ABCofCBC

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www.medicalppt.blogspot.com

ABC of CBC

Is it so simple?

Dr. Sanwar Agrawal MDEkta Institute of child health

Raipur

PUBLISHED BY


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Yes, it is!

The complete blood count (CBC) is a deceptively simpletest to order and interpret

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Provided, You concede that it is difficult to be

simple.

The CBC is a bargain; its cost can be much less than modern imaging studies, butlike the modern imaging studies, its value is lost without appropriate analysis.

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Because of explosion of new technology and information, understanding ofand sophisticated use of the CBC will take on added importance.

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What is CBC?

Measured parameters:

Hemoglobin, MCV, anderythrocyte count

Hemoglobinconcentration ismeasured by absorbancespectrophotometry and

relies on complete lysisof erythrocytes.

Platelets, WBC

Derived parameters:MCH, MCHC, andhematocrit

The hemoglobin, MCV and RBC count are the measured parameters

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The technique

electric impedance

Flow cytometry

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Impedance

Erythrocyte counting and sizedetermination are accomplished

by passage of cells through anelectric field created by directcurrent

As the cells pass singly throughthe field, a small resistance isgenerated that is measured as a

pulse height. From the pulseheight, the size and number oferythrocytes are determined.These data are frequently plotted

as a histogram in the typicalmodern Coulter-type electricimpedance instrument

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Flow cytometry

With the Advia, red cells aresphered in a diluent and then

passed through a laser. Thecells scatter light (atdifferent angles) which is

detected by the instrument(see image to the right). Thelaser detects the number ofcells, cell volume (using lowangle scatter) and internal

content, i.e. hemoglobinconcentration (using highangle scatter) by lightscatter.

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The rule of 3

the measured hemoglobin concentration is

approximately three times the RBC count, and

the calculated hematocrit is three times the

hemoglobin value. When significant deviation

occurs from these approximate calculations,

artifacts in the CBC should be suspected

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ARTIFACTS IN ELECTRONIC

CELL COUNTING

Cause Hb Hct RBC MCV MCHMCHC

Cold agglutinins-- -- --

Hyperleukocytosis-- --

Hyperosmolar plasma- -- -- --

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Spun V/S Automated Hct.

Spun > Automated.

Spherocytes, sickle erythrocytes, hypochromic cells,and reticulocytes, trap more plasma.

To permit close correlation between the manual andautomated hematocrits, a correction factor ofapproximately 3% is applied to the calculation of thehematocrit .

A case can be made to dispense with reporting a calculated hematocrit with theCBC because the hemoglobin value that is measured directly is more reliable

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MCV

The mean cell volume

indicates the volume of the

"average" red cell in a

sample. It is expressed in

femtoliters (fl; 10 -15 liters).With impedance analyzers,

the MCV is measured by

averaging the amplitude of

the pulses created as thecells pass the aperture of the

counter.

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MCV

The instrument usingscattered light also plots thecell volume and hemoglobinconcentration together in a

volume/hemoglobin (V/HC)cytogram.

Normal red blood cells withnormal volume(normocytic) and

hemoglobin concentration(normochromic) are foundin the middle box

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MCV

Agglutinated cells (arrow)with IHA form a distinctcluster (macrocytic normo-chromic) above the normalred cells (arrowhead)

A. Normal RBC V/Hcytogram B. Polychromasia:the polychromatophils forma discrete cluster (arrow)that extends off the normal

red cell population(arrowhead) and ismacrocytic andhypochromic.

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Micro and Macrocytes

Neonate MCV > 100 fl.

After 6 months age : 70+ age in years ( lower

limit of normal)

84+0.6 x ( age in years)= upper limit of

normal.

Corresponding adult values are 80 and 96respectively.

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Mean Cell Hemoglobin

This represents the absolute amount of hemoglobin in

the average red cell in a sample.

The MCH is calculated from the [Hb] and the RBC

using the following equation: MCH (pg) = (Hb x 10) RBC

This value is generally not useful

A low MCH could be due to smaller than normalcells with normal Hb concentration, or normal sized

cells with lower than normal Hb concentration

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Mean Cell Hemoglobin

Concentration (MCHC)

MCHC is the mean cell hemoglobin concentration,

expressed in g/dL

It can be calculated from the [Hb] and the PCV using

the formula: MCHC = (Hb PCV) x 100. The normal value for MCHC is about 33%.

Values for MCHC significantly above the reference

range are not physiologically possible due tolimitations on the solubility of Hb.

For all practical purposes a MCHC > 35% and RDW >14 is Spherocytosis

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Red Cell Distribution Width (RDW)

The RDW is an index of thevariation in cell volumewithin the red cell

population.

RDW = (Standard deviationof red cell volume meancell volume) x 100

Red cell populations withhigher than normal RDWare termed heterogenous;

those with normal RDW arehomogeneous.

Its a measure of

anisocytosis

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RDW..

In some instances, the RDW is the first testresult to increase with changes in red cellpopulation sizes.

In early iron deficiency, there are only lownumbers of microcytic red blood cells. Thiswill increase the standard deviation and theRDW, but the mean cell volume is unchangedbecause there are insufficient numbers ofmicrocytic cells to change the mean volume

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MCV and RDW

MCV low MCV normal MCV high

RDW

normal

Thalassemia

trait

Chronic

disease

Aplastic

anemia

RDW

high

Iron

deficiency

Early

Iron/folate def

Myelofibrosis

Folate/B12

def

IHA

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Microcytosis and raised TRBC

Only 3 conditions: B Thal Trait, Hypoxic

erythrocytosis, Polycythemia vera.

RDW is normal in first and increased in rest.

For pediatric age group: Microcytosis, raised

TRBC and normal RDW is B Thal trait.

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Platelets

A low counts should becross checked manually.

Small RBCS (like in

IDA) may be counted asplatelets ( Oneexplanation forincreased Plt in IDA).

Platelet clumps areresponsible for lowcounts.

1. Platelets

2. Large platelets3. Red blood cells4. RBC fragments5. Debris6. Ghosts

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MPV

Large Platelets: Macrothrombocytes (MPV >10 fl)

Idiopathic Thrombocytopenic Purpura

Bernard-Soulier Disease

May-Hegglin Anomaly Small Platelets: Microthrombocytes (MPV

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WBC

Nucleated red blood cells are counted by allhematology analyzers (regardless of technique) aswhite blood cells. For this reason, the white bloodcell count must be corrected for the number ofnucleated red blood cells in the circulation. To correcta leukocyte count, the number of NRBCs per 100leukocytes is recorded during the differentialleukocyte count. Then a correction is made as

follows: Corrected WBC = nucleated cell count x (100

[nRBC + 100])

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A. NeutrophilsB. MonocytesC. LymphocytesD. Eosinophils

E. Large unstained cellsF. Platelets/noiseG. Debris/noise

In this cytogram, the cellsare displayed as a worm,with the mononuclear cells(lymphocytes andmonocytes) in the head andthe granulocytes

(neutrophils) in the body ofthe worm.

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CBC IS A LAUGHING GAME

THANKS!

For more lectures visit


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