DETECTION OF VIRAL RNA (RT-PCR tests)
- RT PCR targets the following genes
- env – envelope
- N – nucleocapsid
- S – spike
- RdRp – RNA-dependent RNA polymerase
- ORF1 genes
- Cycle threshold
- number of replication cycles required to produced a fluorescent signal
- lower Ct values represent higher viral RNA loads
- Ct value <40 clinically reported as PCR positive
- Ct values in severely ill patients are lower than mild cases
- (+) PCR result reflects only detection of the viral RNA, not necessarily indicate presence of viable virus
- Sn/Sp
- RT-PCR positivity
- BAL specimen (93%)
- sputum (72%)
- nasal swab (63%)
- pharyngeal swab (32%)
- Specificity is 100%
- RT-PCR positivity
DETECTION OF ANTIBODIES
- most sensitive and earliest serological maker is TOTAL ANTIBODIES, which increase from 2nd week of symptom onset
- IgM and IgG ELISA have been found (+) as early as Day 4 of symptom onset
- higher levels in second and third week
- IgG and IgM seroconversion occurred in all patients between third and fourth week
- IgM begins to decline and reach lower levels by week 5 and disappears by week 7
- IgG persists beyond 7 weeks
- >95% specificity for diagnosis of COVID-19
- Sn/Sp
- majority of Ab are produced against NC (most abundant protein of the virus), tests to detect Ab would be most sensitive
- receptor-binding domain of S (RBD-S) protein is the host attachment protein, would be more specific and expected to be neutralizing
References:
Sethuraman, N., Jeremiah, S., & Ryo, A. (2020). Interpreting Diagnostic Tests for SARS-CoV-2. JAMA. doi: 10.1001/jama.2020.8259
Holmes, K. (2003). SARS-Associated Coronavirus. New England Journal Of Medicine, 348(20), 1948-1951. doi: 10.1056/nejmp030078