International Federation of Clinical Chemistry standardization project for measurements of apolipoproteins A-I and B. II. Evaluation and selection of candidate reference materials.

The first phase of an international collaborative study for standardization of test systems for measuring apolipoprotein (apo) A-I and apo B demonstrated that uniformity of apo A-I and apo B measurements can be achieved if suitable common reference materials are used to calibrate the different systems. The objective of the second phase was to evaluate the linearity and parallelism or proportionality of the candidate reference materials selected in phase one and to determine whether any of them could be proposed as international reference materials. We evaluated the proposed reference materials with 37 test systems for apo A-I and 38 for apo B, involving 23 manufacturers and five research laboratories. Two lyophilized preparations were proposed for apo A-I, SP1 from Behringwerke AG and SP2 from Daiichi Pure Chemicals Co., and two liquid preparations were proposed for apo B, SP3 from Behringwerke AG and SP4 from Reagents Applications. The linearity of the candidate reference materials was compared with the linearity of a frozen serum pool or interim serum reference material distributed to all the participants and with that of a fresh serum pool prepared by each participant. SP1 and SP3 exhibited linearity and parallelism similar to that of the fresh frozen serum pool and had among-laboratory CVs less than or similar to those obtained on normolipidemic serum samples (approximately 6% for apo A-I and approximately 7% for apo B).