None of them mean much. Neurotransmitters exist in separate compartments - the blood brain barrier keeps the brain pool quite separate from that of the peripheral nervous system for example. What you see in the blood doesn't reflect the brain while the urine reflects yet another compartment which is influenced by metabolism in the kidneys.
Here for example is a review which looks at the validity of serotonin and dopamine measurements in urine and concludes they are not valid (apart from well defined conditions like phaeochromocytoma).
If you search for studies on other neurotransmitters you will find similar problems.
