Laboratory Measurement of Lipids and LipoproteinS

I. Preanalytical Factors

• Affected by a number of factors:

  • Diet and Lifestyle: Maintain usual habits for 3 days before testing.

  • Condition: Avoid testing during stress or illness (post-MI, surgery, etc.).

  • Fasting: Required for triglycerides and LDL-C calculation; prefer 12-hour fast.

  • Specimen Stability: Serum or EDTA plasma preferred; EDTA stabilizes samples by reducing oxidation.

  • Sample Handling: Minimize hemoconcentration by seating patients before sample collection.

II. Measurements of Lipids

• Importance: Indicators of coronary heart disease (CHD) risk.

• Standardization of lipid analysis crucial for accurate results.

A. Triglycerides

• Measurement Importance: Helps determine genetic/metabolic disorders and CHD risk.

• Sample Preparation: 10-14 hour fasting recommended.

• Measurement Method:

  1. Enzymatic Methods: Use lipase to cleave triglycerides, releasing glycerol.

  2. Key Reaction Diagram:

  • $\text{Triglycerides} \rightarrow \text{Glycerol} + \text{Fatty Acids}$

  • $\text{Glycerol} + \text{ATP} \rightarrow \text{Glycerophosphate} + \text{ADP}$

  • $\text{Glycerol-3-phosphate} + \text{NAD} \rightarrow \text{Dihydroxyacetone} + \text{NADH}$

    • Corrections: Use a double-cuvette or calibration blanking technique to account for free glycerol interference.

    • Desired Levels: <150 mg/dL.

B. Total Cholesterol

• Key Factors Influencing Results:

  • Age, sex, menstruation, stress, and hemolysis can all impact cholesterol levels.

• Measurement Method:

  1. Enzymatic Reactions: Free cholesterol released by cholesterol esterase; oxidized by cholesterol oxidase producing hydrogen peroxide.

  2. Reaction Diagram:

  • $\text{Cholesterol ester} \rightarrow \text{Cholesterol} + \text{Fatty Acids}$

  • $\text{Cholesterol} + \text{O}_2 \rightarrow \text{Cholest-4-en-3-one} + \text{Peroxide}$

• Desired Levels: <200 mg/dL.

III. Methods for Lipoprotein Measurement

• Various methods based on physical properties of lipoproteins.

A. Ultracentrifugation

• Principle: Separate lipoproteins by density.

• Limitations: Requires specialized skills and equipment, not widely used in clinical labs.

B. Electrophoretic Methods

• Usage: Separate lipoprotein classes; limited precision for quantitation.

• Medium: Agarose gel.

C. Polyanion Precipitation Methods

• Process: Use polyanions to precipitate lipoproteins; frequently used for HDL.

• Factors Affecting Results: Reagent concentration, pH, ionic strength.

D. Immunochemical Methods

• Usage: Antibodies to separate lipoproteins, can target atherogenic remnants.

IV. HDL and LDL Cholesterol Measurement

A. HDL-Cholesterol

• Clinical Importance: Important for CAD risk assessment; higher HDL correlates with lower CHD risk.

• Methodologies:

  1. Precipitation Methods: Polyanion used to separate HDL from other lipoproteins.

  2. Desirable Levels: >60 mg/dL (risk categories: 35-59 mg/dL, <35 mg/dL high risk).

B. Methods for LDL-Cholesterol Measurement

• Friedewald Formula:

  • $\text{LDL-cholesterol} = \text{Total Cholesterol} - (\text{HDL-Cholesterol} + \frac{\text{Triglycerides}}{5})$

  • Assumes no lipoproteins other than HDL, LDL, and VLDL; not reliable if triglycerides >400 mg/dL.

• Direct Measurement Methods: Available for cases exceeding triglyceride limits; automated methods enhance efficiency.

• Desirable Levels: <100 mg/dL (classification ranges provided).

V. Apolipoprotein Methods

• Significant in measuring lipid solubility and emerging risk indicators for CHD:

  1. Apo B: Major protein of LDL and VLDL, potentially better indicator of atherogenic particles than LDL-C.

  2. Apo A-I: Major protein of HDL, enables direct measurement.

  3. Lp(a): Variant of LDL, independent CHD risk factor.

• Most assays are immunoassays, sensitive to triglyceride-rich lipoprotein interference.