Is there a blood test to detect leukemia?

Yes.: There are many forms of leukemia. Some are "acute" and present quickly. These can be serious / fatal without treatment. Other types of leukemia are "chronic' and can be present for years without the patient knowing / being diagnosed. One of the first tests used to diagnose any leukemia is a complete blood count and review of a peripheral blood smear under the microscope. Hope this helps.