Clinical laboratory parameters for evaluation of haemostasis
Haemostasis is assessed with a small set of screening tests that together locate the defect: primary haemostasis (vessel and platelets), the intrinsic, extrinsic and common coagulation pathways, and the final step, fibrinogen to fibrin.
Specimen
- Specimen type: plasma.
- Collection: 3.2% sodium citrate tube.
- Minimum volume: the collection tube must be full, to ensure the correct mixing ratio of sodium citrate to venous blood, so that the tube contains 1 part sodium citrate solution to 9 parts blood.
Tests of primary haemostasis
Platelet count and bleeding time assess primary haemostasis (prolonged in thrombocytopenia, von Willebrand disease and aspirin use).
- Bleeding time (Ivy skin method): measures the length of time that bleeding continues after a standardised wound is made on the forearm or ear lobe. Blood is wicked from the incision with filter paper at 30-second intervals until bleeding ceases. It evaluates the vascular and platelet responses associated with haemostasis. With the Ivy method bleeding should stop within 1–9 minutes.
- Platelet count: normal 140–440 x10^9/l. Counts under 50 x10^9/l put a patient at risk of severe bleeding, and counts under 20 x10^9/l may cause spontaneous and fatal intracranial bleeding. An increased platelet count is thrombocytosis; an unexpectedly high count may point to an advanced or metastatic malignancy.