Real-time reverse transcription polymerase chain reaction
(rRT-PCR) of a salivary positive sample. (a)
Schematic illustration of an rRT-PCR result. The
amplification curve for positive samples follows a sigmoid
trend (i.e., the relative fluorescence intensity
increases, with an exponential middle tract, until a
plateau phase). No increase in fluorescence is observed
when the sample is negative. The threshold is placed so to
intersect the amplification curves at the beginning of the
exponential tract. The cycle threshold (Ct) represents the
cycle number at which the amplification curve intersects
the threshold line and is an indicator of the quantity of
the amplified target gene. The lower the Ct value, the
higher the amount of the target gene and then the viral
load. (b) An example of amplification curves
in log scale for a salivary sample that
tested positive for the presence of all 3 genes associated
with SARS-CoV-2 (E, N, and
RdRp). The internal control (IC),
whose viral load is known, is used as comparison to
quantify the viral load of the sample.