Q: Assuming patient has normal renal function, what is a laboratory way to determine that GI bleed is likely to be upper?
If the ratio of blood urea nitrogen to creatinine is greater than 30 the source is more likely from the upper GI tract.
Srygley FD, Gerardo CJ, Tran T, Fisher DA (March 2012). "Does this patient have a severe upper gastrointestinal bleed?". JAMA 307 (10): 1072–9