How the test is performed
A complete blood count (CBC) serves as the starting point in the diagnosis of most diseases, including hemoblastoses and various types of anemia. The material for the test is either capillary blood (collected using a scarificator from the pad of the ring finger of the left hand, puncture depth 2–3 mm) or venous blood, which is drawn from the median cubital vein. For routine examinations, the material is collected in the morning on an empty stomach, though emergency indications allow collection at any time.
A mandatory requirement of the procedure is the use of disposable instruments and thorough skin disinfection with alcohol. Modern automated electronic analyzers are the current standard for counting formed elements. Nevertheless, classical blood smear microscopy is still used to evaluate cell morphology and precisely calculate the differential leukocyte count. Any obtained figures can be interpreted only in conjunction with the actual clinical picture and with mandatory evaluation of changes over time.
Red blood cells: erythrocytes and hemoglobin
Hemoglobin (Hb) reflects the oxygen-carrying capacity of the blood. A decrease in this indicator is the primary laboratory marker of anemia. An increase can be physiological (compensation for hypoxia in high-altitude residents and smokers) or pathological, indicating erythrocytosis.
Erythrocytes normally have a biconcave disc shape, which maximizes the surface area for efficient gas exchange. The normal range in women is $3.9–4.9 \times 10^{12}/\text{L}$, and in men $4.0–5.2 \times 10^{12}/\text{L}$ (higher values in men are due to the stimulation of erythropoiesis by androgens).
- Quantitative changes: An increase in the number of red blood cells (erythrocytosis) can be reactive, secondary (in renal tumors or chronic pulmonary insufficiency), or primary (polycythemia vera). A decrease (erythrocytopenia) is associated with massive blood loss, pronounced hemolysis, or suppressed bone marrow production (infections, chronic kidney disease, cytostatic therapy).
- Qualitative changes: The smear may reveal cells of varying sizes (anisocytosis) or altered shapes, such as sickle or oval cells (poikilocytosis).
Special indices are used for an objective assessment of size and hemoglobin saturation. MCV (mean corpuscular volume) is normally 80–94 fL and allows for the detection of microcytosis or macrocytosis. MCH (mean corpuscular hemoglobin, normal range 27–33 pg) helps classify anemias into hypochromic (e.g., iron-deficiency) and hyperchromic.
Evaluation of regeneration: reticulocytes
An important parameter is the count of reticulocytes — young anucleated forms of erythrocytes that have just emerged from the bone marrow. Their normal proportion is 0.5–1.5%, which reflects the daily physiological turnover of approximately one percent of the cellular pool.
Reticulocytes serve as a reliable marker of the regenerative capacity of the bone marrow. Their increase (reticulocytosis) indicates an adequate organism response to acute blood loss or hemolysis. A decrease is typical for aplastic, iron-deficiency, and megaloblastic anemias.
This parameter is indispensable for monitoring treatment efficacy. For example, during the treatment of $B_{12}$-deficiency anemia, a sharp rise in this indicator ("reticulocytic crisis") should occur within 7–10 days. If the number of young forms increases less than 2–3 times, the prescribed treatment is considered ineffective. Note that in profound impairments of erythropoiesis, normoblasts (nucleated precursors) may appear in the blood, which are normally absent in the peripheral blood.
White blood cells: leukocytes and differential leukocyte count
White blood cells react sensitively to pathological processes. An increase in the total number of white blood cells is called leukocytosis, and a decrease is called leukopenia.
- High neutrophilic leukocytosis is characteristic of bacterial infections, localized purulent processes (lung abscess), massive tissue necrosis (myocardial infarction, burns), and endogenous intoxications.
- Leukopenia most often accompanies viral infections, exposure to ionizing radiation, immunosuppressant therapy, and systemic autoimmune pathologies (such as SLE). The extreme degree of suppressed granulocyte production is called agranulocytosis syndrome, which leads to a life-threatening decrease in the body's resistance to infections.
