Final answer:
The best first test for a person peeing blood is a urinalysis, which analyzes a urine sample to assess the presence of abnormalities like blood and white blood cells.
Step-by-step explanation:
The best first test for a person peeing blood would be a urinalysis. A urinalysis is a routine urine test that can provide important information about kidney function and detect certain abnormalities in the urine. It involves analyzing a urine sample physically, chemically, and microscopically to assess the presence of substances like blood, white blood cells, and proteins.