Serum appears lipemic when the content of neutral fat is elevated. Although normally less than 200 mg. per 100 ml., the neutral fat may reach values of the order of 10,000 mg. per 100 ml. of serum. Colorimetric analytic methods, in which serum is still present in the final test mixture, but not in the blank, result in elevated optical densities in the presence of lipemia. Depending on the method used, falsely elevated values for total protein, thymol turbidity, amylase, transaminase, and so on, can be obtained. A suitable correction in most instances is to make a dilution of the serum in saline solution corresponding to the final dilution of serum in the cuvet. The optical density of this dilution is measured against distilled water, then subtracted from the optical density of the test specimen.
Results of determinations of blood hemoglobin may be falsely elevated as much as 2 or 3 Gm. per 100 ml. in the presence of gross lipemia. A suitable correction is made by means of diluting a second sample of blood with isotonic saline solution and centri-fuging. The optical density or hemoglobin value measured on this control is subtracted from the test result.
Mild lipemia normally persists for several hours after eating and should cause no significant interference in most procedures, with the exception of thymol turbidity determination. Before obtaining blood for study of serum lipid or lipid fractions, a 12hr. fast is recommended. In this connection it may be noted that after a standard breakfast a number of common chemistry determinations reveal no significant change in the blood.5 These include: urea nitrogen, carbon dioxide, chloride, sodium, potassium, calcium, phosphorus, total protein, albumin, creatinine, uric acid, cholesterol, and cholesterol esters. Glucose, of course, increases greatly postprandially.
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