Although currently detection of measles IgM is considered the gold standard assay for diagnosing measles, it is known that IgM assays can lack specificity and also IgM may not be detectable in the first 3 days following rash onset. Measles RT-qPCR has long been used for the confirmation of measles cases, but with more widespread availability of molecular diagnostic tests a negative RT-qPCR on appropriate sample is increasingly being used to discard possible cases. It has the added advantage that samples collected for RT-qPCR can be readily sequenced for genotyping and if following recent vaccination for excluding possible cases.
A paper has been written (Annex A07) highlighting the advantages and disadvantages of this approach. Samples suitable for testing, such as throat swabs, mouth swabs or oral fluid need to be collected rather than serum samples, and sent to the laboratory for timely processing and RNA extraction. Samples should be collected as soon as possible and within two weeks of rash onset. This approach negates that need for additional samples to be collected for genotyping, but currently the assays themselves are more expensive than standard serological assays and easier to implement in laboratories using automated RNA extraction.
Although a molecular approach works well for measles and is increasingly being used especially in Europe, it is not really suitable for discarding rubella cases as the viral loads are much lower, and virus detected in throat swabs and oral fluid or a much shorter duration.