Currently, there are various biomarkers and methods available to detect SARS Coronavirus Type-2 (SARS-CoV-2) in biological specimen, the virus that caused the Coronavirus Disease 2019 (COVID-19). The biomarkers, namely the Viral RNA, Viral Antigen proteins and the Antibodies (IgM & IgG) that produce from the body as an immunological response to the viral infection. The chart below is an illustration of the biomarker levels during the typical time course of COVID-19 infection.
Detection of Viral RNA
- Detecting the RNA nuclei acids of SARS-CoV-2 is recommended on the Envelope (E) gene, which it also shared with other subgenus of sars-beta-coronaviruses (eg. SARS-CoV and Animal’s SARS-related CoV) as first line screening, and confirm with SARS-CoV-2 specific gene, Open Reading Frame 1ab (ORF1ab) gene in RNA-dependent RNA polymerase (RdRP) region. In some protocols, also confirm with Nucleocapcid (N) gene.
- Currently the “gold standard” method (best sensitivity & specificity) is employing Reverse-Transcriptase Polymerase Chain Reaction (rt-PCR) amplification of the viral RNA. It provides the best detection limit and able to detect the infection as earliest as around 2-4 days of infection. However, the whole process needs about 4 – 6 hours.
- Another more rapid nuclei acids amplification method is Isothermal Amplification (AMP), able to produce result within 15 – 80 minutes (depends on system).[iii] However, throughput is much lesser (1 or 2 samples at a time).
Detection of Viral Antigen
- Viral Antigen proteins is another biomarker that detect directly on the virus. However, it is unable to amplify, thus higher concentration level of the virus is required for detection. Current available method is rapid chromatographic immunoassay (commonly called Antigen rapid test kit, Ag RTK). It is an aid for early diagnosis of COVID-19 with patients with clinical symptoms.
- It can be use in a Laboratory or at the Point-of-Care (POC) with Biosafety Cabinet (BSC) as safety requirement. Tested Positive and those tested Negative cases but symptoms persist patient, require to confirm with rtPCR test on new specimen.
Detection of Antibodies
- Generally, body will response with immunological responses after the infection and produce Immunoglobulin M (IgM) in acute phase (onset of symptoms) and Immunoglobulin G (IgG) in convalescence phase. However, Antibody rapid test kit (Ab RTK) usually require a high concentration of antibodies for detection, it only sensitive for patients after 3 ~ 5 days after onset of symptoms (7 ~ 10 days after infection). Thus, currently Ab RTK is not recommended for screening or detection of asymptomatic carriers and person under investigation (PUI).
- However, antibodies testing may be of value when combine with rtPCR test as diagnostic tool. With high throughput system, it is suitable for large scale, whole-population, testing to assess the overall immune response to the virus and as a reference for health policy making. [iii] Laboratory based high throughput system with Enzyme Immunoassay (EIA, ELISA) method or Chemiluminescent Immunoassay (CLIA) method is currently available (But not in Malaysia yet). EIA and CLIA methods have better sensitivity and specificity then Ab RTK.