Is a weak or late (high) CT value of the IC an indicator for PCR inhibition?
FAQ ID -1398

Interpretation of the IC results strongly depends on the analytical PCR data. High pathogen copy numbers in a sample inevitably lead to impairment or, in rare cases, to even a complete failure of the IC PCR (as a consequence of competition between the two PCR systems).

The impairment is reflected in a decrease of the IC fluorescence intensity. However, the CT value is not affected. In contrast, PCR inhibition can lead to an increase of the CT value, i.e. to a significantly less efficient PCR. Since PCR inhibition can lead to false negative results, it is advisable to repeat the particular sample run and/or modify the RNA/DNA extraction procedure. However, the curve shape and CT values of the other ICs of the same run should be taken into account for comparison. If only one IC PCR differs from the others, an inhibition is very likely. If the IC was included in the extraction procedure of the sample material, a CT deviation of the IC may also indicate a partial IC loss and, hence, a deficient DNA/RNA preparation. In this case, the nucleic acid preparation should be repeated