Lab result explainedBlood count (CBC)Last updated November 1, 2026

Basophils on a blood test: what high and low basophils mean

Basophils (absolute basophil count) (BASO) · CBC with differential · reported in ×10³/µL (same as ×10⁹/L); also reported as % of white blood cells

Basophils are the rarest white blood cells, making up less than 1% of the total, and they carry granules of histamine and heparin that they release during allergic reactions and in defence against parasites. On an adult CBC with differential, most US labs call an absolute basophil count of 0–0.1 ×10³/µL (0–100 cells per microlitre), or 0–1% of white cells, normal. The two most common reasons the count is raised are allergic conditions (hay fever, urticaria, food or drug allergy) and an underactive thyroid; the most common reasons it is low are steroids, acute infection or stress and an overactive thyroid, though a low count is rarely meaningful because the normal range starts at zero.

A basophil result that is flagged high is usually a small number, such as 0.2 or 0.3 ×10³/µL, and on its own is seldom the sign of anything serious. Its real diagnostic value is as a companion marker: a persistently raised basophil count alongside a high neutrophil count, high platelets or an enlarged spleen is one of the classic clues to chronic myeloid leukaemia and other myeloproliferative disorders, which is why clinicians look at it even though it is a tiny fraction of the white cells.

This page explains what the count measures and why the percentage can mislead, the reference range, the causes of high and low basophils in order of how often they appear, the markers a clinician reads it beside, and what usually happens next.

BASO at a glance

Basophils are rare white blood cells that release histamine and heparin in allergic reactions and against parasites.
Normal adult absolute basophil count: 0–0.1 ×10³/µL (0–100/µL), about 0–1% of white cells; some labs allow up to 0.2 ×10³/µL or 2%.
High basophils (basophilia, above 0.1–0.2 ×10³/µL) most often mean allergy, hypothyroidism, chronic inflammation or recovery from infection.
Persistent basophilia with high neutrophils, high platelets or an enlarged spleen is a classic clue to chronic myeloid leukaemia and needs a blood film and BCR-ABL test.
Low basophils (basopenia) are usually normal and reflect steroids, stress, acute infection or an overactive thyroid.
Next step: check the absolute count, review allergies and thyroid function, and repeat the CBC; persistent unexplained basophilia goes to a blood film.

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What BASO measures

The basophil result is part of the white cell differential on a complete blood count. Automated analysers identify basophils by their dense, dark-staining granules and report them as a percentage of white cells and as an absolute basophil count, the percentage multiplied by the total white cell count. Because basophils are so scarce, the result is the least precise part of the differential: a 100-cell manual count that finds one basophil reports 1%, and finding two reports 2%, so small swings are expected noise rather than change. Only the absolute count should be interpreted, and only when it is clearly and repeatedly raised.

Basophils are closely related to mast cells, which live in tissues, and both carry receptors for IgE. When an allergen cross-links IgE on their surface, basophils release histamine and other mediators within minutes, producing the itch, swelling and flushing of an allergic reaction. They also help recruit other immune cells in chronic allergic inflammation and parasitic infection. In the marrow they come from the same lineage as neutrophils and eosinophils, which is why disorders of marrow production (myeloproliferative neoplasms) raise all three together.

Normal range for BASO

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (×10³/µL (same as ×10⁹/L); also reported as % of white blood cells)	Notes
Adults, absolute count	0.0–0.1 ×10³/µL (0–100 cells/µL)	Some labs use 0–0.2 ×10³/µL. Basophilia is usually defined as above 0.1–0.2 ×10³/µL on repeat testing.
Adults, percentage	0–1% of white blood cells	Up to 2% on some reports. Use the absolute count; a 2% reading with a low white count can be normal.
Children	0.0–0.1 ×10³/µL	Similar to adults; counts are slightly higher in newborns.
Pregnancy	Similar to adults	Basophils change little in pregnancy; the percentage may look lower as total white cells rise.

