Immunoassay of human plasma apolipoprotein B.

A specific and precise double antibody immunoassay for human plasma apolipoprotein B (apoB) was developed and applied in normolipidemic and hyperlipidemic subjects. The intra-assay coefficient of variation was ca. 9%. The distributions of total apoB and low density lipoprotein (LDL) apoB in a randomly selected, healthy, fasting population (n = 349) was slightly skewed with a mean total apoB of 81 mg/100 ml and LDL apoB of 72 mg/100 ml. The 90th percentile cutoffs for total apoB and LDL apoB were 106 and 97 mg/100 ml, respectively. Regarding total apoB, women showed a statistically significant increase (r = 0.463, p less than 0.001) with age (30-65) and an average annual increment of plasma apoB of 1.1 mg/100 ml. In contrast, men showed only a slight increase of apoB from the 4th to 5th decade, with an average annual increment of 0.7 mg/100 ml (r = 0.201, 0.02 less than p less than 0.05). Similarly, regarding LDL apoB, women showed an increase of 1.0 mg/100 ml/year from the 4th to 7th decade (r = 0.501, p less than 0.001), whereas men's LDL apoB did not increase significantly with age (r = 0.114, 0.2 less than p less than 0.3, for ages 30-49). Six of ten normal young subjects showed essentially no physiological variation in fasting apoB levels over a 10-wk period, whereas four had a variation of ca. 5% or less. LDL apoB represented ca. 90% of the total apoB in normolipidemic and type II plasma samples (86% in type IV samples) but only 68% in type III plasmas (n = 7). The ratios of LDL cholesterol-LDL apoB were similar for the random and hyperlipoproteinemic groups, ranging from a high of 1.8 for type IIa to a low of 1.5 for type IV. The ratio of cholesterol to apoB was significantly elevated (p less than 0.002) in the d less than 1.006 fraction of the type III plasma samples compared to the random and type II groups.