Highly-sensitive PCR tests can continue to detect the virus for up to 90 days, meaning you may test positive well beyond the period of time you need to isolate. This is why individuals who test positive can be granted a 90-day exemption from testing with medical documentation. The Doctor needs to know the date of your first positive test and see that you don't have symptoms or symptoms that are minor ie. cough but not a productive cough. Mexico may or may not have different requirements for a medical professional to give you documentation.