Your doctor may order any of the following blood tests.

A complete blood count, or CBC. Your doctor may order this blood test if he or she suspects an infection is causing your headache. A complete blood count, or CBC, shows if you have an elevated white blood cell count. This could indicate an infection or other disease your body is fighting.

An erythrocyte sedimentation rate, or ESR. If you are older than 65 years, your doctor may take blood to test for giant cell, or temporal, arteritis. Temporal arteritis is an inflammation of the arteries in the temple. This blood test is called an erythrocyte sedimentation rate, or ESR. An ESR tests the speed at which mature red blood cells settle. An elevated rate can indicate many types of disease or inflammation.

A C-reactive protein test. Your doctor may also do a C-reactive protein test. This blood test measures abnormal protein that is detectable in the blood during the active phase of certain illnesses or inflammation.