High basophils (basophilia)

A high absolute basophil count (above about 0.1–0.2 ×10³/µL) most often reflects an allergic or inflammatory process, an underactive thyroid or the recovery phase of an infection, and in these settings it is mild, usually under 0.3 ×10³/µL, and resolves when the cause is treated. Because the count is so small and the measurement imprecise, a single mildly raised result that is not reproduced on repeat should not be over-read.

Basophilia matters most as part of a pattern. A persistently raised count, particularly above 0.3–0.5 ×10³/µL, together with a high neutrophil count, immature white cells on the film, high platelets, raised eosinophils or an enlarged spleen, is a recognised signal of chronic myeloid leukaemia (where basophils above 20% of white cells marks the accelerated phase) or of polycythaemia vera, essential thrombocythaemia or myelofibrosis. In that pattern a blood film and a BCR-ABL blood test are the next step and usually settle the question.

Common causes

Allergic disease
Allergic rhinitis, chronic urticaria, food and drug allergies and atopic dermatitis raise basophils modestly through IgE-driven activation and IL-3 release. The rise is usually small and tracks with symptoms.
Hypothyroidism
Low thyroid hormone slows basophil clearance and is one of the best-recognised non-blood causes of a persistently mild basophilia. It resolves once the thyroid is replaced, so TSH is a standard early test.
Recovery from acute infection
Basophils are suppressed during acute infection and rebound as it resolves, so a mildly raised count a week or two after an illness is expected.
Chronic inflammatory disease
Inflammatory bowel disease (ulcerative colitis especially), rheumatoid arthritis and some chronic skin conditions cause mild basophilia that follows disease activity.
Chronic infections
Tuberculosis, varicella, influenza and some parasitic infections (hookworm, filaria) can raise basophils, usually with other cell types.
Myeloproliferative neoplasms and chronic myeloid leukaemia
CML is the most important cause of marked basophilia and is often the only condition that pushes the count well above 0.5 ×10³/µL. Polycythaemia vera, essential thrombocythaemia and primary myelofibrosis also raise basophils alongside red cells, platelets or immature cells. Basophilia with any of these features needs a blood film and BCR-ABL testing.
Iron deficiency anaemia
Iron deficiency has been linked with a mild rise in basophils in some series, correcting with iron replacement; the mechanism is unclear and the effect small.
Post-splenectomy
Without a spleen, basophils and other cells that would normally be filtered out run slightly higher.
Medications and oestrogen
Oestrogen-containing drugs and antithyroid medications have been associated with mild basophilia; it is uncommon and reversible.
Lab artefact
Automated analysers can misclassify abnormal cells, large platelets or lipid particles as basophils, producing a spurious high count. A manual film review resolves this and is why a single unexpected result is repeated before anything else is done.

Symptoms that can go with it

Often none; the result is found incidentally
Itching, hives, sneezing, flushing or swelling in allergic causes
Fatigue, weight gain, cold intolerance, dry skin or constipation in hypothyroidism
Gut symptoms such as bloody diarrhoea in inflammatory bowel disease
Fatigue, night sweats, weight loss, bone pain, easy bruising or fullness under the left ribs when a myeloproliferative disorder is the cause

Low basophils (basopenia)

A low basophil count, usually reported as zero, is within the normal range and is rarely significant on its own. It most often reflects cortisol: either a steroid medication or the body's stress response to an acute infection, surgery, injury or a severe allergic reaction in progress, all of which clear basophils from the blood within hours. Hyperthyroidism also lowers the count, as thyroid hormone speeds basophil turnover.

Because most labs cannot reliably distinguish 0.00 from 0.02 ×10³/µL, basopenia is not a result that drives investigation. It is simply noted, and the context, particularly steroid use, acute illness or thyroid symptoms, explains it.

Common causes

Corticosteroids
Prednisone and other glucocorticoids drive basophils out of the blood within hours; counts return once the course ends.
Acute infection, stress or injury
The cortisol and adrenaline surge of acute illness, surgery, trauma or burns suppresses basophils along with eosinophils.
Hyperthyroidism
Excess thyroid hormone shortens basophil survival; counts normalise when the thyroid is controlled.
Acute allergic reaction or anaphylaxis
During a major allergic reaction basophils degranulate and leave the blood, so the count can transiently fall even though the problem is allergic.
Ovulation and pregnancy
Counts dip briefly around ovulation and are mildly lower in pregnancy; neither is clinically relevant.
Chemotherapy and radiotherapy
Marrow suppression lowers all white cells, including basophils.

