if a patient being dialyzed needs a pt inr drawn and the heparin was intiated for 5 minutes before being realized would it have a large affect on the pt inr

Did the patient receive a bolus? If not, then I don't think 5 minutes will make much, if any, difference, especially to the INR. You could always wait a little while after turning the heparin off, if time permits, and then draw the specimen.