Of special diagnostic value is the differential leukocyte count — the percentage ratio of various white blood cell types. In acute purulent-inflammatory processes, a so-called shift to the left occurs: young neutrophils enter the bloodstream (band cells, and in severe cases, metamyelocytes and myelocytes).
Changes in the proportion of other cells are also highly specific:
- Eosinophilia indicates allergic conditions or parasitic infestations.
- Monocytosis is characteristic of specific infections (tuberculosis) or systemic diseases (rheumatoid arthritis).
- Lymphocytosis accompanies viral infections and lymphoproliferative disorders.
In malignant tumors, leukemoid reactions may occur — conditions with pronounced leukocytosis and a sharp rejuvenation of the differential count, which visually mimic the picture of true leukemia, but are exclusively reactive in nature.
Platelets, hematocrit, and ESR
Platelets are counted in a smear (counting per 1000 erythrocytes) or by an automated method on a hematological analyzer. A decrease in their number (thrombocytopenia) clinically manifests as bleeding and occurs in Werlhof's disease, DIC syndrome, autoimmune reactions, or during cytostatic therapy. An increase (thrombocytosis) is characteristic of hemoconcentration, myeloproliferative disorders, oncology, or post-splenectomy status (removal of the spleen).
Hematocrit (normal range 40–50%) is the ratio of the volume of formed elements to the volume of the liquid part of the blood (plasma). The indicator increases with blood thickening (pronounced dehydration during vomiting, primary erythrocytoses) and decreases with hemodilution (massive infusion therapy, compensatory reaction a few hours after acute blood loss).
Erythrocyte sedimentation rate (ESR) is a non-specific marker reflecting the physico-chemical properties of the blood. ESR increases in inflammation (due to changes in the plasma protein spectrum — an increase in fibrinogen and gamma-globulin levels), oncology (especially sharp acceleration caused by multiple myeloma with paraproteinemia), as well as in anemias and alkalosis. A decrease in ESR is observed in blood thickening (erythrocytosis) and acidosis.
Briefly: answers to topical questions
What parameters are included in a complete normal differential leukocyte count?
Normally, the differential leukocyte count reflects the percentage ratio of five types of circulating leukocytes: basophils, eosinophils, various forms of neutrophils, lymphocytes, and monocytes.
Normal values for an adult:
| Type of leukocyte | Content (%) |
|---|---|
| Basophils | 0,5–1 |
| Eosinophils | 2–4 |
| Neutrophils: | |
| — Myelocytes | — (none) |
| — Metamyelocytes (juvenile) | 0–1 |
| — Band cells | 3–5 |
| — Segmented cells | 51–67 |
| Lymphocytes | 21–35 |
| Monocytes | 4–8 |
The term "shift to the left" can be remembered as follows: if you imagine the cell maturation scale from left to right (from young to old), the appearance of immature (band, juvenile) forms in the blood pulls the balance toward the "left" (young) edge. This is a reliable marker of acute inflammation.
Class and exam questions
Why does the erythrocyte sedimentation rate (ESR) increase?
ESR increases due to changes in the protein composition of the plasma. This occurs in bacterial inflammation, autoimmune processes, or the appearance of specific proteins (paraproteins) in malignancies. ESR also accelerates when the number of erythrocytes itself decreases (anemias).
What do reticulocytes mean in a test?
These are young, newly released erythrocytes from the bone marrow. They show how actively the bone marrow is producing new cells, which is fundamentally important for evaluating the body's response to anemia treatment.
Can hematocrit decrease after hemorrhage?
Yes, but this does not happen immediately, rather a few hours after acute blood loss. The decrease is associated with the compensatory inflow of fluid from tissues into the vascular bed, leading to blood dilution (post-hemorrhagic reaction).
What else to study on this topic
- A graphical method for evaluating erythrocytes: plotting the Price-Jones curve
- Pathological inclusions in erythrocytes (basophilic stippling) in poisonings
- Morphological elements of the LE-cell phenomenon in systemic lupus erythematosus
- Pathogenetic mechanisms and differential diagnosis of leukemoid reactions
- Effect of paraproteinemia on the physicochemical properties of plasma in multiple myeloma
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