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Chemistry

Lipids

Lipid testing is fundamental in assessing cardiovascular risk and identifying metabolic disorders. This comprehensive overview integrates the underlying biochemical theory with laboratory testing procedures, result interpretation, and disease state correlation

Biochemical Theory and Physiology

  • Lipid Functions
    • Energy Storage: Triglycerides are the primary form of energy storage
    • Cell Structure: Phospholipids and cholesterol are essential components of cell membranes
    • Hormone Synthesis: Cholesterol is a precursor for steroid hormones and bile acids
  • Lipid Metabolic Pathways
    • Lipolysis: Breakdown of triglycerides to release fatty acids and glycerol
    • Fatty Acid Oxidation: Breakdown of fatty acids to generate energy
    • Lipogenesis: Synthesis of fatty acids from acetyl-CoA
    • Cholesterol Synthesis: Complex process to produce cholesterol
    • Lipoprotein Metabolism: Assembly, transport, and breakdown of lipoproteins
  • Lipoproteins
    • Chylomicrons: Transport dietary triglycerides
    • VLDL: Transport triglycerides from the liver
    • LDL: Transport cholesterol to tissues
    • HDL: Transport cholesterol from tissues back to the liver (reverse cholesterol transport)
  • Key Enzymes
    • Lipoprotein Lipase (LPL): Hydrolyzes triglycerides in lipoproteins
    • HMG-CoA Reductase: Rate-limiting enzyme in cholesterol synthesis
  • Hormonal Regulation
    • Insulin: Promotes lipid storage and inhibits lipolysis
    • Glucagon and Epinephrine: Promote lipolysis
    • Cortisol: Affects lipid metabolism and distribution

Laboratory Test Procedures

  • Lipid Panel
    • Total Cholesterol (TC): Enzymatic methods
    • Triglycerides (TG): Enzymatic methods
    • HDL Cholesterol (HDL-C): Direct or indirect methods
    • LDL Cholesterol (LDL-C): Calculated (Friedewald equation) or direct methods
  • Apolipoproteins
    • Apo A-1 (HDL component): Immunochemical methods
    • Apo B (LDL, VLDL, IDL component): Immunochemical methods
  • Lipoprotein (a) [Lp(a)]
    • Immunochemical methods
  • Lipid Electrophoresis
    • Separation of lipoproteins based on electrical charge and size
  • Special Precautions
    • Patient Preparation: Fasting (9-12 hours), medication considerations
    • Specimen Collection: Proper tube type, order of draw, and tourniquet time
    • Specimen Processing: Prompt centrifugation and storage
  • Interfering Substances
    • Lipemia, bilirubin, hemoglobin, medications

Test Result Interpretation

  • Reference Intervals
    • Total Cholesterol: < 200 mg/dL
    • Triglycerides: < 150 mg/dL
    • HDL Cholesterol: > 40 mg/dL
    • LDL Cholesterol: Varies depending on risk factors
  • Clinical Significance
    • Elevated LDL-C and non-HDL-C increase CVD risk
    • Low HDL-C is an independent risk factor for CVD
    • Elevated triglycerides increase CVD and pancreatitis risk
  • Factors Affecting Interpretation
    • Fasting status, medications, medical conditions, ethnicity, age, gender, pregnancy

Disease State Correlation

  • Atherosclerosis
    • Elevated LDL-C, non-HDL-C, and Apo B; low HDL-C
  • Coronary Artery Disease (CAD)
    • Elevated LDL-C, triglycerides, and Lp(a); low HDL-C
  • Hyperlipidemia and Dyslipidemia
    • Elevated total cholesterol, LDL-C, and triglycerides; low HDL-C
  • Metabolic Syndrome
    • Elevated triglycerides and fasting glucose; low HDL-C
  • Acute Pancreatitis
    • Elevated triglycerides (> 500 mg/dL)
  • Lipoprotein Disorders (e.g., Familial Hypercholesterolemia, Tangier Disease)
    • Elevated LDL-C (FH), very low HDL-C (Tangier)
  • Non-Alcoholic Fatty Liver Disease (NAFLD)
    • Elevated liver enzymes, triglycerides, and fasting glucose
  • Clinical Management
    • Lifestyle Modifications: Diet, exercise, weight loss, smoking cessation
    • Medications: Statins, fibrates, niacin, omega-3 fatty acids
    • Assessment of Cardiovascular Risk: Use risk assessment tools (e.g., Framingham Risk Score, ASCVD Risk Estimator)