Pat Letendre’s Medlab-L list has had several posts lately about how to freeze serum or plasma specimens for long-term storage. Some have asked for specific guidelines, so I contacted members of the Precision BioLogic technical staff for advice. Precision BioLogic, as many coagulation specialists know, ships frozen pooled normal plasma, reference plasma, and single factor-depleted plasmas for routine and special coagulation testing. Precision’s plasmas are particularly stable, as referenced in Nougier C, Sobas F, Nguyen TK: Analytic variability due to change of deficient plasma vials: application to one-stage clotting factor VIII assay. Blood Coagul Fibrinolysis 2011;22:151-4.
I received a response from CEO Steve Duff, who commented there is, regrettably, no standard or guideline for specimen freezing, but made the following recommendations:
- A household freezer holds a temperature of approximately –17°C. This temperature provides a “slow freeze,” which is acceptable when the analytes to be preserved are not temperature labile. This may be minimally acceptable for coagulation factor VIII, which deteriorates at about 10% per hour at room temperature.
- Most household freezers are “self-defrosting,” which means they are designed to periodically reach temperatures at or slightly above 0°C. This prevents frost buildup, but damages plasma analytes, particularly proteins, as specimens are allowed to thaw and refreeze. When a lab manager selects a household freezer, it must have no defrost cycle, or the defrost cycle must be disabled.
- Many labs are equipped with –70°C freezers or ultra-low –80°C freezers. These provide a “fast freeze” speedy enough to preserve most labile proteins. Samples held in –70°C or –80°C freezers maintain their integrity for extended periods.
- If an analyte is extremely labile, “snap-freezing” may be necessary. Snap-freezing employs liquid nitrogen at –196°C, and is the method Precision and other frozen material distributors uses for freezing all their CRYO-check and like plasma products. Snap-freezing is seldom necessary for managing routine specimens.
- Plasmas shipped to coagulation reference laboratories must be shipped with at least 5 lbs of dry ice to maintain integrity.
I would like to hear from our members if you have other recommendations regarding specimen freezing.