Blood and lab tests
What Are Basophils on a Blood Test?
Basophils · say it: BAY-so-filsAlso written: BASO, basophil count, baso%
Short answer
Basophils are the rarest white blood cells. They release histamine during allergic reactions and appear on a full blood count.
What it means
Basophils are the least common type of white blood cell. They carry granules filled with chemicals such as histamine, which they release during allergic reactions and inflammation. Histamine is the same substance that causes itching, sneezing and swelling in hay fever.
Because basophils are so few, the numbers on a complete blood count (CBC) are often tiny, sometimes shown as 0.0 or 0.1. The lab still reports them to complete the white cell breakdown. A flag tells the doctor the count is outside the lab's range, but counts this small can shift with how the machine happened to count the sample.
Basophils are sometimes confused with mast cells, which hold similar chemicals but live in tissues such as the skin and gut rather than in the blood. Basophils are also one of the three granulocytes, alongside neutrophils and eosinophils.
Where you may see it
- The white cell differential in a CBC, labeled "BASO" or "BASO%"
- Patient portal blood results, listed beside eosinophils and monocytes
- Hematology reports that describe a blood smear looked at under a microscope
📄 How it looks on a report
Basophils 0.1 x10^9/L
🙅 What it does not mean
- A reading of zero does not mean your body has no basophils. They are so rare that a machine may count none in the small sample it measured.
- A slightly high basophil count is not, on its own, a sign of a serious blood disease. Doctors look at it together with the rest of the blood count.
- Seeing basophils listed does not mean the doctor ordered an allergy test. They are part of every standard white cell breakdown.
🗒️ Questions to ask your doctor
🧾 Reports where you will see “Basophils”
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🔗 Read more from trusted sources
- “Basophils” on MedlinePlus ↗
US National Library of Medicine
- MedlinePlus lab test guides ↗
US National Library of Medicine
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General information about a medical word, not advice about your own results. Ask the clinician who ordered the test what it means for you.