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What is thromboplastin?

Dear George,

This is back to basics but, believe it or not, some of us are coming up with different ideas of what exactly “thromboplastin” is and exactly which cells it is normally a part of. Could you please define it for us and clear up the confusion?

Thank You, Anne Dagostino

Hello, Anne. Thanks for your question. One must be “of an age” to be able to answer.

Thromboplastin is the term given to what Hougie called “a large lipoprotein complex…found as a cell-membrane component in almost all cells of the body.” [Hougie C. The Biochemistry of Blood Coagulation. In Triplett DA, Laboratory Evaluation of Coagulation. ASCP Press 1982]. Hougie also applied the term “tissue factor.” Thromboplastin was assigned “factor III” in the coagulation scheme. The number is now largely abandoned, and for many years we’ve more or less used thromboplastin and tissue factor synonymously.

Now we know tissue factor to be an integral membrane glycoprotein of 44,000 mw located in the subendothelial adventitia, especially in fibromyocytes. Contrary to what we believed in 1982, tissue factor is not found in endothelial cell membranes. Tissue factor is closely associated with membrane phospholipids and triggers the activation of factor VII upon exposure when the adventitia is damaged.

Most prothrombin time reagents are called thromboplastin, and until the advent of recombinant technology, all PT thromboplastins were ethanol extractions of rabbit (USA) or human (Europe) brain tissue. These preparations contain tissue factor, phospholipids and calcium chloride.

This technology still exists, but now most manufacturers synthesize tissue factor and add it to measured amounts of chemically produced phospholipids, mostly phosphatidyl serine, plus calcium chloride.

Partial thromboplastin, by the way, is an extract of rabbit brain containing phospholipids but no tissue factor. In the original partial thromboplastin time (PTT), the reagents were rabbit brain phospholipids and calcium chloride, added sequentially. This test was slow and imprecise, so a negatively charged particulate activator was added: Celite, Kaolin, or ellagic acid, giving us the “activated” partial thromboplastin time or APTT. The particulate activator triggers activation of factor XII. We no longer employ the original test, so most of us have taken to calling the APTT the PTT.

I hope this answers your question. Geo

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