Long-Term Stability Studies on the WHO IRP for Thromboplastin (Human Plain BCT/253)

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.

Thromboplastin standards

The Prothrombin Time (PT) test is used for monitoring of treatment with Vitamin K-antagonists (VKA). The result of the PT test should be expressed as the International Normalized Ratio (INR). Calculation of INR is based on the availability of International Standards (IS) for thromboplastin and a calibration model. Calibration of a new PT test system is performed with the appropriate IS and fresh plasma samples of healthy (normal) volunteers and patients treated with VKA. The calibration model is based on the assumption of a linear relationship between the log(PT)'s obtained with the new PT system and the reference IS for both normal and patients' samples. Patients' samples for calibration should be selected by rejecting samples beyond the 1.5-4.5 INR range. Outliers should be rejected defined as points with a perpendicular distance greater than three residual standard deviations from the line of relationship. Selection of patients' samples and rejection of outliers result in a reduction of the between-laboratory variation of calibration. In addition to monitoring of VKA, the PT is used for management of patients with chronic liver disease. Likewise, INR(liver) should be based on calibration with an IS using samples from patients with chronic liver disease.