Packed cell volume (PCV)

Packed cell volume (PCV) is the proportion of blood occupied by RBCs, 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 or erythrocyte volume fraction (EVF).

METHOD OF DETERMINATION

Blood is mixed with the anticoagulant ethylenediaminetetraacetic acid (EDTA) or heparin and filled in hematocrit or Wintrobe tube (110 mm long and 3 mm bore) up to 100 mark. The tube with the blood is centrifuged at a speed of 3000 revolutions 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. In the laboratories with modern equipments, hematocrit is not measured directly but calculated indirectly by autoanalyzer. It is determined by multiplying RBC count by mean cell volume. However, some amount of plasma

is always trapped between the RBCs. So, accurate value is obtained only by direct measurement of PCV.

SIGNIFICANCE OF DETERMINING PCV

Determination of PCV helps in:

1. Diagnosis and treatment of anemia

2. Diagnosis and treatment of polycythemia

3. Determination of extent of dehydration and recovery from dehydration after treatment

4. Decision of blood transfusion.

NORMAL VALUES OF PCV

Normal PCV:

In males = 40% to 45%

In females = 38% to 42%

VARIATIONS IN PCV

INCREASE IN PCV

PCV increases in:

1. Polycythemia

2. Dehydration

3. Dengue shock syndrome: Dengue fever (tropical

disease caused by flavivirus transmitted by mosquito

Aedes aegypti) of grade III or IV severity.

DECREASE IN PCV

PCV decreases in:

1. Anemia

2. Cirrhosis of liver

3. Pregnancy

4. 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 the calculations derived from RBC count, hemoglobin content of blood and PCV.

IMPORTANCE OF BLOOD INDICES

Blood indices help in diagnosis of the type of anemia.

DIFFERENT BLOOD INDICES

Blood indices include:

1. Mean corpuscular volume (MCV).

2. Mean corpuscular hemoglobin (MCH).

3. Mean corpuscular hemoglobin concentration (MCHC).

4. Color Index (CI).

1. Mean Corpuscular Volume (MCV)

MCV is the average volume of a single RBC and it is expressed in cubic microns (cu μ). Normal MCV is 90 cu μ (78 to 90 cu μ). 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. In pernicious anemia and megaloblastic anemia, the RBCs are macrocytic in nature. In iron deficiency anemia the RBCs are microcytic.

2. Mean Corpuscular Hemoglobin (MCH)

MCH is the quantity or amount of hemoglobin present in one RBC. It is expressed in micromicrogram

or pictogram (pg). Normal value of MCH is 30 pg (27 to 32 pg).

3. Mean Corpuscular Hemoglobin Concentration (MCHC)

MCHC is the concentration of hemoglobin in one RBC. It is the amount of hemoglobin expressed in relation to the volume of one RBC. So, the unit of expression is percentage. This is the most important absolute value in the diagnosis of anemia. Normal value of MCHC is 30% (30% to 38%).

When MCHC is normal, the RBC is normochromic. When the MCHC decreases, the RBC is known

hypochromic. In pernicious anemia and megaloblastic anemia, RBCs are macrocytic and normochromic or hypochromic. In iron deficiency anemia, RBCs are microcytic and hypochromic. A single RBC cannot be hyperchromic because, the amount of hemoglobin cannot increase beyond normal.

4. Color Index (CI)

Color index is the ratio between the percentage of hemoglobin and the percentage of RBCs in the blood.

Actually, it is the average hemoglobin content in one cell of a patient compared to the average hemoglobin contentin one cell of a normal person. Normal color index is 1.0 (0.8 to 1.2). It was widely used in olden days. However, 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.