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.
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