Creatinine

Normal Range <120 mmol/l

  • most reliable simple test of GFR
  • ideally should be fasting sample as dietary intake may increase concentration
  • concentration related to muscle mass
    • lower in children, small adults
      • normal value may not indicate absence of renal problems
    • Transient increase after exercise
    • massive increases in crush injuries, rhabdomyolysis
  • falls in pregnancy due to increased GFR
Increased Creatinine
  • impaired GFR
  • High muscle mass
  • Acute muscle damage
  • minimal increase from high meat meal
  • Transient increase after excercise
 Reduced creatinine
  • small muscle mass
  • pregnancy
  • SIADH

Creatinine clearance

= ([creatinine] urine x volumeurine ) / ([creatinine]plasma x timecollection)

  • volume of plasma which is theoretically cleared of substance per minute
  • plasma and urine creatinine need to be in same units
  • Inulin (NOT inSulin) is more accurate as a small quantity of creatinine secreted into tubule (similarly a small quantity of urea diffuses in) but requires exogenous source of inulin so not used in practice
 

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