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Understand Complete Blood Count

Raymond Lepage, PhD
Raymond Lepage, PhD
Senior Scientific Advisor

The complete blood count (CBC) is one of the most common laboratory tests. Here is some information to help you understand what it involves.

A CBC may be prescribed as part of a general health assessment or as a result of specific symptoms or signs such as fatigue, pallor, sensitivity to infections, phlebitis or abnormal bleeding.

The sample for a CBC is collected in a lavender-capped tube containing an anticoagulant. No fasting or other specific preparation is needed. Analysis for a CBC often involves three steps. The first step is done on an automated analyzer, which provides basic data on the number and percentage of the three types of cells that make up the blood: red blood cells, white blood cells and platelets. In the event of a significantly abnormal result, a medical technologist conducts a second analysis manually under a microscope. In rare cases, a blood smear will enable a hematologist to reanalyze results that are harder to interpret.

The results of a CBC are interpreted in two parts. First, we examine separately the information for each of the three cell types. Since these cells are all produced in the bone marrow, depending on the situation, we will then look at all three groups of cells together.

Red blood cells

Lines 2 to 6 in the results of your CBC all refer to red blood cells (RBCs), also known as erythrocytes.

Number: The number of RBCs is first calculated by the automated analyzer and the result is reported in thousands of billions (1012) RBCs per litre of blood. Normal or reference values vary depending on age and sex. The values on your report are already adjusted according to your demographic information. Men typically have higher counts of red blood cells, hemoglobin and hematocrit. A significant drop in the number of RBCs is usually a sign of anemia, while a significantly elevated result is a sign of polycythemia, also called polyglobulia.

Hemoglobin / hematocrit

The hemoglobin and hematocrit results provide basically the same information as the red blood cell count. Hemoglobin gives red blood cells their typical colour and is responsible for carrying oxygen from the lungs to tissues where oxygen is essential for metabolism, i.e. the various chemical reactions happening in cells that produce energy from nutrients. Hemoglobin is also partly responsible for eliminating, via the lungs, the CO2 (carbon dioxide) produced by cellular metabolism. A low hemoglobin level is therefore associated with poor oxygen delivery to tissues and slower cell metabolism, leading to the typical symptoms of anemia, such as pallor, weakness, fatigue, etc. The hematocrit test indicates the volume of blood occupied by red blood cells. This volume is typically 40% (0.40) for women and slightly higher for men. Again, a below-normal hematocrit is a sign of anemia, while an elevated hematocrit is often a sign of polycythemia or polyglobulia. Symptoms associated with polycythemia, (headaches, dizziness, tinnitus (perception of noise or ringing in the ears,) vision problems, tingling fingertips, etc.) are due in large part to poor blood circulation of blood that is too thick, while an excessive number of red blood cells leads to abnormal red coloration of the skin and mucous membranes, sometimes accompanied by burning or itching.


The MCV, or mean corpuscular volume, and the RDW or red cell distribution width both refer to the size of red blood cells. The average blood cell volume is expressed in femtolitres (10-15 L). Measuring the volume of red blood cells provides valuable diagnostic information in cases of anemia. Anemia with abnormally small red blood cells (microcytic anemia) does not have the same causes (most often iron deficiency) as anemia with large red blood cells (macrocytic anemia), which is most often caused by a lack of vitamin B12. As for anemia with a normal cell volume (normocytic anemia), this occurs primarily following a loss of blood and may be caused by digestive bleeding, the rupture or hemolysis of many red blood cells due to congenital causes, or the development of autoantibodies. Healthy bone marrow will quickly make up for any lack of RBCs by producing new red blood cells. These are called reticulocytes and are larger than more mature red blood cells. The presence of these cells is therefore normal and desirable in the case of anemia. The presence of red blood cells of different sizes is called anisocytosis. The RDW or red cell distribution width provides an idea of the variation in RBC volumes. The results are reported as a percentage variation and the higher the result, the greater the variation in RBC sizes.


The MCH (mean corpuscular hemoglobin) level indicates the amount in picograms (10-12 g) of hemoglobin in each red blood cell, while the MCHC (mean corpuscular hemoglobin concentration), expressed in grams per litre (g/L), refers to the mean hemoglobin concentration per litre of red blood cells. These two measures partly take into account the number of RBCs, the hemoglobin levels and the cell sizes. These indicators are closely related to the coloration of red blood cells. For example, anemia will be characterized by RBCs that have little colour (hypochromic anemia), have a normal colour (normochromic anemia), or are darker coloured (hyperchromic anemia). The presence of red blood cells of different colours in a blood sample is called anisochromia.

We find the same information when we do the CBC under a microscope. In this case, instead of using numerical values, we evaluate the degree of anisocytosis or anisochromia, both expressed as a number of “plus” signs, from zero to ++++.

Abnormal red blood cells

When indicated, a microscopic examination of whole blood can reveal the presence of abnormal forms or colors of red blood cells. Your report can therefore indicate the presence of drepanocytes, elliptocytes, codocytes, keretinocytes, etc. You will find in our Glossary a definition and the origin, genetic or pathologic of each of these abnormal forms of red blood cells.