Creatinine is useful. But creatinine is late.
This is one of the most important concepts in critical care nephrology. A patient can have significant kidney injury before the creatinine looks impressive.
Creatinine is a concentration, not a real-time kidney monitor. It depends on creatinine production, volume of distribution, renal clearance, muscle mass, and time.
In early shock, glomerular filtration may fall quickly, but serum creatinine may rise slowly. This delay can create false reassurance.
Urine output often changes earlier. Oliguria may reflect decreased renal perfusion, neurohormonal sodium retention, tubular injury, venous congestion, obstruction, or medication effects.