How do you differentiate between acute renal failure and chronic renal failure?

A recent acute cause like severe bleeding and hypovolaemia or acute urinary tract obstruction usually point to an acute type of renal failure. Chronic Diabetes or hypertension, poly-cystic kidney disease etc on the other hand makes the chronic renal failure more likely.

An ultrasound can be helpful, as a chronic renal disease can lead to a shrunken kidney. So the size of the kidney can be helpful.

Serial measurement of serum creatinine is the preferred measure, but generally if in doubt, it is better to treat as acute renal failure, which is usually reversible, as opposed to the chronic form which is not.

Other biochemical abnormalities like hyperphosphataemia, hyperkalemia and low erythropoetin can also favor the chronic form of renal failure, but are generally less reliable.