Dodgy procedures

You are working in a transfusion laboratory with one other person -your supervisor- late on Friday night. It is a very busy night, and there are many requests coming in. One of the procedures involved in confirming compatible blood requires washing of the plasma sample a minimum of 4 times. Your supervisor tells you that he thinks you will be fine if you only wash the samples 2 or 3 times, as there is no way anyone would ever find out, and time is of the essence. You know that washing the sample less than 4 times will cause false positives.