I received this question today via the American Society for Clinical Laboratory Sciences consumer forum from a hospital pharmacist:
Our lab says that heparin drips have to be turned off for 30 minutes before the PTT is drawn, even though they draw the specimen from the opposite arm. Is this true and what is the reason?
A summary of my response:
Hello. Here is a helpful reference on specimen protocol for IV unfractionated heparin monitoring: Johnston M. Monitoring Heparin Therapy, in Kitchen S, Olson JD, Preston FE. Quality in Laboratory Hemostasis and Thrombosis. Blackwell, 2009.
IV unfractionated heparin begins with a bolus dosage followed by weight-adjusted continuous infusion. The first specimen for PTT monitoring is collected at least 4-6 hours after administration of the bolus so the plasma heparin level may equilibrate to the infusion dosage. It must be collected during continuous infusion within the first 24 hours. Heparin binds a variety of endothelial cell and platelet receptors and plasma proteins. The degree of binding varies by patient, but on average, heparin’s plasma half-life upon discontinuing the drip is 30 to 60 minutes. The disappearance half-life also rises in a non-linear fashion relative to dose. If you discontinue the drip for 30 minutes prior to collection, the result will not reflect the continuous IV heparin dosage but will be reduced by some unpredictable percentage between 25% and 50%, introducing a random preanalytical error.
My references don’t provide a specific prohibition, but discontinuing the drip would be contraindicated.
To influence your clinicians, you can use the Johnston reference, plus Hirsh J, Bauer KA, Donati MB, et al. Parenteral Anticoagulants. Chest 2008;133:141S-159S. The Chest article is available as a free download from the American College of Chest Physicians.
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