Variation in Different Lipoprotein Fractions if Estimated by Different Methodologies.
BACKGROUND
Determining the levels of lipids and lipoprotein fractions is important in assessing the risk of coronary artery disease. The levels of total cholesterol, triglycerides and high-density lipoproteins are determined directly by enzymatic assays. In the case of low-density lipoproteins and very-low-density lipoproteins, Friedewald's formula has been in use since 1972. According to the formula, the level of very-low-density lipoprotein is equal to that of triglycerides divided by five, while that of low-density lipoprotein cholesterol is equal to the total cholesterol level minus the concentrations of high-density and very-low-density lipoproteins. The determination of low-density lipoprotein by this equation involves three independent lipid analyses, each of which may introduce errors. Besides, other lipoproteins, like intermediate-density lipoproteins and chylomicrons, are not accounted for. This study was done to compare the values of low-density and very-low-density lipoproteins by Friedewald's formula and to determine the values of high-density and low-density lipoproteins by homogenous assays with electrophoretic separation of lipoproteins.


RESULTS
Sixty adult patients with triglyceride levels of less than 400 mg/dL were evaluated. The level of low-density lipoprotein was measured enzymatically, using cholesterol esterase, after selective micellary solubilization of low-density lipoprotein cholesterol by a non-ionic detergent. High-density lipoprotein cholesterol was estimated similarly, after binding anti-human ss-lipoprotein antibody to low-density lipoprotein, very-low-density lipoprotein and chylomicrons. Electrophoretic separation of serum lipoproteins was done on agarose gel, and the bands scanned by a densitometer using a 570-nm filter. The values of low-density and very-low-density lipoproteins were also calculated using Friedewald's formula, following the spectrophotometric assay of serum cholesterol and triglycerides. Using the paired t-test, we found that the values of low-density lipoprotein as calculated by Friedewald's formula were significantly different from those determined by direct assays (p < 0.001) and electrophoresis (p < 0.001). Further, the values determined by direct assay and electrophoresis did not differ significantly (p = 0.53). The high-density lipoprotein cholesterol values as determined by direct assay and electrophoresis differed significantly (p < 0.001) from each other. The level of very-low-density lipoprotein as calculated by Friedewald's formula was significantly different from that estimated by electrophoresis (p < 0.001).


CONCLUSIONS
Though Friedewald's equation is widely used in the case of triglyceride levels below 400 mg/dL, the values arrived at are erroneous if there are alterations in intermediate-density lipoproteins, as reported in diabetes mellitus, chronic renal failure, coronary heart disease and certain other disorders. This study shows that even at low triglyceride levels, the calculated values of various lipoproteins differ from those measured by direct spectrophotometric and electrophoretic assays.
Srivastava
Madhulika
M
Dr Lal Pathological Laboratories, Connaught Place, New Delhi, India.
Arora
Sarika
S
Nigam
Puneet K
PK
Singh
Ritu
R
Lal
Arvind
A