Guidelines for organisation and management of external quality assessment using proficiency testing. Expert Panel on Cytometry of the International Council for Standardization in Haematology

Long-Term Analytical Performance of Hemostasis Field Methods as Assessed by Evaluation of the Results of an External Quality Assessment Program for Antithrombin

Background: It is important for a laboratory to know the stability of performance of laboratory tests over time. The aim of this study was to adapt from the field of clinical chemistry a method to assess the long-term analytical performance of hemostasis field methods.

Methods: The linear regression model was used to compare the laboratory results with the consensus mean value of a survey. This model was applied to plasma antithrombin activity using the data for 82 laboratories, collected between 1996 and 1999 in the European Concerted Action on Thrombosis (ECAT) external quality assessment program. The long-term total, random, and systematic error were calculated. The variables introduced to define the long-term performance in this model were the long-term analytical CV (LCVa) and the analytical critical difference (ACD), which indicates the minimum difference necessary between two samples measured on a long-term time-scale to consider them statistically significantly different.

Results: The systematic error (bias) ranged from 4.5 to 103 units/L. The random error ranged from 24.4 to 242 units/L. For the majority of the laboratories, random error was the main component (>75%) of the total error. The LCVa, after adjustment for the contribution of the bias, ranged from 2.8% to 48%. The ACD ranged from 78 to 1290 units/L with a median value of 190 units/L. No statistically significant differences were observed for either LCVa or ACD between the two different measurement principles for antithrombin activity based on the inhibition of either thrombin or factor Xa.

Conclusions: This linear regression model is useful for assessing the total error, random error, and bias for hemostasis field methods. The LCVa and ACD for measurement on a long-term time-scale appear to be useful for assessing the long-term analytical performance.

External Quality Assessment Schemes: need for recognised requirements

Programs for Accreditation of clinical laboratories consider participation in External Quality Assessment Schemes (EQAS) a key element in the evaluation of testing procedures and improving them. One of the main functions of EQAS is to assess whether laboratories perform tests competently. It is therefore of utmost importance for laboratories to participate in EQAS that are in line with formally recognised requirements. Specific proposals have been made on how to design and execute EQAS by International Working Groups, but there seems to be no consensus on the best strategies to use and quality specifications to set out. The Clinical Pathology Accreditation (CPA) Program for EQA Scheme Accreditation (CPA-EQA) is the only program in Europe to provide a formal recognition of the quality of EQAS activities. The present paper reports on the experience of the Centre of Biomedical Research which is following an accreditation process for their own schemes in line with the CPA-EQA program and a proposal to set requirements that Italian schemes must follow to be recognised as valid and effective.