Packed cell volume and blood indices

Packed cell volume (PCV) is the volume of red cells in blood expressed in percentage. It is the volume of RBCs packed at the bottom of a hematocrit tube when the blood is centrifuged. It is also called ‘hematocrit value’.

The PCV is an easily obtained measurement for the detection of anemia or polycythemia and can be useful in estimating the changes in hemodilution and hemoconcentration.

An abnormally low hematocrit may suggest anemia, a decrease in the total amount of red blood cells, while an abnormally high hematocrit is called polycythemis. Both are potentially life-threatening disorders.

Method of determination of packed cell volume:

Apparatus required:

• Wintrobe or hematocrit tube
• Centrifuge machine
• Pasteur pipette
• Whole blood sample with the anticoagulant (Ethylenediaminetetraacetic acid or heparin)

Procedure:

• Fill the Wintrobe or hematocrit tube (110 mm long and 3 mm bore) up to 100 mark (which represent 100%) with the help of Pasteur pipette.
• Place the tube in the centrifuge machine and balance it with a tube, filled with water.
• Centrifuge at a speed of 3000 revolution per minute (rpm) for 30 minutes.
• RBCs packed at the bottom form the packed cell volume and the plasma remains above this. In between the RBCs and the plasma, there is a white buffy coat, which is formed by white blood cells and the platelets.
• Measure the percentage of red cells packed at the bottom.

Normal value:

• In male  :  40% - 45%
• In female  :  38% - 42%

Significance of determination of PCV:

• Diagnosis and treatment of anemia
• Diagnosis and treatment of polycythemia
• Determination of extent of dehydration and recovery from dehydration after treatment
• Decision of blood transfusion

Variation in PCV:

Elevated level of PCV:

Polycythemia:- A condition in which the bone marrow produces excessive numbers of red cells resulting elevated hematocrit.

Dehydration:- Dehydration occurs when body does not have as much water as it needs.

Dengue shock syndrome:-  Dengue fever (tropical disease caused by flavivirus transmitted by mosquito Aedes aegypti) of grade III or IV severity. Hemoconcentration can be detected by an escalation of over 20% in hematocrit levels that will come before shock.

Decrease in blood plasma increases PCV.

Capillary leak syndrome:- It is characterized by the leakage of blood plasma through capillary walls from the blood circulatory system. It leads to abnormally high hematocrit counts.

Chronic obstructive pulmonary disease (COPD) is associated with hypoxia may increase the production of red blood cells. This increase is mediated by the increased levels of erythropoietin by the kidneys in response to hypoxia.

At higher altitudes, there is a lower oxygen supply in the air and thus hematocrit levels may increase over time.

Lowered level of PCV:

Hemolysis:- The situation where the RBCs are destroyed permanently by the immune system. This occurs due to certain organ damages and inherited abnormalities of RBCs.

Anemia:- A condition in which low number of red blood cells are present in the body.

Cirrhosis of liver:- It is a late stage of scarring (fibrosis) of the liver caused by many forms of liver diseases and conditions. The scar tissue blocks the flow of blood through the liver and slows down the ability of liver.

Hemorrhage due to ectopic pregnancy (pregnancy due to implantation of fertilized ovum in tissues other than uterine wall), which is characterized by vaginal bleeding.

Blood indices

Blood indices are part of complete blood count. They are used to diagnose the type and cause of anemia.

The relationship between the hematocrit, the hemoglobin level and the RBCs are converted to blood indices through mathematical formulas.

Blood indices include:

• Mean corpuscular volume (MCV)
• Mean corpuscular hemoglobin (MCH)
• Mean corpuscular hemoglobin concentration (MCHC)
• Color Index (CI)

Mean corpuscular volume (MCV)

There are three main types of corpuscles (blood cells) in blood- red blood cells, white blood cells and platelets. MCV is the average volume of red blood cells in blood. It is expressed in cubic microns (cu μ). The normal range of MCV is 78 to 90 cu μ.

MCV blood test measures the average size of your red blood cells, also known as erythrocytes. If your red blood cells are too short or too large, it could be a sign of a blood disorder such as anemia, a vitamin deficiency or other medical conditions.

When MCV is normal, the RBC is called normocyte. When MCV increases, the cell is known as a macrocyte and when it decreases, the cell is called microcyte.

MCV is calculated as follows:

When the result shows that red blood cells are smaller than normal, it indicates thalassemia, iron-deficiency anemia or other types of anemia.

When the result shows that red blood cells are larger than normal, it indicates deficiency of vitamin B12 and folic acid, liver disease, hyperthyroidism and many more medical conditions.

Mean corpuscular hemoglobin (MCH)

MCH is the average amount or quantity of hemoglobin present in single RBC. It is expressed in micro-microgram or pictogram (pg). Normal range of MCH is 27 to 32 pg.

MCH is calculated as follows:

The amount of hemoglobin per RBC depends on hemoglobin synthesis and the size of the RBC. MCH decreases when hemoglobin synthesis is reduced, or when RBCs are smaller than normal, such as in case of iron-deficiency anemia.

Mean corpuscular hemoglobin concentration (MCHC)

MCHC is the average concentration of hemoglobin in a single red blood cell. So, the unit of expression is percentage. This is the most important absolute value in the diagnosis of anemia. Normal range of MCHC is 30% to 38%.

MCHC is calculated as follows:

When MCHC is normal, the RBC is normochromic. When the MCHC decreases, the RBC is known hypochromic. Cause of high MCHC includes- macrocytic anemia, autoimmune hemolytic anemia, severe burns, liver disease and medications. Cause of low MCHC includes- peptic ulcers, iron deficiency, lead poisoning, hemolysis and many medical conditions.

Color Index (CI)

Color index is the ratio between the percentage of hemoglobin and the percentage of RBCs in the blood. Normal color index is 0.8 to 1.2.

Color index is calculated by dividing the hemoglobin percentage by the RBC count percentage.

It is useful in determining the type of anemia. It increases in macrocytic (pernicious) anemia and megaloblastic anemia. It is reduced in iron deficiency anemia. And, it is normal in normocytic normochromic anemia.