From our colleague and frequent contributor Heather DeVries, Indiana University Health. Greetings George! One of our labs contacted me about PTT specimens coming from outpatient clinics, where they do not have centrifuges. Before coming into our system, this lab did a small study (30 specimens) to validate 24-hour stability of nonheparinized specimens for PTT testing. This goes against everything I know about the PTT , but when it comes to outpatients, I can see why they did the study. I wonder how other large systems address this? We do have a fair number of specimens that we cancel from outreach during the evening shift--the whole blood specimens are couriered here, but greater than 4 hours old when received. Thanks, Heather.
Hello, Heather, this is a challenging issue. I'm with you, uncentrifuged specimens for the PTT assay must be maintained at room temperature, spun, and analyzed within 4 hours to avoid deterioration in von Willebrand factor, factor VIII, factor IX, and protein S activities. The documentation is probably on your shelf or in your computer: Clinical and Laboratory Standards Institute (CLSI). Collection, Transport, and Processing of Blood Specimens for Testing Plasma-Based Coagulation Assays and Molecular Hemostasis Assays: Approved Guideline-Fifth Edition. CLSI document H21-A4. The CLSI guideline is based on solid research, especially data reported in Adcock DM, Kressin IX, Marlar RA. The effect of time and temperature on routine coagulation tests. Blood Coag Fibrinolysis 1998; 9: 463–70 and reviewed in Favaloro EJ. Preanalytical variables in coagulation testing. Blood Coag Fibrinolysis 2007; 18: 86–9. Further, specimens from patients on unfractionated heparin must be centrifuged and analyzed within one hour, whereas specimens for PT are good unspun and held at ambient temperatures for up to 24 hours.
I would distrust a 30-specimen sample, as it may not provide enough abnormals to reach a generalizable conclusion. I'd suggest you help your small outpatient clinics to equip themselves with an inexpensive but reliable desktop centrifuge, which would enable them to follow the CLSI guideline.
One additional reference I recommend for your shelf is Ernst DJ, Ernst C. The Lab Draw Answer Book, Second Edition. Center for Phlebotomy Education, 2017. The book offers authoritative specimen collection standards in a Q&A format from Catherine and Dennis Ernst, your fellow Indianians, and international blood collection experts.