When is a False Positive Not a False Positive?

Readers will know that analysis of foetal DNA circulating in maternal blood yields a very accurate (sensitive and specific) test for Down’s syndrome and other trisomies (where there are extra copies of one of the chromosomes in the normal pair). But some false positives arise. A recent report identifies two cases where the extra chromosome arose not from the foetus, but from an error in cell division propagated in a clone of metastatic cells from a previously undiagnosed cancer.[1]

Years ago, I tried to find foetal trisomies by harvesting foetal cells from blood, and my friend and colleague, Peter Selby, tried to diagnose metastasising cancer by harvesting cancer cells from the circulation. We were both looking in the wrong place – we should have been going directly for free circulating DNA, not cells containing DNA.

— Richard Lilford, CLAHRC WM Director


  1. Romero R, Mahoney MJ. Noninvasive Prenatal Testing and Detection of Maternal Cancer. JAMA. 2015; 314(2): 131-3.

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