Probably cleared by immune cells.

Renal clearance is another possibilty.

Dear Victor, I  would like to ask the question other Wise

how long the ctDNA will present in the serum after the blood has been taken from a patient?

Dear Abderrahmane, I think the ctDNA will be detectable as long as the serum does contain it and is well preserved.

Dear Victor,

Chan KC et al. PNAS 2013; and Yu SC et al Clin Chem 2013, reported that the ctDNA id cleared by the spleen, liver and the kidney.

I hope this will help

Ya, I understand this. But by what exact mechanisms?