Interpreting Diagnostic Tests for COVID-19
1. PCR: In most individuals with symptomatic infection, viral RNA in a nasopharyngeal swab measured by ΔCt (cycle threshold) becomes detectable as early as day 1 of symptoms and peaks within the first week of symptom onset. The ΔCt is the number of replication cycles required to produce a fluorescent signal, with lower ΔCt values representing higher viral RNA loads. A ΔCt value <40 is clinically reported as PCR positive. PCR positivity starts to decline by week 3 and becomes undetectable subsequently. In severely ill hospitalized patients ΔCt values are lower than in mild cases and PCR positivity may persist beyond 3 weeks when most mild cases will yield a negative result. However, a “positive” PCR reflects only the detection of viral RNA and does not necessarily indicate presence of viable virus. 2. Antibodies: In case series to date IgM and IgG seroconversion occurs in all patients between the third to fourth week of clinical illness onset. Thereafter IgM begins to decline, reaches very low levels by week 5 and almost disappears by week 7, whereas IgG persists beyond 7 weeks. From Frank Hansen‘s 6/5/20 post in COVID-19 Anaesthesia and Intensive Care Experience Sharing Facebook group.