Complete Blood Count

A complete blood count (CBC) is a simple blood test that is commonly ordered as part of a routine medical assessment. As the name suggests, it is a count of the different types of cells found in the blood. The test can diagnose and monitor many different diseases, such as anemia, infection, inflammatory diseases, and malignancy. The following table gives an example of CBC values, but note that the reference ranges and the units used may differ, depending upon the laboratory that carried out the test.

Table 1. Complete blood count

Blood component Abbreviation used Reference range SI Reference range

White blood cells WBC 4500-11,000/mm³ 4.5-11.0 x 10⁹/L

Red blood cells* RBC Male: 4.3-5.9 million/mm³
Female: 3.5-5.5 million/mm³ Male: 4.3-5.9 x 10¹²/L
Female: 3.5-5.5 x 10¹²/L

Hemoglobin* HGB Male: 13.5-17.5 g/dL
Female: 12.0-16.0 g/dL Male: 2.09-2.71 mmol/L
Female: 1.86-2.48 mmol/L

Hematocrit* HT Male: 41%-53%
Female: 36%-46% Male: 0.41-0.53
Female: 0.36-0.46

Mean corpuscular volume MCV 80-100 µm³ 80-100 fl

Mean corpuscular hemoglobin MCH 25.4-34.6 pg/cell 0.39-0.54 fmol/cell

Mean corpuscular hemoglobin concentration MCHC 31%-36% Hb/cell 4.81-5.58 mmol Hb/L

Platelets Platelets 150,000-400,000/mm³ 150-400 x 10⁹/L

*Values differ depending upon altitude.

Red blood cell count detects anemia

A CBC measures the following features of RBCs:

the total amount of hemoglobin (Hb) in the blood
the number of RBCs (RBCs)
the average size of a RBC (MCV)
the amount of space RBCs take up in the blood (hematocrit)

The CBC also includes information about RBCs that is calculated from the other measurements, e.g., the amount (MCH) and concentration (MCHC) of hemoglobin in RBCs.

The number of RBCs and the amount of hemoglobin in the blood are lower in women than in men. This is because of the menstrual loss of blood each month. Below a certain level of hemoglobin, a patient is said to be anemic, suggesting a clinically significant drop in oxygen carrying capacity. Anemia is not a diagnosis but a symptom of an underlying disease that has to be investigated.

A clue to the cause of anemia is the average size of RBC (mean corpuscular volume, MCV). Causes of a high MCV include a deficiency of B₁₂ or folate vitamins in the diet. B₁₂ is found in red meat therefore, a deficiency of B₁₂ is especially common in vegetarians and vegans. Conversely, folate is plentiful in fresh leafy green vegetables, therefore, a deficiency of folate is common in the elderly, who may have a poor diet.

Low MCV anemia is common and may be a result of hereditary blood disorders, such as thalassemia, but is most often caused by a deficiency of iron. For example, women of reproductive age may lose too much iron through heavy menstrual bleeding and are prone to this form of anemia, known as iron-deficiency anemia.

Hematocrit is the percentage of RBCs in relation to the total volume of blood

The hematocrit measures the fraction of the blood that is made up of RBCs. It reflects the combination of the total number of RBCs, and the volume that they occupy.

One of the changes seen in pregnancy is a drop in hematocrit. This occurs because although the production of RBCs does not change greatly, the plasma volume increases, i.e., the RBCs are "diluted". Alternatively, a low hematocrit can reflect a drop in RBC production by the bone marrow. This may be attributable to bone marrow disease (damage by toxins or cancer) or due to a decrease in erythropoietin, a hormone secreted by the kidney that stimulates RBC production. Decreased RBCs may also be the result of a reduced life span of the RBCs (e.g., chronic bleeding).

A high hematocrit value may truly reflect an increase in the fraction of RBCs (e.g., increased erythropoietin attributable to a tumor of RBCs called polycythemia rubra vera), or it may reflect a drop in the plasma component of the blood (e.g., fluid loss in burn victims).

The number of WBCs increases in infection and tumors

The WBC count is a count of the number of WBCs found in one cubic millimeter of blood.

An increased number of WBCs is most commonly caused by infections, such as a urinary tract infection or pneumonia. It may also be caused by WBC tumors, such as leukemia.

A decreased number of WBCs is caused by the bone marrow failing to produce WBCs or by an increased removal of WBCs from the circulation by a diseased liver or an overactive spleen. Bone marrow failure may be caused by toxins or by the normal bone marrow cells being replaced by tumor cells.

The WBC differential part of the CBC breaks down the WBCs into five different types: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Finding out the count of each type of WBC gives more information about the underlying problem. For example, in the early stages of an infection, most of the increase in WBCs is attributable to the increase in neutrophils. As the infection continues, lymphocytes increase. Worm infections can trigger an increase in eosinophils, whereas allergic conditions, such as hay fever, trigger an increase in basophils.

The number of platelets indicates whether bleeding or clotting is likely

Normally, one cubic millimeter of blood contains between 150,000 and 400,000 platelets. If the number drops below this range, uncontrolled bleeding becomes a risk, whereas a rise above the upper limit of this range indicates a risk of uncontrolled blood clotting.

Source:

Blood Groups and Red Cell Antigens - published by National Library of Medicine, USA.