An Overview of PCR Testing

How virus RNA is isolated and identified

Basic RT-PCR Testing

Testing for COVID-19 involves a polymerase chain reaction (PCR) assay*, which attaches fluorescent markers to genetic material from the virus and generates millions of copies to create a fluorescent signal if the virus is present. 

Testing Process

  1. A throat or nasal swab is taken,  designed to capture virus genetic material. 
    • SARS-CoV-2 is an RNA virus, so its genetic material is more transient and fragile than DNA. 
    • Samples should ideally be transported to testing laboratories on ice or in special media to prevent them from degrading. 
  2. Once at the lab, the RNA must first be converted to DNA using an enzyme called reverse transcriptase.
  3. Then, specific sequences of DNA (primers) designed to recognize complementary virus sequences are added
  4. Another enzyme is added —usually a modified form of Taq polymerase—which can make a copy of a short length of viral DNA. 
  5. This process (3 and 4) is repeated for 20-30 cycles, exponentially amplifying the amount of viral DNA so that it can be detected. 
Different tests target different parts of the SARS-CoV-2 genome, and the World Health Organization (WHO) has issued their own protocol specifying recommended primers and procedures.