Unit Converter
Basophils (BASO)
(Percentage of Basophils – Proportion – Fraction of Total WBCs)
Synonyms
- BASO %
- Basophil percentage
- Relative basophil count
- Basophil proportion
- Basophil fraction (of 1.0)
Units of Measurement
- % (percentage of total WBCs)
- Proportion of 1.0 (decimal form)
- Fraction (e.g., 0.005 = 0.5%)
Description
Basophils are the least abundant granulocytes, comprising <1% of circulating white blood cells.
The relative basophil count (BASO %) indicates the percentage of basophils among total leukocytes, not the absolute number.
BASO % is used to:
- Screen for myeloproliferative disorders
- Assess allergic inflammation
- Interpret CBC differentials
- Detect changes in thyroid or immune conditions
Physiological Role of Basophils
Basophils contain granules with:
- Histamine – vasodilation, allergic symptoms
- Heparin – anticoagulant
- Leukotrienes – bronchoconstriction
- Cytokines (IL-4, IL-13) – drive Th2 immune response
Basophils participate in:
- Type I hypersensitivity
- Chronic allergic inflammation
- Helminth immunity
- Immune modulation with mast cells
Clinical Significance
High BASO % (Relative Basophilia)
May indicate:
1. Myeloproliferative Neoplasms (Most Important)
- Chronic Myeloid Leukemia (CML) → hallmark finding
- Polycythemia vera
- Essential thrombocythemia
- Primary myelofibrosis
2. Allergic Conditions
- Chronic urticaria
- Atopic dermatitis
- Asthma
- Drug hypersensitivity
3. Chronic Inflammation
- Ulcerative colitis
- Rheumatoid arthritis
4. Endocrine Disorders
- Hypothyroidism
- Estrogen therapy
Low BASO % (Relative Basopenia)
Usually not clinically significant.
Seen in:
- Acute infections
- Hyperthyroidism
- Stress (cortisol surge)
- Chronic steroid therapy
- Pregnancy
- Severe allergic reactions (initial degranulation phase)
Reference Intervals
(Based on Tietz 8E + ISLH + ASH hematology data)
Basophils (%)
- 0 – 1% (normal)
Basophils (proportion of 1.0)
- 0.000 – 0.010
Examples:
- 0.5% = 0.005
- 1.0% = 0.010
- 1% → basophilia (if persistent)
Important
BASO % is less diagnostic than the absolute basophil count, but is always part of the CBC differential.
Diagnostic Uses
1. CBC Differential Interpretation
Helps identify immune shifts:
- Allergy → ↑ BASO %
- Infection → often ↓ BASO %
- Steroid effect → ↓ BASO %
2. Screening for Hematologic Disorders
Persistent BASO % elevation prompts:
- Peripheral smear
- Absolute basophil count
- Bone marrow studies (if needed)
- BCR-ABL testing (for CML)
3. Endocrine & Metabolic Associations
- Hypothyroidism → ↑ BASO %
- Hyperthyroidism → ↓ BASO %
Analytical Notes
- Automated hematology analyzers calculate BASO % using flow cytometry.
- Ensure no clots or platelet clumps (may alter differential).
- EDTA samples stable for ~24 hours.
- BASO % should always be interpreted with absolute basophil count (ABC).
Clinical Pearls
- BASO % > 1% is uncommon → evaluate for MPN, especially CML.
- BASO % can be misleading when total WBC is low or high; always check absolute basophils.
- Thyroid dysfunction significantly affects basophil proportions.
- In allergic disease, BASO % elevation is usually mild (<2%).
- BASO % is a relative measure, not a marker of total basophil burden.
Interesting Fact
Basophils make up such a small fraction of WBCs that early hematology textbooks called them “the forgotten granulocyte”, yet they play a powerful role in allergy and immune signaling.
References
- Tietz Clinical Chemistry & Molecular Diagnostics, 8th Edition - Hematology.
- ISLH Guidelines - Differential Leukocyte Count.
- ASH Hematology Reference Ranges.
- Mayo Clinic Laboratories - CBC Interpretation.
- ARUP Consult - Basophilia & Differential Diagnosis.
- NIH / MedlinePlus - WBC Types.
- Hematology Textbooks - Basophil Physiology & Pathology.