Final answer:
When a phlebotomist uses an expired tube, informing the supervisor and following their instructions is the correct action to ensure test result integrity and patient care.
Step-by-step explanation:
If a phlebotomist discovers that she has used an expired lavender top tube upon returning to the lab after drawing an in-house patient's blood, the correct course of action would be to inform the supervisor and follow their instructions. The use of an expired tube could compromise the integrity of the test results, potentially leading to erroneous laboratory data, which could affect patient care. The supervisor will be able to provide guidance on whether to redraw the specimen or take any other necessary steps to ensure accurate test results.