Table 1: Factors that affect the measured serum concentrations of creatinine and urea

      Creatinine     Urea
   
Increase   Decreased GFR   Decreased GFR
    Increased skeletal muscle mass   Dehydration or low urine flow
   

Drugs that reduce tubular secretion
e.g. cimetidine and trimethoprim

   rate
    High protein intake   Gastrointestinal bleed
    Positive analytical interference
e.g. ketone and cephalosporins		   High catabolic state
    High meat intake      
             
Decrease   Decrease in muscle mass   Low protein intake
    Negative analytical interference
e.g. bilirubin		   Liver disease