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Test Code 608 Ristocetin Cofactor Assay

Specimen Containers and Fixatives

3.2% Sodium Citrate Blue Top Tube (2nd Tube Method)

Specimen Required

NorthShore Lab Services: draw 5.0 mL blood (see CLIENT SPECIAL INSTRUCTIONS), centrifuge twice and submit two 1 mL aliquots of frozen plasma in plastic tubes.

Hospital setting: submit 5.0 mL blood – see CLIENT SPECIAL INSTRUCTIONS.

Client Special Instructions

The specimen must be double centrifuged for special coagulation testing (e.g. lupus, protein C,

S etc.). Centrifuge specimens immediately (must be spun within 2 hours of collection). Carefully

remove plasma from cells avoiding platelets or red cells. Dispense into a plastic tube and centrifuge

plasma for 10 minutes. Remove the top portion of the plasma leaving approximately 250μl in the

bottom to discard. The double – centrifuged plasma should be aliquoted (1ml each) into clearly

labeled plastic tubes. Plastic tubes should be labeled with the patient's name, date of drawing, and

specimen number. The specimen should never come in contact with glass. If plasma cannot be sent to Evanston Special Coag lab within 4 hours of collection, freeze specimen and send it on dry ice.

Day(s) and Time(s) Performed


Monday – Friday (7AM – 3PM), 48 hours

Reference Values

50-150% Activity

This test is used for the differential diagnosis between Hemophilia and von Willebrand’s disease and to evaluate variant forms of von Willebrand’s disease. Variant forms of von Willebrand’s disease necessitate a series of clinical and laboratory evaluations including patient and family history, platelet function analysis, von Willebrand’s antigen and multimer analysis. Plasma from von Willebrand’s, disease patient lacks a factor causing platelet aggregations in response to ristocetin. This is associated with high molecular weight fraction of factor VIII antigen.

Method Name

Platelet aggregation

CPT Code Information