Neonates can develop bruising and/or purpura due to defects in platelets, coagulation factors or both. Coagulation tests should be interpreted with caution because in the term infant the concentration of vitamin K-depen-dent proteins (II, VII, XI and X together with protein C and protein S) are 50% of normal adult values, contact factors (XI, XII, PK and HMWK) are 30-50% of adult concentrations, and all are lower in pre-terms. Factors VIII, V and vWF are normal. Thrombin inhibitors antithrombin (AT, previously called antithrombin III) and HCII are 5 but a2-M is t. Platelet count is the same as adults in both term and pre-term infants. There are technical problems in obtaining uncontaminated (heparin from catheters or IV lines) and adequate venous samples from neonates, causing in vitro inhibition or pre-test activation of clotting factors or dilution due to short sampling and thus excess anticoagulant. All can give spurious results.
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