A variable proportion of the total apolipoprotein A-I (apo A-I) present in plasma or high density lipoproteins (HDL) is normally detectable by immunochemical methods. This has been attributed to masking of some of the immunoreactive sites of apo A-I by lipid in the intact HDL particle. This difficulty has been circumvented by heating or delipidation. We find that exposure of plasma to concentrations of urea greater than about 7.0 M in the barbital buffer used to dilute plasma samples for estimation by electroimmunoassay enables the complete detection of Apo A-I, as judged by comparison with samples delipidated with tetramethylurea. The need for time-consuming heating or delipidation is avoided.
