New blood tests based upon genetic testing are starting to be marketed to clinicians to use during diagnosis, or to enhance screening programmes, which may have great potential to improve patient outcomes.

Tests like BCtect for Breast Cancer and ADtect for Alzheimer's, being marketed to clinicians in the UK at the moment, involve single blood samples and are good examples of where gene testing has the potential to improve diagnosis and care. However, we weren't really sure what the full story was, so we started digging into the tests a little further.

The two tests we've looked at are based upon analysis of blood samples for characteristic changes in the activity of selected genes, rather than mapping full sets of genes. Without getting hugely technical, the uniform chemical nature of RNA (a nucleic acid similar to DNA) makes it a good "bio marker" and therefore useful for diagnosis.

The tests extract RNA and then covert it into cDNA (a collection of complementary, or cloned, DNA synthesised from mature RNA) and you pretty much need to be a genetic scientist to fully get to grips with the conversion process. The process takes a few days, so the results take slightly longer than traditional blood testing.

The tests then assume that the over expression of mRNA of a certain type will be associated with an increased risk of disease, in these cases, Breast Cancer or Alzheimer's. The sensitivity (proportion of actual positives correctly identified) and specificity (proportion of negatives correctly identified) of the tests are relatively low though, at around 75% for each test (which implies one in four test results may not be wholly accurate).

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