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Low Phospholipid PTT Reagent

Another excellent question from “Elhosenymohamed”

How can the dilution of the phospholipid component of the partial thromboplastin time (PTT) reagent detect  antiphospholipid antibodies?

Hello, again, Dr. Mohamed, and thank you for your question.

Most of the PTT reagents we use for monitoring unfractionated heparin or for performing routine coagulopathy screens are formulated with a relatively high concentration of phospholipid. There is enough phospholipid in the reagent to completely neutralize a potential lupus anticoagulant and still have enough left over to support clotting in the PTT reaction. This is desirable, as 1 to 2% of the general population has a lupus anticoagulant, most of which are transient and benign. A high phospholipid reagent prevents the need for follow-up on what is most likely a benign condition.

If, however, there is a clinical reason to test for a lupus anticoagulant, such as a venous thrombotic event or repeated miscarriages, we would choose a low phospholipid PTT reagent, for example, Stago’s Sta-Clot LA for lupus anticoagulant detection. The small concentration of phospholipid means that a lupus anticoagulant would neutralize most of the phospholipid available and the remaining phospholipid would be inadequate to support the PTT reagent. Thus, the PTT would be prolonged in response to the lupus anticoagulant. This is the basis for the PTT screen for lupus anticoagulant.

The confirmatory test involves adding phospholipid to the low phospholipid reagent and repeating the assay. If the PTT shortens to near its original result, lupus anticoagulant is confirmed. Geo.

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