Eosinophil percentage: causes of high and low results

Eosinophils are one of the five types of white blood cell counted in the differential that accompanies a full blood count. They are made in the bone marrow and carry granules that release chemicals when they are activated. Lab Tests Online UK says that eosinophils increase during allergic reactions and some parasitic infestations such as worms.

The eosinophil percentage is the share of the total white cells that are eosinophils on the day the sample was taken. If your total white count is 8 and eosinophils make up a small slice, the percentage will be low, but if a different cell family drops away the percentage can climb without any real change in eosinophil numbers. That is why most reports pair it with an absolute count, and why clinicians treat the absolute count as the dependable value.

The differential is produced automatically with the full blood count, so you rarely request an eosinophil figure deliberately. It becomes the focus when a clinician is thinking about allergic disease, a drug reaction, an unexplained rash, breathing symptoms that come and go, long-standing bowel symptoms, or a recent trip abroad.

A persistently raised eosinophil count is called eosinophilia. Because it can have many causes, the approach is detective work: ask about allergies and medicines, ask where you have travelled, and review the rest of the blood count for clues.

There is nothing to prepare. The blood is taken from a vein in the arm in the usual way. Lab Tests Online UK notes that age, eating, physical activity and stress can shift white cell differential values slightly, and that long-term exposure to some toxic chemicals and smoking can raise the chance of an abnormal differential.

Eosinophil numbers also follow a daily rhythm in many people, being lower at certain times of the day, and corticosteroid medicines reduce them. For that reason, repeat tests are best taken under similar conditions where possible, and clinicians should know if you are taking steroid tablets or inhalers.

In its full blood count guide, Lab Tests Online UK lists the situations in which eosinophils may be high: allergic conditions such as asthma, hay fever or eczema, drug allergies, some parasitic infections, vasculitis, Hodgkin lymphoma and bone marrow disorders. Its differential page adds inflammation of the skin and certain bone marrow malignancies.

In day-to-day practice the common explanations come first. Seasonal hay fever, uncontrolled asthma, eczema flares and reactions to a newly started medicine account for most mild rises. Travel to parts of the world where parasitic worm infections occur is a further consideration, and a stool test or blood test for parasites may be requested when the history fits.

The less common explanations tend to come with other clues. Very high or rising counts, unexplained fevers, weight loss, rashes that do not settle or organ symptoms prompt specialist assessment. In those cases a haematologist or the relevant organ specialist may be involved.

A low eosinophil result is frequent and usually unimportant, partly because the healthy level is already a small proportion of white cells. Steroid treatment, acute stress and some acute infections can all reduce the figure for a time. Doctors do not usually pursue a low eosinophil count on its own when the remainder of the blood count is healthy.

For a small rise in someone with known allergic disease, the answer is often reassurance and better control of the underlying condition. For an unexplained rise the clinician may repeat the test, review your medicine list, request stool samples or specific serology, and look at inflammatory markers and kidney and liver tests.

If the count stays high without explanation, a referral to haematology or to a respiratory, skin or gastroenterology service may follow, depending on your symptoms. Bring a list of recent travel, supplements and over-the-counter products to that appointment.