Short answer
A wellness test estimates everyday tendencies, how you handle caffeine, carbs, or exercise, and needs no prescription. A medical test answers a specific clinical question and is ordered through a doctor.
Confusing the two is the most common mistake people make. A wellness or lifestyle test is low-stakes, direct-to-consumer, and about daily choices. A diagnostic test is high-stakes, clinician-ordered, and about a defined medical question. Both are legitimate; they are simply different tools for different jobs. Knowing which one you actually need saves money and disappointment.