Pitfalls of urinary protein/crt ratio

Nephrologists often use a rule of thumb that the urine protein: creatinine ratio—a unitless number—approximates the 24-h protein excretion in grams. This approximation is based on the assumption that the 24-h urine creatinine excretion is approximately 1000 mg.
There are few instances when this might not be true and hence in those cases the Total Protein/creatinine ratio can be misleading and give false results.

1. AKI:- As there is decreased clearance of creatinine, the total protein/crt ratio can have now a falsely lower denominator and hence leading to a larger ratio. This can possibly mistake some cases of just pure AKI with AKI now with nephrotic syndrome. 

2. Increased Muscle mass: In cases of gross increase in muscle mass, the ratio can grossly underestimate the total ratio. 

3. Severe CKD

A nice reference is in CJASN from 2009