Erythrocyte Indices

The Erythrocyte or RBC indices are three values that will show up on a laboratory form which gives the results of your CBC.

These are not actually tests themselves, but rather calculations that are made from the three main red blood cell test, i.e. the RBC count, the haemoglobin and the hematocrit tests.

The erythrocyte indices include the mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC).

(As a bit of interesting, if unimportant information, these were first worked out and used to classify anaemias by Maxwell Wintrobe in 1934).

The calculations:-

MCV = hematocrit ÷ rbc count
MCH = hemoglobin ÷ rbc count
MCHC = hemoglobin ÷ hematocrit

Why they are done

Anaemia does not have a single cause. It includes a variety of conditions with the same symptom - the person's blood does carry as much oxygen as it should.

To find out what disorder is causing the anaemia you need to know what type of anaemia it is.

Just knowing how many cells one has tells you nothing about the quality of those cells. The erythrocyte indices reflect the size and hemoglobin content of individual red blood cells.

Abnormal erythrocyte indices are helpful in quickly narrowing down the potential causes, and thus classifying, the different types of anaemia. This allows the medical practitioner to prescribe the correct treatment.

How anaemias are classified using the erythrocyte indices:-

1. MCV, MCH and MCHC are all normal - normocytic, normochromic anaemia (RBC's size and hemoglobin content are within normal limits), most often caused by acute blood loss
2. MCV, MCH, and MCHC are all decreased - microcytic, hypochromic anaemia (RBC's are smaller and their hemoglobin content is much less) - most often caused by iron deficiency but sometimes by impaired production of hemoglobin.
3. Increased MCV with variable MCH and MCHC - macrocytic anaemia (RBC's are larger ) - most often caused by Vitamin B12 deficiency (due to pernicious anaemia) and folic acid deficiency.

Mean corpuscular volume (MCV)

This is probably the most important of the erythrocyte indices.

It calculates the average size of individual red blood cells and thus categorises them, eg:-

Normal size cells are called normocytic
Smaller size cells are called microcytic
Larger size cells are called macrocytic

The calculation is done by dividing the hematocrit by the total RBC count, eg:-

MCV (fl) = hematocrit ÷ rbc count

Say your HCT results are 0.413 and your red cell count is 4.32 x 10*12/L:-
MCV (fl) = 0.413 ÷ 4.32 x 10*12/L
Then your MCV will be ________________

OOPS - If, like me, you are unable to get an answer to 4.32 x 10*12/L on your home calculator then you can try a different way of getting to the same result.

Take your HCT and multiply it by 1000.

Then divide the answer by your red cell count, forgetting the '10*12/L' bit.

The answer will be a good enough estimation.

For example:-

0.413 x 1000 = 413
413 ÷ 4.32 = 95.6
MCV = 95.6fl

Normal range

MCV values are higher in newborns and infants.
Adult men 80-98 fl (femoliters)
Adult Women: 96-108 fl

What abnormal results mean:-

1. A low MCV means the red blood cells are microcytic.

This may be due to :-

a) Iron deficiency anaemia,
b) Lead poisoning.
c) Genetic diseases
Thalassemia major and thalassemia minor - an inability to manufacture sufficient quantities of globin chains.
Sideroblastic anaemia - an inherited disorder that prevents red blood cells from making sufficient hemoglobin.
d) Copper deficiency.
e) Vitamin deficiency eg. B6 and C
f) Low stomach acid (Hypochlorhydria) - hampers the uptake of certain minerals and vitamins.
g) Rheumatoid arthritis.
h) Haemolytic anaemia - a disorder in which the red blood cells are destroyed faster than the bone marrow can produce them.
i) Having one's spleen removed (splenectomy)

