- Examples: Asymptomatic individuals undergoing screening (e.g., port of entry, workplace, event testing).
- Interpretation:
- Negative antigen test: No confirmatory RT-PCR needed.
- Positive antigen test: Requires RT-PCR confirmation due to the risk of false positives.
📌 Antigen tests are best used in symptomatic individuals with high pre-test probability. In asymptomatic individuals, confirmatory RT-PCR is recommended for positive results.
COVID-19 Antigen Testing Process
✅ Sample Collection & Handling
- Antigen tests use respiratory specimens (same as RT-PCR tests).
- Collection should follow infection prevention and control (IPC) protocols.
- Correct sample handling, storage, and processing are essential for reliable results.
- Trained personnel must perform and interpret antigen tests to avoid misdiagnosis.
✅ Specimen Collection & Biohazard Safety
- Healthcare workers collecting samples must wear PPE.
- Once the sample is placed in the buffer tube, the virus is inactivated.
- No advanced biohazard laboratory equipment is required for antigen testing.
📌 Proper sample collection and test execution improve result accuracy.
Reporting of COVID-19 Antigen Test Results
✅ Mandatory Reporting Requirements in South Africa
- All SARS-CoV-2 antigen test results (positive and negative) must be reported to the National Institute of Communicable Diseases (NICD).
- Any laboratory or facility outside the National Health Laboratory Service (NHLS) must comply with this reporting requirement.
- COVID-19 is a notifiable medical condition—clinicians must report all positive cases to the NICD.
📌 Accurate reporting ensures effective COVID-19 surveillance and response planning.
Ampath Antigen Testing Solution
✅ Ampath’s SARS-CoV-2 Antigen Testing Services
- Ampath is an ISO 15189:2012 accredited laboratory, ensuring high-quality testing.
- Trained personnel collect samples at Ampath depots, workplaces, or alternate sites.
- COVID-19 antigen testing is available on-site or off-site, tailored to customer needs.
- All antigen tests used by Ampath have:
- South African Health Products Regulatory Authority (SAHPRA) approval.
- Passed internal validation according to WHO standards.
✅ Automatic RT-PCR Confirmation Available
- Clients can request antigen testing with or without automatic confirmatory RT-PCR.
- Confirmatory RT-PCR is generally recommended for:
- Positive antigen results in low pre-test probability scenarios.
- Negative antigen results in high pre-test probability scenarios.
✅ Electronic Test Ordering & Reporting
- Ampath provides electronic test ordering and reporting for improved efficiency.
- All antigen test results are automatically reported to the NICD.
📌 Ampath offers comprehensive, accredited antigen testing solutions, including electronic reporting and on-site services.
Key Takeaways for Clinicians & Employers
✅ Antigen tests detect viral proteins and provide rapid results.
✅ RT-PCR remains the gold standard but has longer turnaround times.
✅ Antigen tests are best used in symptomatic individuals during the first week of illness.
✅ Pre-test probability must be considered when interpreting results.
✅ Positive antigen tests in asymptomatic individuals should be confirmed by RT-PCR.
✅ Ampath’s antigen testing service includes SAHPRA-approved tests, on-site sample collection, and automatic NICD reporting.
📌 For antigen testing enquiries or corporate screening solutions, contact Ampath Laboratories.