COVID-19 Antibody Testing

PATHCHAT Edition No. 68
August 2020
Please contact your local Ampath pathologist for more information.

Authors:

• Dr. Cathy van Rooyen (Immunologist)
• Dr. Marieke Brauer (Clinical Virologist)

Introduction

✅ What is SARS-CoV-2 Antibody Testing?

• Determines whether a patient has previously been exposed to SARS-CoV-2, the virus causing COVID-19.
• Not used for diagnosing active infection.
• Helpful in epidemiological research and assessing immune response.

📌 PCR remains the gold standard for diagnosing active COVID-19 infections.

Symptoms of COVID-19

✅ Common Symptoms:

• Fever.
• Cough.
• Difficulty breathing.
• Flu-like symptoms.

✅ Incubation Period:

• 2 to 14 days (median of ~5 days).

📌 Some individuals may have mild or no symptoms while still being infected.

Antibody Response to SARS-CoV-2

✅ Stages of Immune Response:

• Incubation Period: No circulating antibodies.
• Acute Infection:
  • IgM antibodies appear first.
  • IgG follows a few days later.
• Seroconversion (Day 14–16 after symptom onset):
  • IgM and/or IgG detectable in ~90% of infected individuals.

📌 Antibody development varies between individuals and may wane over time.

Indications for SARS-CoV-2 Antibody Testing

✅ Recommended Uses:

1. Epidemiological Research & Community Surveillance:
   • Healthcare settings.
   • Care homes.
   • Workplaces.
2. Outbreak Investigation & Prevention.
3. Limited Clinical Use (≥14 Days After Symptoms Onset):
   • Diagnosing past infection when PCR was negative or not performed.
   • Assessing suspected COVID-19 in children with multisystem inflammatory syndrome (PIMS).

📌 Antibody tests should not be used for diagnosing active infection.

Who Should NOT Be Tested?

❌ Patients With Suspected Active COVID-19 Infection

• PCR is the appropriate diagnostic test for current infection.

📌 Serology cannot confirm active infection and should not replace PCR testing.

Specimen Collection for SARS-CoV-2 Antibody Testing

✅ Sample Requirements:

• Single blood sample (clotted tube – red or yellow top).
• No special handling required.
• Specimens can be collected at any routine Ampath lab or depot (except those within hospital facilities).

📌 Proper sample collection ensures accurate antibody testing results.

Interpretation of SARS-CoV-2 Antibody Test Results

✅ Negative Antibody Test:

• IgG antibodies to SARS-CoV-2 not detected.
• Possible explanations:
  • Test performed too soon after infection (<14 days).
  • Patient had mild/asymptomatic infection and did not develop detectable antibodies.
  • Antibodies have waned (some evidence suggests they decline after ~3 months).

✅ Positive Antibody Test:

• Indicates likely past SARS-CoV-2 infection.
• Does NOT confirm protective immunity or duration of immunity.
• Active infection should still be ruled out using PCR if clinically indicated.

📌 The presence of IgG antibodies does not guarantee long-term immunity against reinfection.

Key Considerations for COVID-19 Antibody Testing

✅ Limitations & Considerations:

• Antibody tests are most reliable when performed ≥14 days after symptom onset.
• False positives are rare but possible due to cross-reactivity with other coronaviruses.
• False negatives may occur if testing is done too early or in patients with weak immune responses.
• Clinical history and PCR testing should guide interpretation of results.

📌 Antibody testing should be used alongside other clinical and epidemiological data.

Contact Information

✅ For Assistance with Interpretation:

• Local pathologists or Ampath Immunology Department (National Reference Laboratory).
• Contact: 012 678 0614/3.

📌 Laboratory professionals are available to assist with result interpretation and clinical decision-making.