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Renal Physiology

GFR

  • How to measure renal function in clinical practice
  • Serum creatinine and serum cystatin C are the primary biomarkers used to estimate GFR
    • Nonlinear relationship
      • Every time GFR halves, the serum creatinine doubles
    • PCT meds that decrease creatinine excretion:
      • Cimetidine, Trimethoprim, Cobicistat, Dolutegravir, Bictegravir, Rilpivirine
  • Non-progressive rise in serum creatinine (0.2-0.3 mg/dL)
    • More pronounced in CKD
  • Do not affect GFR
  • Do not need to stop unless (hematuria, pyuria, proteinuria increase)
  • Could measure Cystatin C
    • Cystatin C less affected by age, sex, muscle mass
  • BUN
    • Affected by protein intake and catabolic rate
    • Increases with glucocorticoids, hemorrhage, or trauma

False Creatinine Elevations

  • Increased Production (0.5-1.0)
    • Menstrual and Diurnal variations, race, and diet (meat)
    • Intense Exercise
    • Increased meat intake or Creatine Supplements
    • Fenofibrates (8-18% change), Rhabdomyolysis
    • Corticosteroids (10%)
  • Interference
    • Antibiotics (Cephalosporins especially cefoxitin and cefazolin)
    • Barbiturates, N-acetylcysteine, chemo drugs (Flucytosine)
    • Dopamine (-)
    • Ketone bodies
  • Decreased Tubular Secretion (15%)
    • Trimethoprim, Cimetidine, H2 blockers, Ranitidine
    • Pyrimethamine, Salicylates, Cobicistat, Calcitriol
      • Does not affect eGFR

Indications for Kidney Biopsy

  • Glomerular hematuria
  • Severely Increased Albuminuria
  • Acute on chronic kidney disease
  • Kidney Transplant dysfunction or monitoring