Creatinine

• Non-protein end product of creatinine metabolism.
• Formed from skeletal muscle and excreted in constant amounts.
• Completely filtered by kidney (marker of GFR)
• Production proportional to muscle mass
• Relatively unaffected by diet, exercise or urine volume
• More sensitive test of renal damage.  Elevation of levels almost exclusively secondary to renal impairment
• Clearance is an excellent indicator of kidney efficiency in clearing creatinine
  • Clearance levels become abnormal when >50% of nephron units have become damaged

Causes of increased serum creatinine

1. Renal impairment (>50% nephrons affected) – any cause of renal failure, basically
   • Pre-renal
     • Decreased perfusion to the kidney
     • Dehydration, hypovolaemia, shock
   • Renal
     • Intrinsic renal lesion
     • Nephrotoxic drugs
   • Post renal obstruction
2. Nephrotoxic drugs
   • NSAID, aspirin, indomethacin
   • Ampicillin, rifampicin, cephalosporin, aminoglycosides
   • Frusemide, Thiazides, acetazolamide, mannitol
   • Lithium, iron, lead, copper, gold
3. Metabolic
   • Acromegaly
   • Hyperthyroidism
   • Very large muscle mass (athletes) (without renal compromise)
   • NOT affected by dietary protein intake
4. Fictitious
   • Interfere with colorimetric analysis (Glucose, fructose, ascorbic acid)

Causes of a Decreased Creatinine

• Pregnancy
• Conditions characterised by muscle wasting