Hi George. Why is it not important to double-spin D-dimer specimens for platelet poor plasma (PPP) when there’s a delay in testing? Is this because D-dimers are not affected by labile factors as other coag testing? Looking forward to your answer.
Teresa Lovejoy, MT (ASCP)
Senior Medical Technologist, Hematology
Children’s of Alabama. Birmingham, AL
Hi, Teresa, and thank you for your question. I guess you are busy moving your lab into that beautiful new hospital building on Sixth Avenue South these days. The purpose for preparing PPP, defined as plasma with a platelet count <10,000/mcL, (some recommend <5,000/mcL) is to ensure no platelet secretions affect coagulation test results. Platelets may become activated in vitro and secrete, for example platelet factor 4, which neutralizes heparin and artifactually shortens the PTT. Platelets also secrete factor V, VIII, and von Willebrand factor, and release plasma membrane phospholipids that interfere with lupus anticoagulant testing. The longer a non-PPP specimen stands, the more the platelets secrete, and platelet materials become especially critical when plasma is frozen, as cells become ruptured.
I find no studies to confirm this, but the D-dimer concentration seems to be unaffected by platelets, as D-dimer is a product of fibrin crosslinking and fibrinolysis. Consequently, it is not necessary to prepare PPP for a D-dimer assay; however, there is no harm in it, either. Since most labs prepare PPP for all coagulation testing, most D-dimer assays probably are performed using PPP. Geo.