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Clinical Chemistry Comprehensive Exam Notes

Renal Function & Kidney-Related Chemistry

  • Key Waste Products & Where They Originate
    • Urea: principal nitrogenous waste of protein catabolism; synthesized in the liver via the urea cycle ⇒ transported in blood to kidneys for excretion.
    • Creatinine: by-product of muscle creatine phosphate breakdown; filtered mainly by glomerulus (minimal tubular re-absorption) ⇒ excellent endogenous marker of GFR.
    • Uric acid: end-product of purine metabolism; excess causes crystal deposition in joints → gout.
  • Major Diagnostic Tests
    • BUN/Urea test – evaluates renal clearance of urea; ↑ in renal failure, dehydration, high-protein diet.
    • Creatinine clearance – compares serum & 24-h urine creatinine to estimate glomerular filtration \text{C}{Cr}=\frac{U{Cr}\times V}{P{Cr}} (mL/min). • eGFR – automated calculator that adjusts for age, sex, race. • Osmolality – total solute (ions+molecules) concentration in serum or urine (ref 280–300\,\text{mOsm/kg H2O}); dehydration, SIADH, DKA alter result.
    • Sweat chloride by pilocarpine iontophoresis – confirms cystic fibrosis when Cl⁻ > 60\,\text{mmol/L}.
  • Electrolyte Overview
    • Primary extracellular cation: Sodium (Na⁺).
    • Normal K⁺ range: 3.5–5.0\,\text{mmol/L}; hyperkalemia → arrhythmia risk.
    • Hypernatremia etiologies include Cushing syndrome (↑ cortisol → Na⁺ retention).
    • Anion Gap with K⁺: \text{AG}=(Na^+ + K^+) - (Cl^- + HCO_3^-) → for given patient \text{AG}=5.5\,\text{mmol/L} (135+3.5−105−28).

Blood Glucose Regulation & Diabetes

  • Hormonal Control
    • Pancreatic β-cells produce insulin (↓ glucose), α-cells produce glucagon (↑ glucose via glycogenolysis).
    • Negative feedback maintains euglycemia.
  • Pathophysiology
    • Type 1 DM: autoimmune β-cell destruction → absolute insulin deficiency.
    • Gestational DM screen: 50-g oral glucose drink, plasma collected at 1 h.
    • HbA1c goal in treated diabetics: < 7 % (reflects 2–3-month glycemic history due to 120-day RBC life span).
    • Ketone production: occurs when lipids, not glucose, are primary fuel (starvation, DKA) → β-hydroxy-butyrate, acetoacetate, acetone.

Cardiac & Vascular Markers

  • Troponins (cTnT / cTnI)
    • Most sensitive & specific marker for myocardial injury; detectable in conditions beyond MI (e.g., myocarditis, CHF, renal failure).
  • BNP (Brain-type Natriuretic Peptide)
    • Ventricular stretch peptide; diagnostic/prognostic for congestive heart failure (CHF).
  • CRP (C-Reactive Protein)
    • Acute-phase protein; high-sensitivity CRP (hs-CRP) stratifies cardiovascular risk.
  • Sample Integrity Issues
    • Hemolysis falsely elevates K⁺, LD, AST, etc.; most dramatic for potassium.

Lipid & Lipoprotein Metabolism

  • Functions & Forms
    • Lipids provide cell-membrane structure & electrical insulation for neurons.
    • Exogenic cholesterol: dietary origin vs. endogenic (hepatic synthesis).
  • Standard Lipid Panel
    • Total Cholesterol, HDL, LDL (calculated by Friedewald: \text{LDL}=\text{Chol}-\text{HDL}-\frac{\text{Trig}}{2.2} mmol/L when trig < 4.5 mmol/L).
    • Chylomicrons cause gross lipemia (milky serum); can be cleared by ultracentrifugation prior to testing.

Liver Function & Jaundice

  • Key Enzymes
    • ALT & AST (transaminases) – hepatocellular damage (viral hepatitis).
    • ALP & GGT – cholestasis/obstruction.
    • LD (esp. LD-5) – hepatic & muscular injury.
  • Types of Jaundice
    • Pre-hepatic: hemolytic anemia ↑ unconjugated bilirubin.
    • Hepatic: viral hepatitis.
    • Post-hepatic: obstruction (↑ conjugated bilirubin).
  • Total protein/albumin
    • Hyperproteinemia can be physiologic (dehydration).
  • Clotting
    • Liver synthesizes most coagulation factors; INR may be ordered for hepatic assessment.

