Basophils (BASO#): absolute count in blood

What are basophils (BASO#)

Basophils are the rarest white blood cells — they make up less than 1% of all leukocytes. Despite their small number, basophils play an important role in allergic and inflammatory reactions. They contain granules that hold histamine and heparin — substances involved in allergic responses and the regulation of blood clotting. BASO# shows the absolute count of basophils per unit volume of blood.

This marker is measured as part of a complete blood count with differential. The result is reported in thousands of cells per microlitre (10³/µL).

Reference ranges

The absolute basophil count in adults is typically 0 to 0.2 × 10³/µL (or 0–0.2 × 10⁹/L). Many healthy individuals have basophil counts close to zero, so even a small increase can appear dramatic as a percentage. Always interpret your result against the reference range provided by your laboratory.

Why basophils may be elevated

An elevated basophil count (basophilia) is rare. The most common causes are allergic reactions — especially chronic allergic reactions to food or medication. Reduced thyroid function (hypothyroidism) may be associated with mild basophilia.

Myeloproliferative disorders, such as chronic myeloid leukaemia, can cause a significant increase in basophils. Iron deficiency is also sometimes linked to a slightly elevated basophil count. Inflammatory bowel diseases (Crohn’s disease, ulcerative colitis) may influence basophil counts.

Why basophils may be low

Basopenia (very low basophil count) is difficult to evaluate because the normal count is already very low. Acute infections, acute stress, and the use of glucocorticoids can temporarily reduce basophil counts to undetectable levels. Ovulation and pregnancy are also associated with a physiological decrease in basophils.

In most cases, a low basophil count has no clinical significance and does not require further investigation.

What to do with your result

If your basophil count is elevated, your doctor will first assess other components of the blood differential — particularly neutrophils, eosinophils, and platelets. A persistent elevation without a clear allergic cause may prompt additional haematological investigations.

Managing allergies, reducing stress, and having regular health check-ups help maintain immune system balance.

Monitoring over time

Because the absolute count is very small, fluctuations in basophils between tests can appear dramatic as a percentage, even though they are clinically insignificant. It is important to monitor the trend across several tests and to evaluate results alongside the full white blood cell differential. If your doctor suspects a haematological condition, tests are repeated regularly.

Questions for your doctor

1. Is my basophil count within normal limits, taking into account my other blood markers? 2. Could elevated basophils be related to an allergy or thyroid function? 3. Are additional tests needed (e.g., iron, thyroid hormones)? 4. How often should I repeat the test if my basophils are outside the normal range?

*This is not medical advice. Consult your doctor regarding any health decisions.*