Done full blood test they all fine except one luc which is a tiny bit low what does luc mean and what does it mean if it's low?

Leukopenia?: I'm not entirely sure what you're asking. My best guess is that you're asking about leukocytes. If you're leukocytes are low, the cells in your bloodstream which fight infection are decreasing in number. This is called leukopenia. This can because by a lot of things, and requires follow up with your doctor who ordered the test. Some things that cause leukopenia include medications, infections, autoimmune disorder,or bone marrow problems.