Symptoms that can go with it

None attributable to the low count itself
Palpitations, weight loss, heat intolerance or tremor if hyperthyroidism is the cause
Fever or the features of an acute illness when it reflects stress or infection

Read it alongside

Neutrophils — marked neutrophilia with basophilia is the classic combination in chronic myeloid leukaemia; basophilia with normal neutrophils favours allergy, thyroid or inflammation.
Eosinophils — raised together, they point to allergy, parasites or a myeloproliferative disorder; eosinophils alone favour allergy or drug reaction.
Platelets — high platelets with basophilia suggests a myeloproliferative neoplasm rather than a reactive cause.
TSH — hypothyroidism is a common, treatable cause of mild persistent basophilia and hyperthyroidism of a low count.
Hematocrit and hemoglobin — a high haematocrit with basophilia raises polycythaemia vera; anaemia with basophilia raises CML or myelofibrosis.
Blood film — distinguishes true basophils from misclassified cells and shows immature granulocytes, which change the picture entirely.
Total IgE — supports an allergic explanation when basophils and eosinophils are both mildly raised.

What usually happens next

Check that the absolute count, not just the percentage, is raised; a 2% reading with a normal white count may be within range.
Repeat the CBC with differential in 4–6 weeks with a manual film review; many mild basophil flags are not reproduced.
Ask for a TSH if not already done, and review allergy symptoms, recent infections and inflammatory conditions.
If basophilia persists above 0.2 ×10³/µL, or appears with high neutrophils, high platelets, a high haematocrit, immature cells or an enlarged spleen, expect a BCR-ABL test and JAK2 mutation test and a haematology referral.
A low count needs no action beyond noting steroids, acute illness or thyroid symptoms.

Diet, lifestyle and natural levers with evidence

What can move BASO once the cause is known. Nothing here replaces finding the cause.

Identify and avoid allergic triggers

Moderate evidence

Basophil counts and activation fall when the driving allergen is removed or treated: in chronic urticaria and allergic rhinitis, effective allergen avoidance, antihistamines and allergen immunotherapy reduce basophil reactivity and histamine release, measured by basophil activation tests in clinical trials. Diet elimination only helps when a specific food allergy has been confirmed by testing.

Avoid broad elimination diets without a confirmed allergy; they rarely help and carry nutritional risk.

Treat hypothyroidism and support thyroid nutrition

Moderate evidence

Replacing thyroid hormone in hypothyroidism normalises the basophil count, and this is the natural lever with the clearest effect on a persistently mild basophilia. Correcting a documented iodine or selenium deficiency supports thyroid function, but supplementing either without a measured deficiency can worsen thyroid disease.

Do not take iodine or selenium supplements for a basophil result; test thyroid function first and treat the thyroid, not the number.

Correct iron deficiency

Preliminary evidence

Small case series describe mild basophilia resolving with iron replacement in people with iron deficiency anaemia. The evidence is thin, but iron deficiency is common and easily measured with ferritin, so checking it is reasonable when basophils are mildly raised with anaemia.

Take iron only for a confirmed deficiency; excess iron is harmful and will not lower a basophil count that has another cause.

Quercetin and vitamin C for allergic symptoms

Preliminary evidence

Quercetin inhibits histamine release from basophils and mast cells in laboratory studies, and small human trials report modest improvement in allergic rhinitis symptoms. Vitamin C has been reported to lower histamine levels in small studies. Neither has been shown to lower the basophil count itself, and both are adjuncts for allergy symptoms rather than treatments for basophilia.

Quercetin can interact with some antibiotics and blood thinners; keep doses moderate.

Anti-inflammatory diet and weight management

Preliminary evidence

Chronic low-grade inflammation and inflammatory bowel disease activity are associated with mild basophilia, and Mediterranean-style eating and weight loss lower inflammatory markers such as CRP in randomised trials. No trial has used the basophil count as an outcome, so the effect on this marker is inferred rather than shown.

