Whether Medicare covers a laboratory testdoesn't depend on whether it's "routine" -- in fact, Medicare doesn't cover many routine lab tests -- but whether it's "medically necessary." Having the test ordered by your doctor is the first part of this process of deciding whether it's medically necessary. But Medicare must agree with your doctor that the lab test is necessary and proper, given your specific condition, and considering whatever other previous testing you've had. If the particular lab test is highly unusual for your condition, or if Medicare determines that other testing has been sufficient and that this particular test is not necessary, it might refuse to cover it. Or, it might refuse to cover it if the test itself is considered experimental, or is not approved for the particular purpose your doctor has in mind. On the other hand, if Medicare agrees with your doctor that the test is medically necessary and proper given your condition, it will cover it even if it's not routine for your particular condition.
Talk to your doctor about the question of Medicare coverage for this test. If your doctor thinks there might be some doubt whether Medicare will cover the test, ask that he or she contact Medicare in advance and ask for a coverage determination (a decision by Medicare about whether they will approve the test). Medicare may ask your doctor for more specific information, which he or she can then provide to Medicare, increasing the odds that it will cover the test. For more discussion of how to help get your care covered, see How Can I Increase the Odds That Medicare Will Cover My Medical Service? on this site.