LIPIDS

Lipids Overview

  • Organic substances insoluble in water but soluble in organic solvents (petroleum, ether, benzene, chloroform).

  • Commonly referred to as fats.

  • Require special transport mechanisms (lipoproteins) for circulation in blood.

Main Forms of Lipids in Humans

  • Fatty Acids

  • Triglycerides

  • Cholesterol

  • Phospholipids

Lipoproteins

  • Major Lipoproteins:

    • Chylomicrons

    • Very Low-Density Lipoprotein (VLDL)

    • Low-Density Lipoprotein (LDL)

    • High-Density Lipoprotein (HDL)

  • Minor Lipoproteins:

    • Intermediate-Density Lipoprotein (IDL)

    • Lipoprotein(a) (Lp(a))

Laboratory Considerations

  • Position

  • Application of Tourniquet

  • Type of Specimen

  • Handling and Storage

  • Choice of Anticoagulant

  • Fasting Requirement

  • Others

Patient Preparation

  • Fasting:

    • 12-14 hours fasting required for Triglycerides (TAG) and LDL-C measurements.

    • Non-fasting samples can only measure Total Cholesterol (TC) and HDL-C.

  • Diet:

    • Concentrations of LDL and HDL cholesterol decline transiently after eating.

  • Posture:

    • Patients should be seated for 5 minutes before sampling.

Cholesterol

  • Accounts for almost all sterol in plasma; exists as a mixture of unesterified and esterified.

  • Chemical Methods for Measurement:

    • Liebermann-Burchard Reaction

    • Salkowski Reaction

    • Abell-Kendall Method

  • Enzymatic Methods:

    • Cholesteryl ester + H₂O → cholesterol + fatty acid

    • Cholesterol + O₂ → cholest-4-en-3-one + H₂O₂

    • H₂O₂ + phenol + 4-aminoantipyrine → quinoneimine dye + 2H₂O

Advantages and Disadvantages of Enzymatic Methods

  • Advantages:

    • Less subject to interference.

    • Consumes only microtiter quantities of sample.

    • Rapid results.

  • Disadvantages:

    • Not specific for cholesterol (cholesterol oxidase can react with other sterols).

    • Reducing substances can interfere by consuming H₂O₂.

    • Sample turbidity and bilirubin can also interfere, as well as ascorbic acid.

Diagnostic Significance of Cholesterol

  • Evaluates risk for atherosclerosis, myocardial infarctions, and coronary arterial occlusions.

  • Used in thyroid, liver, renal function tests, and DM studies.

  • Aids in diagnosis and management of lipoprotein disorders.

  • Monitors effectiveness of lifestyle changes and stress management.

Cholesterol Reference Range

  • Normal Range: 140 – 200 mg/dL

  • Increased Levels in:

    • Hypothyroidism

    • Atherosclerosis

    • Myocardial Infarction (MI)

    • Diabetes Mellitus (DM)

  • Decreased Levels in:

    • Various forms of liver disease.

Triglycerides (TAG)

  • Esters of glycerol and 3 fatty acids, constituting 95% of adipose tissue.

  • Chemical Methods for Measurement:

    • Van Handel and Zilversmith Colorimetric Method

    • Hantzsch Condensation Reaction (Fluorometric Method)

  • Enzymatic Method:

    • TAG → glycerol + fatty acids

    • Glycerol + periodic acid → formaldehyde

    • Formaldehyde + chromotropic acid → blue-colored compound

    • Formaldehyde + diacetyl acetone + NH₃ → 3,5-diacetyl-1,4-dihydrolutidine

Triglycerides Reference Range

  • Normal Range: 60 – 150 mg/dL

  • Elevated Levels Associated with:

    • Atherosclerosis

    • Hypertension

    • Coronary artery disease

    • Diabetes Mellitus

    • Hypothyroidism

    • Certain types of Hyperlipoproteinemias

    • Alcoholism

    • Storage diseases (Gaucher, Niemann-Pick)

    • Pancreatitis

Diagnostic Significance of Triglycerides

  • Evaluates suspected atherosclerosis and body’s ability to metabolize fat.

  • Fasting TAG ≥ 200 mg/dL indicates risk for coronary artery disease.

High-Density Lipoprotein (HDL)

  • Reference Range: 40-75 mg/dL

  • Reduces atherosclerotic process.

  • Test Methodology:

    • Precipitation of LDL and VLDL using dextran sulfate magnesium chloride or heparin sulfate-manganese chloride, with analysis of the supernatant using enzymatic technique.

    • Homogeneous assay uses antibody to apo B-100 to bind LDL and VLDL, analyzing only HDL.

Low-Density Lipoprotein (LDL)

  • Reference Range: 50-130 mg/dL

  • Directly associated with atherosclerosis and coronary artery disease.

  • Friedewald Formula:

    • Indirect measurement; invalid for TAG > 400 mg/dL: LDL = TC - (HDL + TAG/5).

  • Homogeneous assay using detergents to block HDL and VLDL to isolate LDL.

Factors Affecting Plasma Lipid and Lp Concentration

  • Age

  • Sex

  • Diet

  • Medication

  • Seasonal Variation

  • Fasting Status

  • Venous Occlusion

  • Type of Anticoagulants

  • Pregnancy