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Overfilled Tubes


I want to hear from you regarding the effects of overfilled coagulation tubes on prothrombin time (PT), partial thromboplastin time (PTT), D-dimer, and fibrinogen. Thank you, Elvin Tiamzon.

Hello, Elvin Tiamzon, and thank you for your question. While there are a number of definitive articles on “underfilling” 3.2% sodium citrate whole blood collection tubes, there are none on overfilled tubes. This provides me the opportunity to speculate freely.

In my experience, overfilled citrate tubes are regrettably commonplace. In most instances, overfilling occurs when a phlebotomist first collects blood in a syringe, then applies pressure to the syringe plunger while transferring to the citrate tube. The proper way to transfer is to passively allow tube vacuum to draw the blood from the syringe. The tube is properly filled when blood transfer ceases. Applying pressure overfills the tube. Removing the closure and needle and transferring through the syringe tip is also likely to cause overfilling.

Overfilling theoretically risks clotting, as the ratio of additive anticoagulant to whole blood may be insufficient. In practice, however, we seldom detect clots in overfilled tubes, though we nonetheless reject overfilled tubes on general quality assurance principles. Any clotted tube must be rejected, of course, as the results of all parameters cannot be predicted. I would guess that if you performed a PT, PTT, D-dimer, or fibrinogen on a non-clotted but overfilled tube, the results would parallel those from a properly collected tube, not that I advocate running tests on specimens from overfilled tubes.

Once I learned of an overfilled tube the phlebotomist had submitted after adding blood from one underfilled tube to blood from another. This piecemeal specimen, of course, was the same as an underfilled tube: the excessive anticoagulant to whole blood ratio prolonged the PT and PTT and generated a falsely reduced fibrinogen result. I don’t know the outcome of the D-dimer on the specimen.

All cases of overfilling or underfilling provide us with opportunities to educate the phlebotomist about proper coagulation specimen collection.

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