Two types of samples served as the background control for determination of the C(t) cut off. The 1 st type of negative control was 50 ng of human cellular DNA. The 2 nd type of negative control was negative donors' plasma. Data were collected in separate experiments from 8 human cellular DNA controls and 3 negative donors' plasma. To standardized the C(t) cut off for all primers the threshold was set at the PCR machine default setting. Based on a false positive rate of less than 5% the following method [14] was used to estimate the C(t) cut off from the range of C(t) obtained with negative samples for all 24 primer sets in the array: