blood plasma is commonly used as test samples for verifying the maternal rhd status. blood plasma can also be used for determining the foetal rhd status if the mother is rhd- as maternal blood plasma contains maternal dna and trace amounts of foetal dna. blood samples can be obtained through venipuncture of the mother. since plasma and other components of blood have different densities, centrifugation of blood samples with added anticoagulant (such as edta) can segregate blood contents into multiples layers. blood plasma can then be isolated from the other components for rhesus factor testing. the method of extracting foetal dna from maternal blood plasma is considered to be a type of non-invasive prenatal testing.