Long-term stability of frozen pooled plasmas stored at -70°C, -40°C, and -20°C for prothrombin time and International Normalized Ratio (INR) assessment.

Background: Frozen pooled plasmas may be used for quality assessment and local calibration of prothrombin time (PT) and International Normalized Ratio (INR) measurement systems. The purpose of the present study is to estimate the long-term stability of frozen pooled plasmas stored for at least three years.

Methods: Six pooled plamas with different INR levels were stored at -70°C, -40°C, and -20°C for various time periods up to 1453days. PT of stored samples were measured with two thromboplastin reagents on a single automated coagulation instrument. INR was calculated from PT and plotted against plasma storage time. Linear regression lines of INR on storage time were used to estimate the percentage increase of INR.

Results: INR of plasma stored at -40°C or -20°C increased significantly with time. INR of plasma stored at -70°C did not change significantly in 11 out of 12 cases. The INR change of pooled plasmas stored at -70°C for 3years varied between 0.07% and 2.03%.

Conclusion: Long-term storage of plasmas at -40°C or -20°C should be avoided. Pooled plasmas stored at -70°C can be used for quality assessment and local calibration for at least 3years.