Thyroid & Other Hormones

  • Hormones Defined: chemical messengers secreted by glands → bloodstream → distant target organs.
  • Thyroid Hormones
    • T4 = thyroxine, T3 = triiodothyronine; regulate basal metabolic rate; assessed by immunoassays.
  • β-hCG
    • Glycoprotein from placenta; quantitative serum testing confirms/monitors pregnancy & trophoblastic tumors.

Immunoassay Technologies

  • ELISA (Enzyme-Linked Immunosorbent Assay)
    • Principle: antigen bound to surface, patient antibody binds, enzyme-labeled conjugate (e.g., alkaline phosphatase) produces measurable color.
    • Used for hormones, drugs, infectious markers.
  • Nephelometry
    • Measures light scattered by antigen–antibody complexes ⇒ quantifies proteins (IgG, CRP).
  • Tumor Markers
    • CA-125 – ovarian cancer monitoring.
    • AFP, CEA, PSA, etc. (connections to future lectures).

Spectrophotometry & Photometry Fundamentals

  • Visible Spectrum: ~400 nm (violet) – 700 nm (red); 300–340 nm = ultraviolet (invisible to naked eye).
  • Beer–Lambert Law: A = \varepsilon b c ⇒ absorbance proportional to concentration.
  • Instrument Components (proper order)
    1. Light source → 2. Collimator (lens) → 3. Prism/monochromator → 4. Slit (selector) → 5. Cuvette (sample) → 6. Detector (photocell) → 7. Digital display.
  • Applications
    • Photometry: broader term for any color-based quantitative measurement.

Specimen Collection & Pre-Analytical Variables

  • Tube Types & Additives
    • Red top (no anticoagulant) → serum + clot after centrifuge.
    • SST/PST: polymer gel; if gel fails to migrate ⇒ re-spin, aliquot into new tube.
    • Grey top: NaF (glycolysis inhibitor) + K oxalate anticoagulant; preserves glucose.
  • Handling
    • Amber microtainer protects neonatal bilirubin from light.
    • Remove serum from cells promptly (except SST) to prevent cellular metabolism ↓ analytes.
  • Hemolysis & Icterus
    • Hemolyzed serum: pink/red; Icteric: brownish-yellow (bilirubin); Lipemic: milky.

Calculated/Observed Chemistry Values

  • Anion Gap Example shown earlier.
  • Serum Osmolality Example: derive by formula 2(Na^+)+\frac{Glucose}{18}+\frac{BUN}{2.8} (mg/dL units) – future practice.

Acid–Base & Gas Parameters

  • Blood pH
    • Normal 7.35–7.45; acidosis if < 7.35.
  • Bicarbonate (HCO₃⁻) – major metabolic buffer; total CO₂ content of serum.

Hematology Tie-ins

  • RBC Life Span: 120 days → relevance for HbA1c & bilirubin production (breakdown leads to jaundice).
  • Hemolysis Definition: rupture of RBCs releasing Hb into plasma; pre-analytical error.

Therapeutic Drug Monitoring (TDM)

  • Information Required: patient/date, exact time of draw, last dose time, drug name/dose/route.
  • Example Drugs: digoxin, phenytoin (not methadone – statement in transcript says false).

Pancreatic Enzymes & GI Tests

  • Amylase & Lipase: elevated in acute pancreatitis; lipase more specific.
  • Fecal Occult Blood Test: guaiac-impregnated paper reacts with peroxidase activity of Hb → blue color.
  • Chylomicron Removal: ultracentrifugation to clarify sample.

Miscellaneous High-Yield Facts

  • Hormone Regulation: negative feedback loops (e.g., TSH–T4 axis).
  • Glycogenolysis: breakdown of glycogen to glucose; stimulated by glucagon & epinephrine.
  • Electrolyte Disorders: hyperkalemia definition; hypernatremia causes.
  • Cardiac vs. Non-Cardiac Causes of Troponin Elevation: renal failure, sepsis, strenuous exercise.
  • Light-Scattering vs. Light-Absorbance: Nephelometry vs. Spectrophotometry.
  • Enzymes & Pathology Summary: ALT/AST (liver), CK-MB (heart, historical), LD (multi-organ), ALP (bone, liver).

Ethical & Practical Implications

  • Correct specimen handling prevents erroneous results that could mislead treatment (e.g., pseudo-hyperkalemia).
  • Accurate TDM documentation ensures dosage safety, averting toxicity.
  • hs-CRP & lipid profiling guide preventive strategies in cardiovascular disease, aligning lab practice with population health.