Effect of reagent pH on determination of high-density lipoprotein cholesterol by precipitation with sodium phosphotungstate-magnesium.

When determining high-density lipoprotein cholesterol by use of sodium phosphotungstate-magnesium precipitation method, I found that the pH of the sodium phosphotungstate reagent was a critical factor in the method. Unless the pH of the reagent was less than 7.6, the precipitation of low-density lipoprotein and very-low-density lipoprotein was incomplete. When the specimen pH was between 7.35 and 8.65, the pH of the serum of plasma did not influence the completeness of precipitation. Optimum concentrations of precipitation reagents, determined after the pH of the sodium phosphotungstate reagent was standardized to pH 6. 15, were 40 g/L for sodium phosphotungstate and 2 mol/L for MgCl2. The distribution of high-density lipoprotein cholesterol in a healthy adult population was skewed to the left for women (n = 34; mean = 660 mg/L) and bi-modal for men (n = 44; mean = 460 mg/mL). The central 95% reference interval was 280 to 880 mg/L for women and 250 to 750 mg/L for men.