Serum or Plasma Substance Concentration (Eosinophil Count)

What Is an Eosinophil?

Eosinophils are a type of immune cell that plays a role in defending the body against parasites, managing allergic reactions, and reducing inflammation. They are part of the white blood cell family and are typically present in small amounts in healthy blood. When levels rise, it may signal an underlying condition requiring further evaluation.

Why Might This Test Be Ordered?

Your healthcare provider may order this test to investigate symptoms such as persistent itching, skin rashes, asthma, chronic sinusitis, or unexplained fatigue. It can also help diagnose conditions like allergic reactions, parasitic infections, certain cancers (e.g., leukemia), or autoimmune disorders.

What to Expect Before the Test

No special preparation is usually needed. If you’re having blood drawn, you may want to drink plenty of fluids the day of the test to help reduce discomfort. If you’re having a urine sample collected, follow any specific instructions your provider gives you.

What Happens During the Test?

Blood is drawn from a vein in your arm, or a urine sample may be collected. The sample is then analyzed in a lab to count the number of eosinophils present. Results are typically reported as the absolute count of eosinophils per microliter (µL) of blood.