Radioimmunoassay of apolipoprotein A-I in serum and EDTA plasma, and effects of freeze-thaw cycles.

To measure apolipoprotein A-I, we use an RIA kit (Ventrex Laboratories, Inc., Portland, ME) for which serum is recommended, and avoidance of repeated freeze-thaw cycles. With the reported association of alterations in apolipoprotein A-I concentrations in plasma with both heart (1) and liver disease (2), those interested in retrospectively measuring apolipoprotein A-I with this kit have been limited by these restrictions. We examined whether use of EDTA-treated plasma and of serum samples subjected to repeated freeze-thaw cycles would in fact affect apolipoprotein A-I measurement with this kit. Blood was sampled from eight healthy nonfastung volunteers into plain and EDTA-coated evacuated tubes. The order of tube filling was randomized to avoid a bias due to first-draw effect. After clot formation in the plain tubes, all tubes were centrifuged (2500 rpm, 10 miii, 10#{176}C). We transferred 50-ML aliquots of sera and EDTA plasma to several sets of microcentrifuge tubes for assay of apolipoprotein A-I. One set containing serum was frozen once at -20 #{176}C; a second set was frozen, allowed to thaw at room temperature, and refrozen. All nonfrozen aliquots were kept at 8#{176}C until assay. All assays, performed in duplicate according to the supplier’s instructions, were done together on the same day as the specimens were drawn. The following tabulation summarizes the values for fresh serum and EDTA plasma for these subjects.