Final answer:
A patient on bisphosphonate medication for 2 years developing rapid cycling should have their serum lithium levels measured.
Step-by-step explanation:
If a patient who is on bisphosphonate medication (such as Li) for 2 years develops rapid cycling, a laboratory test that should be done is measuring the serum lithium levels.
Rapid cycling is a term used to describe the condition where a person with bipolar disorder experiences four or more episodes of depression, mania, hypomania, or mixed states within a year.
Measuring the lithium levels is important as it helps determine if the rapid cycling is related to the medication. High lithium levels may indicate toxicity, which can cause rapid cycling among other symptoms.