Final answer:
Hemoglobin has a high affinity for oxygen, allowing even a small amount of hemoglobin to effectively transport oxygen throughout the body.
Step-by-step explanation:
Hemoglobin is a protein found in red blood cells that transports oxygen in the bloodstream. Only a small amount of hemoglobin is required to get a positive result in tests because hemoglobin has a high affinity for oxygen, meaning it readily binds oxygen molecules. This allows even a small amount of hemoglobin to effectively transport oxygen throughout the body.