Final answer:
The first sample from a heparin or saline lock must be discarded by the phlebotomist to avoid dilution with the lock's fluids and ensure accurate laboratory test results.
Step-by-step explanation:
A phlebotomist must discard the first sample drawn from a heparin or saline lock because it may be diluted with the heparin or saline from the lock itself. This pre-drawn fluid can interfere with the accuracy of certain laboratory tests, as it may alter the composition of the blood sample. It's critical to obtain a sample that reflects the patient's true physiological state, so discarding the initial sample ensures that subsequent samples are valid and untainted by anticoagulants or other fluids. Laboratory results obtained from such samples are crucial in informing medical decisions, including diagnosis, treatment, and management of medical conditions.