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Full blood count

A full blood count (FBC) or complete blood count (CBC) is a test requested by a doctor or other medical professional that gives information about the cells in a patient's blood.

Automated blood count

Blood for a FBC is usually taken into an EDTA tube to stop it from clotting. The blood is well mixed (though not shaken) and put through a machine. The machine, called an automated analyser, counts the numbers and types of different cells within the blood. The machine prints out, and/or sends to a computer, the results.

Blood counting machines work by sampling blood, and sucking a standard amount through narrow tubing. Within this tubing, there are sensors that count the number of cells going through it, and can identify the type of cell.

The two main sensors used are light detectors, and electrical impedance.

Because an automated cell counter samples and counts so many cells, it gives a very precise estimate. However, with certain abnormal cells in the blood, they may be identified incorrectly, and not be as accurate as a manual count. Automated blood counting machines include the Sysmex XE-2100 and the Celldyne range.

Manual blood count

Counting chambers, that hold a specified volume of diluted blood (as there are far too many cells if it is not diluted) are used to calculate the number of red and white cells per litre of blood.
To identify the numbers of different white cells, a blood film is made, and a large number of white cells (at least 100) are counted. This gives the percentage of cells that are of each type. By multiplying the percentage, with the total number of white blood cells, an estimate of the absolute number of each type of white cell can be obtained.

Manual counting has the advantage in that it can identify blood cells that may be misidentified by an automated counter. It is, however, subject to human error, and has a much smaller sample size. Additional factors, such as the quality of the blood film, also play a greater part.

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Results from a blood count

A complete blood count will normally include:

Red cells

Total red blood cells - The number of red cells is given as an absolute number per liter.

Hemoglobin

The amount of hemoglobin in the blood, expressed in grams per liter. (Low hemoglobin is called anemia.)

Hematocrit

Hematocrit or packed cell volume (PCV) - This is the fraction of whole blood volume that consists of red blood cells. Usually three times the Hgb.

Red cell indices

(Wintrobe Indices) - Wintrobe introduced a very useful method to show certain characteristics of red cells.

A. Mean Corpuscular Volume (MCV)

the average volume of the red cells, measured in femtolitres. This concept utilizes the effect that the average sized RBC has on the hematocrit. If the average RBC size is increased, the same number of RBCs will have slightly larger cell mass and thus a slightly increased hematocrit reading. The opposite happens if the average RBC size is smaller than normal. The MCV is, therefore, calculated from the hematocrit and RBC count as follows:

Reference values are 80-100 m3

If <80, cells are microcytic; if >100, cells are macrocytic.

Anemia is classified as microcytic or macrocytic based on whether this value is above or below the expected normal range. Other conditions that can affect MCV include thalassemia and reticulocytosis.

B. Mean Corpuscular Hemoglobin (MCH)

The average amount of hemoglobin per red blood cell, in picograms. This concept is an estimate of the amount of hemoglobin in the average red cell. This is done by comparing the blood hemoglobin level to the RBC count as follows:

Reference values are 27-31 pg

C. Mean Corpuscular Hemoglobin Concentration (MCHC)

Mean cell hemoglobin concentration (MCHC) - the average concentration of hemoglobin in the cells. This concept estimates the concentration of hemoglobin in the average RBC. It is different from MCH in that the average RBC concentration of hemoglobin depends on the RBC size and the actual amount of hemoglobin contained in the RBC. The MCHC is calculated as follows:

Reference values are 32-36 g/dL

Examination of a well-made peripheral blood smear will give most of the same information as will the indices. The indices are not a substitute for examination of the peripheral blood smear, but they may be helpful in confirming impressions. The indices are only as accurate as the various counts and procedures that went into their calculation.

Red cell distribution width (RDW)

A measure of the variation of the RBC population

White cells

Total white blood cells - All the white cell types are given as a percentage and as an absolute number per litre.

A complete blood count with differential will also include:

• Neutrophil granulocytes - May indicate bacterial infection. If >8,000: infection, some inflammatory states, stress, steroids, other drugs, myeloproliferative disease. Absolute neutrophil count (ANC) <500: increased risk for infection; ANC <100: infection certain (if neutropenia persists).
• Lymphocytes - Higher with some viral infections and CLL. <1,500: lymphopenia; >4,000: lymphocytosis; increased in convalescent phase after bacterial or viral infection, lymphoproliferative disease.
• Monocytes - This is raised in glandular fever. Increased in acute and chronic infection, inflammation, some myeloproliferative disorders, chronic myelomonocytic leukemia (CMML).
• Eosinophil granulocytes - Increased in parasitic infections. Increased in allergic states, medications, parasites, chronic myeloid leukemia (CML), metastatic/ necrotic tumors.
• Basophil granulocytes Increase is very rare (CML).

Left shift: increased production of WBCs in the bone marrow results in more immature forms released into the bloodstream.

A manual count will also give information about other cells that are not normally present in peripheral blood, but may be released in certain disease processes.

Platelets

Platelet numbers are given, as well as information about their size and the range of sizes in the blood.

Thrombocytopenia: <20,000/mm³, serious; <10,000/mm³, potentially life-threatening

Examination of Wright's Stained Peripheral Blood Smear

This procedure allows visualization of alterations in size, shape, and structure of individual red cells and white cells, which may have diagnostic significance in certain diseases. Examination of the peripheral smear is essential in assessing neoplastic hematologic disorders (e.g. leukemias), and in those disorders which mimic neoplastic disorders. There are obviously many limitations. For example, a peripheral blood smear cannot prove presence of anemia per se, which must be detected by means of the hemoglobin, hematocrit, or RBC count.

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