A blood count is the examination of the blood for its various components. The blood count is the most common blood test of all, as changes in the blood count occur in a wide variety of diseases. A small blood count can be distinguished from a large blood count, which then includes a differential blood count in addition to the small blood count. In the differential blood count, the various subgroups of leukocytes (white blood cells) in a stained blood smear are counted and evaluated as a percentage of each other:For adults
| Leukocytes | Absolute values | Percentage |
| Neutrophil segment nucleated granulocytes | 3,000-5,800/μl | 50-70 % |
| Neutrophil rod-nucleated granulocytes | 150-400/μl | 3-5 % |
| Eosinophil granulocytes | 50-250/μl | 1-4 % |
| Basophilic granulocytes | 15-50/μl | 0-1% |
| Lymphocytes | 1,500-3,000/μl | 25-45 % |
| Monocytes | 200-800/μl | 2-10 % |
For children
| Leukocytes | Absolute values | Percentage |
| Neutrophil segment nucleated granulocytes | 2,000-7,800/μl | 25-65 % |
| Neutrophil rod-nucleated granulocytes | 0-1,200/μl | 0-10 % |
| Eosinophil granulocytes | 80-600/μl | 1-5 % |
| Basophilic granulocytes | 0-120/μl | 0-1 % |
| Lymphocytes | 2,000-6,000/μl | 25-50 % |
| Monocytes | 80-720/μl | 1-6 % |
For infants
| Leukocytes | Absolute values | Percentage |
| Neutrophil segment nucleated granulocytes | 2,250-9,750/μl | 22-65 % |
| Neutrophil rod-nucleated granulocytes | 0-1,500/μl | 0-10 % |
| Eosinophil granulocytes | 90-1,050/μl | 1-7 % |
| Basophilic granulocytes | 0-300/μl | 0-2 % |
| Lymphocytes | 1,800-10,500/μl | 20-70 % |
| Monocytes | 630-3,000/μl | 7-20 % |
Legend
- Basophilic granulocytes (basophils for short) – also serve as parasite defenses, are triggers of allergic reactions, and make inflammatory reactions.
- Eosinophil granulocytes (short: eosinophils) – serve parasite defense and are triggers of allergic reactions.
- Neutrophil granulocytes (short: neutrophils) with their subgroups provide phagocytosis (“freshening activity of the cells”) of pathogens
- Segmental nucleated granulocytes (neutrophil granulocytes).
- Rod-nucleated granulocytes (penultimate maturation stage of granulopoiesis/development of granulocytes).
- Lymphocytes – include the B cells, T cells and natural killer cells (NK cells) and belong to the leukocytes.
- Monocytes – are the precursor of macrophages (“scavenger cells”).
Definitions
- Agranulocytosis: (almost) complete absence of neutrophil granulocytes in the blood count.
- Basophilia: increase in basophilic granulocytes in the blood picture.
- Eosinopenia: decrease in eosinophilic granulocytes in the blood count.
- Eosinophilia: increase in eosinophilic granulocytes in the blood count.
- Leukopenia: decrease in leukocytes in the blood.
- Leukocytosis: increase in leukocytes in the blood.
- Lymphopenia: decrease in lymphocytes in the blood picture.
- Lymphocytosis: increase in lymphocytes in the blood picture.
- Monocytosis: increase in monocytes in the blood picture.
- Neutropenia: decrease in neutrophil granulocytes in the blood count.
- Neutrophilia: increase in neutrophil granulocytes in the blood count.
Blood count changes in infections
| Normal leukocyte count | Brucellosis, malaria, syphilis (stage II), toxoplasmosis, circumscribed tuberculosis, sleeping sickness, |
| Leukocytosis | Bacterial infections in general, amoebic liver abscess, miliary tuberculosis, rheumatic fever, sepsis |
| Leukopenia | Viral diseasesBrucellosis, malaria, visceral leishmaniasis (synonyms: kala-azar; oriental bump; also known as dum-dum fever or black fever), typhoid fever, and paratyphoid fever, |
| Neutropenia | Brucellosis, malaria, visceral leishmaniasis (synonyms: kala-azar; oriental bump; also known as dum-dum fever or black fever), tuberculosis |
| Toxic neutrophils | Bacterial infections |
| Lymphocytosis | Epstein-Barr virus, cytomegalovirus, other viral diseasesbrucellosis, syphilis, toxoplasmosis, tuberculosis |
| Monocytosis | Bacterial endocarditis, granulomatous disease, syphilis, tuberculosis, |
| Eosinophilia | Acute Fasciola hepatica infection, disseminated coccidioidomycosis, Katayama fever, muscular sarcocystosis, strongyloidiasis, trichinosis |
| Eosinopenia | Typhus abdominalis |
| Thrombocytopenia | Acute HIV infection, dengue fever, Lyme disease, leptospirosis, malaria, rickettsiosis, sleeping sickness, sepsis,visceral leishmaniasis (in the setting of pancytopenia (synonym: tricytopenia: deficiency in all three cell series of hematopoiesis: leukocytopenia (decrease in white blood cells), anemia (anemia), and thrombocytopenia (platelets)). |
For further notes on the following topics, see leukocytes below:
- Leukocytosis or leukopenia.
- Differentiation: is leukocytosis reactive or malignant (“malignant”)?
- Delimitation: is a left shift reactive or pathological (“pathological”)?
- Right shift