The long-term within- and between-laboratory variability for assay of antithrombin, and proteins C and S: results derived from the external quality assessment program for thrombophilia screening of the ECAT Foundation

A stable laboratory performance is important for comparability and transferability of laboratory data both within and between laboratories. The lack of a reference system within hemostasis hampers laboratories in establishing their laboratory performance over a prolonged period of time. Therefore, based on data from an external quality assessment program, we evaluated the between laboratory variation (CVBETWEEN) and the long-term within-laboratory variation (LCVa) for antithrombin, and proteins C and S. We evaluated the CVBETWEEN for the period 1996-2001, including the results of 64-240 laboratories from 23 different surveys (protein S activity 15 surveys). We observed a quite high CVBETWEEN and a broad range for each analyte. The CVBETWEEN was significantly higher for antithrombin and protein S for samples with low levels similar to heterozygous deficiencies. We also evaluated the LCVa, including the results of 136 laboratories. The lowest LCVa [median and 95% content interval (Cl)] was observed for antithrombin (7.6%; 3.6-35.5%), intermediate values for protein C activity and antigen (8.6%; 3.5-25.3% and 10.8%; 4.8-33.1%, respectively) and highest values for the protein S variables (13.4%; 6.4-50.6% for total protein S antigen, 14.1%; 6.5-79.1% for free protein S antigen and 17.2%; 7.2-84.3% for protein S activity). We concluded that the main reason for the high CVBETWEEN is the long-term within-laboratory variability. Application of linear regression on data of an external quality assessment program is a useful model to demonstrate per analyte per laboratory the long-term variability (LCVa). It is concluded that improvement of the long-term within-laboratory test performance is the first priority in hemostasis to yield important improvements in the comparability and transferability of laboratory data.