Important criteria for the design of ECs include (i) the possibility to use quantified or semi quantified controls (in order to manage the detection level of the diagnostic test used), (ii) the possibility to distinguish amplicons obtained from ECs from those obtained from the detected pathogen (in order to detect false positives due to accidental amplification of EC) and (iii) the use of relevant nucleic acids (ie, RNA and DNA molecules for RNA and DNA viruses, respectively).