LAUSR

Low density Lipoprotein Cholesterol (LDLc) is considered as a major risk factor in the development of cardiovascular disease and is the primary basis for diagnosis, treatment and risk classification in patients with hyperlipidemia. This makes it very important to estimate serum LDLc with great precision and accuracy. The ultracentrifugation procedure or b-quantification (BQ) is considered as the referencemethod for LDLc. It is time consuming, expensive and requires a large serum volume. The two commonly used methods of LDLc estimation are – Direct Homogenous Assay, and the Indirect CalculationMethod. Direct assays are found to be reasonably specific and free from endogenous interference. However, these direct methods are not perfect, as they have some disadvantages such as over/under estimation of LDLc, failure of many commercial methods to meet the NCEP total error goals especially in diseased individuals, and high cost. The indirect method of LDLc estimation is by calculation using the landmark equation proposed by Friedewald et al. using other lipid parameters viz. total cholesterol(TC),high density lipoprotein cholesterol(HDLc) and triglycerides(TG). This equation is LDLc(mg/ dl) = TC – HDLc – TG/5. However, Friedewald et al. themselves found that this formula was not applicable to plasma samples containing chylomicrons, patientswith Type III hyperlipoproteinemia and plasma containing TG >400mg/dl. Furthermore, each component’s (TC,TG and HDLc) analytical error is added, hence it is difficult to achieve the recommended total analytical error goal of 12%. Additionally, the equation requires fasting serum sample. Despite these limitations this formula is the method of choice for routine quantification of LDLc by most of the clinical laboratories due to its simplicity, reliability and cost effectiveness, especially in developing countries. To overcome the foresaid limitations of Friedewald formula(FF), several modified equations have been suggested and their advantages and disadvantages have been discussed. These modified formulas did provide some improvement in overcoming the limitations of Friedewald equation but many modified formulas failed to give better estimate of LDLc in sera containing TG >400mg/dl. The different modified formulas have been validated in different populations and each formula was found suitable for a particular population. Some recent studies have shown Friedewald equation to be still better than several modified formulas. Therefore, it is important to evaluate these modified formulas in Indian population as the studies on the validation of the modified formulas and even Friedewald formula in Indian population are only a few. 1. Indian studies