BASO on a blood test stands for basophils, a type of white blood cell reported on a complete blood count (CBC) with differential. According to Cleveland Clinic, basophils help defend the body against allergens, pathogens, and parasites; they release histamine (which affects blood flow and allergy symptoms) and heparin (which helps prevent unwanted clotting).
Basophils are normally the least common white blood cells. Cleveland Clinic states a typical basophil count is about 0.5% to 1% of total white blood cells, or roughly 0 to 300 basophils per microliter of blood in healthy adults. A result outside the lab’s reference range does not diagnose a specific disease by itself; clinicians interpret it with the rest of the CBC and the patient’s symptoms.
Source: Basophils: Function, Range & Related Conditions — Cleveland Clinic