Creatinine

Normal: 0.5 - 1.5 mg/dl  (44 - 132 μmol/L)

Physiology

Creatinine is present in the blood and excreted in the urine.  Each day, about two percent of the creatine phosphate in muscle is converted to creatinine by an irreversible reaction.

Low creatinine: less than 0.5 mg/dL

Causes:

Creatinine may be low in patients who are malnourished or have decreased muscle mass.  Fluid overload may initially dilute creatinine causing a falsely low concentration.

Correction: Check for underlying causes : fluid overload, low protein intake, inability to metabolize protein due to liver failure.

TPN relevance: Assists the clinician in determining the fluid and renal status of the patient.

High creatinine: levels above 1.5 mg/dL

Because creatinine is a byproduct of muscle metabolism, unlike BUN, creatinine is not influenced by changes in protein intake.  An increase in creatinine usually reflects a decrease in renal function.

Assessment: Check the BUN to creatinine ratio, the normal ratio is 10:1.  A ratio of 10 to 20 suggest intrinsic renal damage.  A BUN:creatinine ratio greater than 20 suggests pre-renal causes, e.g., dehydration, GI bleeds, excessive protein intake.

Correction: Check underlying causes: fluid status of the patient/dehydration, factors influencing protein catabolism and reduced renal function. Check for GI bleeding.

TPN relevance: Assist the clinician in determining the fluid and renal status of the patient.