Serum creatinine is a poor marker of renal reserve. A rise in serum creatinine > 0.120 mmol/L may not occur until more than 75% of renal function is lost. It is worth calculating creatinine clearance (using Cockroft and Gault equation) adjusting drug dosages accordingly
Creatinine clearance ml/sec = (140 – age) weight (in kg) (x 0.8 for females)
48869 x Serum Cr (mmol/l)
The minimum urine output required to excrete obligatory daily solute load is 0.5 ml/kg/hr.
Urine electrolyte analysis is of little use in Intensive Care to diagnose the aetiology of renal failure but maybe useful in specific electrolyte abnormalities.
Urine sediment: Unhelpful unless a specific reason exists (true vasculitis, nephritis).
Ultrasound may be useful where pre-existing pathology is suspected, or the renal vasculature has been compromised by surgery or trauma.
Post renal pathology is uncommon in the intensive care unit but should be excluded by ultrasound where there is any doubt about the cause of renal failure.