Finally, in the context of diagnostic virology laboratories, our study shows that standard extraction and amplification techniques used for the molecular diagnosis of human pathogens led to a significant proportion of 'unresolved' results, which cannot be identified if an internal control is not used. In the absence of an internal control, such samples are commonly identified as 'negative', which is with hindsight incorrect: in our hands, detecting amplification inhibition using phages as internal controls, and testing tenfold dilutions of the nucleic acid extracts demonstrated that some of these samples were actually positives. This cost-effective and convenient strategy can therefore be used for enhancing the quality of routine molecular diagnosis, but it may also be adapted in other contexts such as testing of large numbers of animal or environmental samples.