2. A high MCV means the red blood cells are macrocytic.

This may be due to:-

a) Hereditary anaemia
b) Megaloblastic anaemias which include
Pernicious anaemia
Folic acid deficiency
Vitamin B12 deficiency - pernicious anaemia
c) Reticulocytosis (reticulocytes are immature cells with a relatively large size compared to a mature red blood cell)
d) Diseases or disorders such as:-
Aplasia - a condition in which RBC precursors in bone marrow are nearly absent.
Myelofibrosis - fibrosis of the bone marrow.
Hyperglycemia - an increase in plasma glucose.
Cold agglutinins - abnormal proteins in the blood which at high level can cause a form of anaemia.
Manganese deficiency.
e) Liver disease
f) Hypothyroidism
g) Drugs (anti-convulsants)
h) Treatment with the anti-HIV drugs eg. AZT (Retrovir or Zidovidune).

3. An MCV within the normal range means the cells are normocytic.

The patient may still present with anaemia.

This may be due to :-

a) An acute haemorrhage.

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Mean corpuscular hemoglobin (MCH)

This calculation tells you the amount of haemoglobin in a single red blood cell.

It is done by dividing the haemoglobin by the total RBC count, eg:-

MCH (pg) = hemoglobin ÷ rbc count

Say your Hemoglobin results are 13.9g/dL and your red cell count is 4.32 x 10*12/L:-
MCH (pg) = 13.9 ÷ 4.32 x 10*12/L
Then your MCH will be ________________

OOPS - If, like me, you are unable to get an answer to 13.9 ÷ 4.32 x 10*12/L on your home calculator then you can try a different way of getting to the same result.

Take your Hemoglobin and multiply it by 10.

Then divide the answer by your red cell count, forgetting the '10*12/L' bit.

The answer will be a good enough estimation.

For example:-

13.9 x 10 = 139
139 ÷ 4.32 = 32.18
MCH = 32.2 pg/cell (pg/cell = picograms per cell)

Normal range

Normal 27.5 - 33.5 pg/cell

What abnormal results mean:-

The MCH pretty much mirrors the results of the MCV and is increased in and decreased in the same conditions as the MCV.

It is not as useful a test as the MCV for evaluating types of anaemia but does suggest abnormal haemoglobin synthesis.

Decreased MCH, termed hypochromasia, is associated with microcytic anaemia since small cells tend to have less hemoglobin.

Increased MCH, termed hyperchromasia, is associated with macrocytic anaemia since larger cells tend to have more hemoglobin.

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Mean corpuscular hemoglobin concentration (MCHC)

This calculates the average amount of haemoglobin per red blood cell

Or to be more scientific, 'the average haemoglobin concentration per unit volume (100 ml) of packed red cells'.

A low MCHC means that a 100ml of packed red blood cells contain less haemoglobin than normal and a high MCHC means that there is more haemoglobin in 100ml of packed red blood cells.

The calculation is done by dividing the haemoglobin by the hematocrit, eg:-

MCHC (g/dL) = hemoglobin ÷ hematocrit

Say your Hemoglobin results are 13.9g/dL and your hematocrit is 0.413:-

13.9 ÷ 0.413 = 33,65
MCHC = 33.7 g/dL

Normal range

32 to 36 gm/dL
(this is sometimes reported in % i.e. 32-36%)

What abnormal results mean

MCHC is decreased in the same conditions as the MCV, for example:-

Decreased MCHC values, termed hypochromia, may be due to:-

a) Iron deficiency anaemia
b) Thalassemia
c) Blood loss
d) Vitamin B6 deficiency

There is a condition where the MCV and MCHC differ.

This occurs in Pernicious anaemia, where the MCV is high but the MCHC is normal.

In the disease Spherocytosis, a relatively rare disorder, in which the RBC's are small and round (rather than the normal biconcave) and therefore hold more hemoglobin, there is an increase in the MCHC.

As any increase in MCHC is limited by the amount of haemoglobin that will fit inside a RBC it is not very common to find any significant increase and thus the MCHC is used to distinguish a patient with Spherocytosis.



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