When to see a clinician promptly

A basophil count above 0.3 ×10³/µL, or any basophilia alongside a neutrophil count above 15 ×10³/µL, high platelets, a high haematocrit or immature cells reported on the film.
Persistent basophilia on two tests a month or more apart with no allergic or thyroid explanation.
Fatigue, night sweats, unexplained weight loss, bone pain, easy bruising or fullness under the left ribs with any abnormal count.
Itching after hot showers, burning or redness of the hands and feet, or blood clots with a high count, which can point to a myeloproliferative disorder.
Hives, swelling of the lips or throat, wheeze or dizziness after a food, drug or sting, which need emergency care regardless of the basophil number.

Frequently asked questions

Is a basophil count of 0.2 bad?

A count of 0.2 ×10³/µL (200/µL) is at or just above the upper limit on most reports. In someone with hay fever, hives, an underactive thyroid or a recent infection it is expected and not harmful. If the rest of the blood count is normal, the usual approach is a repeat test and a TSH; only a persistent rise, or basophilia with other abnormal counts, needs more.

What is a basophil percentage of 2% telling me?

Usually very little. Basophils are so scarce that counting one or two cells in a hundred gives 1% or 2%, and the difference is measurement noise. What matters is the absolute count: 2% of a white count of 5 is 0.1 ×10³/µL, which is normal. Look at the absolute number before worrying.

Can allergies raise basophils?

Yes. Allergic rhinitis, chronic urticaria, food and drug allergies and eczema all mildly raise basophils because IgE-bearing basophils are central to allergic reactions. The rise is typically small (under 0.3 ×10³/µL), often accompanied by a mild rise in eosinophils, and settles when the allergy is controlled.

What cancers cause high basophils?

Chronic myeloid leukaemia is the one most strongly associated with basophilia, and a basophil count that keeps rising is used to track its progression. Other myeloproliferative neoplasms (polycythaemia vera, essential thrombocythaemia, myelofibrosis) and, rarely, acute basophilic leukaemia or mastocytosis also raise basophils. In these conditions other parts of the blood count are nearly always abnormal too; isolated mild basophilia with a normal count is rarely cancer.

Does hypothyroidism really raise basophils?

Yes. An underactive thyroid is one of the best-documented non-haematological causes of a persistently mild basophilia, and the count normalises once thyroid hormone is replaced. That is why a TSH is a routine early test for an unexplained basophil result, especially with fatigue, weight gain or cold intolerance.

What does low basophils mean?

Almost always nothing, because the normal range starts at zero. A count of zero is usually explained by steroid medication, an acute infection, physical stress, an overactive thyroid or an allergic reaction in progress. It does not need investigation on its own.

Can basophils be high from a lab error?

Yes, and it is one of the more common reasons for an unexpected flag. Analysers can count large platelets, lipid droplets or abnormal cells as basophils, especially when a sample has been stored before processing. A repeat sample with a manual film review by a technologist is the standard check and often returns a normal result.

References

Blood differential test. MedlinePlus, US National Library of MedicineView source
Sticco KL, Lynch DT. Basophilia. StatPearls, NCBI Bookshelf (2023)
Riley LK, Rupert J. Evaluation of patients with leukocytosis. American Family Physician (2015)View source
Siracusa MC, Kim BS, Spergel JM, Artis D. Basophils and allergic inflammation. Journal of Allergy and Clinical Immunology (2013)
Arber DA, Orazi A, Hasserjian R, et al.. The 2016 revision to the World Health Organization classification of myeloid neoplasms and acute leukemia. Blood (2016)
Chirumbolo S. State-of-the-art review about basophil research in immunology and allergy: is the time right to treat these cells with the respect they deserve?. Blood Transfusion (2012)
Blood basics. American Society of HematologyView source
Hypothyroidism (underactive thyroid). Mayo ClinicView source
Iron: fact sheet for health professionals. NIH Office of Dietary SupplementsView source

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.