That's a good question.
Looking at this: http://apps.who.int/iris/bitstream/10665/65957/1/WHO_DIL_LAB... for EDTA (a very common anticoagulant "preservative") it is 1.2 to 2.0 mg/mL, so a decent range. But, this is one source and it will also depend on what the test is looking for.