Biomedical Engineering Reference
In-Depth Information
unspecific signal. To exclude that possibility, the minimal threshold setting used
is 0.2. The maximum threshold of 0.675 still gives good data, although the reac-
tion is at the end of the exponential phase and efficiencies are decreasing.
22.
At DNA concentrations that result in a C
T
of 30.0 or higher (at the threshold of
0.2), deviations occur in the amplifications, sometimes resulting in
C
T
-values
uncharacteristic for the sample karyotype. Using a cutoff of 29.0 is a security
measure to guarantee balanced amplification of both targets.
The cutoff of 25.0 is a result of the baseline setting. Also, the amplification of
genomic DNA of very high concentrations can result in lower and varying effi-
ciencies. None of the 100 amniotic fluid samples tested had a high enough con-
centration to result in such a low C
T
.
∆
23.
To make the test more stringent, the maximum average
C
T
-value can be set to
a smaller value. An alternative stringency requirement could be to demand that
the results of a minimal number of thresholds per sample are indicative of the
normal karyotype or one of the trisomies (e.g., 10 of total 12 values per triplicate).
∆∆
24.
It is usually possible to exclude one of the three karyotypes.
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