Our September 2022 Quick Question was, “You collect blood for a heparin assay. How soon must you centrifuge the specimen?” Answers from our 39 respondents:
- 1 hour: 74% 
- 4 hours: 21% 
- 24 hours: 5% 
- Refrigerate indefinitely:
Most of us adhere to the CLSI Guideline H21-A9, Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays, 5th Edition. The guideline specifies that specimens collected for measuring therapeutic heparin must be centrifuged and the plasma removed from the cells within one hour of collection. In vitro platelets release heparin-neutralizing platelet factor 4 [PF4], falsely reducing the heparin activity and compromising the assay result. The plasma must be “platelet poor” [platelet count less than 10,000/uL] and if the heparin assay is delayed, the specimen must be frozen until the assay is completed. In a busy facility, this guideline is difficult to meet, and there are some preliminary data suggesting that if the laboratory employs the chromogenic anti-Xa heparin assay, the storage time may be extended to up to 4 hours.
The 24-hour rule applies to the PT, not the heparin assay, and of course, whole blood refrigeration is not indicated.