Any creatine-loading must be done at least 3 days before the scheduled U/A because it takes 2-3 days for the body to metabolize creatine into creatinine. Creatinine, being the end waste product, is whats present in urine and tested by the labs.

Aspirin is old tech. Back in the day, it was used as an interferant for only one type of test: the first-generation EMIT screen. Since then, a second-gen EMIT was introduced to counter the interference issues caused by aspirin. As a result, the first-gen EMIT is seldom used anymore (if, at all).