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George received a comment about platelet-poor plasma [PPP] from friend and colleague, Heather DeVries, Indiana University Health. Here is a paragraph from Fritsma GA. Laboratory evaluation of hemostasis, Chapter 41 in Keohane EM, Otto CN, Walenga JM. Rodak’s Hematology: Clinical Principles and Applications, 6th Edition, Elsevier, 2020. ISBN 978-0-323-53045-3:

Platelet-Poor Plasma for Clot-Based Testing

“Clot-based plasma coagulation tests require PPP with a platelet count of less than 10,000/μL.[i] Sodium citrate-anticoagulated whole blood is centrifuged at 1500 ´g RCF for 15 minutes in a horizontal-head centrifuge to produce supernatant PPP. Alternatively, a HemoCue StatSpin type of centrifuge that generates 4400 ´ g RCF can produce PPP within 3 minutes. The advantage of the horizontal centrifuge head is that it produces a straight, level plasma-blood cell interface, making it possible for automated coagulometers to sample from the supernatant plasma of the blood collection tube (the “primary” tube). Angled centrifuge heads cause platelets to adhere to the side of the tube. If the “angle-spun” tube is allowed to stand, the adherent platelets drift back into the plasma and release granule contents. Each hemostasis laboratory manager establishes the correct centrifugation speed and times locally. Centrifugation must yield documentable PPP from specimens with elevated initial platelet counts.

In the special hemostasis laboratory the manager may choose a double-spin approach. The primary tube is centrifuged and the plasma is transferred to a secondary plastic tube, which is labeled and centrifuged again. The double-spin approach may be used to produce PPP with a plasma platelet count of less than 5000/μL, which some laboratory directors prefer for lupus anticoagulant (LAC) testing and for freezing.

A plasma platelet count greater than 10,000/μL affects clot-based test results. Platelets become activated in vitro and release microparticles and the membrane phospholipid phosphatidylserine, which trigger plasma coagulation and interfere with LAC testing. Platelets also secrete fibrinogen, factors V and VIII, and VWF (Chapter 10). These may desensitize PT and PTT assays and interfere with clot-based coagulation assays. In addition, platelets release PF4, a protein that binds and neutralizes therapeutic heparin in vitro, falsely shortening the PTT and interfering with heparin management.

The hemostasis laboratory manager arranges to perform PPP platelet counts on coagulation plasmas at regular intervals to ensure that they are consistently platelet poor. Many managers select 10–12 specimens from each centrifuge every 6 months, perform plasma platelet counts, and document that their samples remain appropriately platelet poor, even if the initial platelet count is elevated.”

[i] Pierangeli SS, Harris EN. Clinical laboratory testing for the antiphospholipid syndrome. Clin Chim Acta 2005;357:17–33.

The formula for converting revolutions per minute [RPM] to relative centrifugal force [RCF] is RCF = 1.12 x radius [mm] x [rpm/1000]2. The radius for each centrifuge head may be found in its package insert. Click here for an automated conversion function. The authoritative reference for PPP management is Clinical and Laboratory Standards Institute [CLSI] H21-A5. Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays–Approve Guideline–Fifth Edtion, 2008. ISBN 1-56238-000-0. Chair Dorothy M. Adcock, MD.

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