The diagnosis of prerenal failure is made by finding concentrated urine and signs of a cause, when the urine output is inadequate.
Examine the patient for dehydration - dry tongue, skin turgor - peripheral perfusion - cold hands - blood pressure - hypotension or check JVP, CVP.
Check observation charts - look for negative fluid balance, check if there has been any period of hypotension or excess blood less.
The urine must be tested. If there is normal renal function then the urine will be concentrated (specific gravity over 1020).
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