Blood test and gene sequencing combined to detect cancer

[News]

Scientists at the Johns Hopkins Kimmel Cancer Center have combined the ability to detect cancer DNA in the blood with genome sequencing technology in a test that could be used to screen for cancers, monitor cancer patients for recurrence and find residual cancer left after surgery.

More... (http://www.news-medical.net/news/20121129/Blood-test-and-gene-sequencing-combined-to-detect-cancer.aspx)

[gdpawel]

Basically, this blood profiling uses 'negative selection' to isolate alleged circulating tumor cells (CTCs). What that means is methods to 'selectively' remove circulating normal cells, such as monocytes, lymphocytes, neutrophils, circulating endothelial cells, etc. The problem is that these normal cells outnumber circulating tumor cells by a factor of a million to one, and no 'negative selection' procedure (or combination of procedures) can possibly strip away all the normal cells, leaving behind a relatively pure population of tumor cells.

What you have to do is to use a 'positive selection' procedure, meaning selectively extracting the tumor cells out of the vastly larger milieu of normal cells. The problem is, when you do this, there is only a teeny tiny yield of tumor cells. Circulating tumor cells are found in frequencies on the order of 1-10 CTC per mL of whole blood in patients with metastatic disease. For comparison, a mL of blood contains a few million white blood cells and a billion red blood cells.

So, from a typical 7 ml blood draw into a purple top tube, you are going to get, on average, 7 to 70 tumor cells -- total. This may be sufficient for certain molecular type tests (although the degree to which this tiny sample of cells is representative may be questioned).