A composition and method suitable for separating and segregating extracellular DNA in a cell-containing sample, in particular, a blood sample is described. The composition comprising a thixotropic barrier gel and a stabilizing agent in aqueous solution at a concentration of at least 400 mM is capable of establishing and maintaining separation between intracellular and extracellular DNA in blood over time by means of physical barrier wherein when the composition is mixed with whole blood and centrifuged, plasma is separated from the packed cell layer by the thixotropic barrier gel and the blood cells are separated away from the plasma.
