## Rule of 3s

The measured hemoglobin concentration is 3 times the RBC count, and the calculated hematocrit is 3 times the Hb level. A significant deviation means artifacts in the value estimated or the RBCs are smaller or larger than the normal.

- HCt = 3 X Hb
- RBC count = Hb/3

## The units

- Hematocrit: Percentage (%)
- MCV: fL (femtolitres i.e. 10^-15 litres)
- MCH: pg (picogram i.e. 10^-12 grams)
- MCHC: g/L

1 ml = 1000 cu.mm

1 L = 1000 ml

1 L = 1000000 cu.mm

Hb (g/dL) X 10 = Hb (g/L)

## Hematocrit

In simple words, hematocrit is the percentage of your blood that is made up of Red Blood Cells (RBCs).

HCt = (Total blood volume – Total Plasma volume)/Total blood volume

## Mean corposcular volume (MCV)

Mean volume of 1 RBC (Total RBC volume/Total number of erythrocytes or RBC)

MCV = (HCt X 10)/RBC count per Litre

Normal: 80-100 fL

Apply rules of 3 in the formula above: MCV = (3 X Hb X 10)/Hb/3 = 90. Hence, MCV is around 90 fL in average. Normal value is 90±10 fL.

HCt = MCV X RBC count X 0.1

## Mean corposcular hemoglobin (MCH)

Mean hemoglobin concentration in 1 RBC (Hemoglobin concentration/Total number of erythrocytes or RBC)

MCH = (Hb X 10)/RBC count per Litre

Normal: 27-33 pg

Apply rules of 3 in the formula above: MCH = (3 X RBC count X 10)/RBC count = 30. Hence, MCH is around 30 pg in average. Normal value is 30±3 pg.

## Mean corposcular hemoglobin concentration (MCHC)

Mean hemoglobin concentration in RBCs (Hemoglobin concentration/proportion of RBC)

MCHC = (Hb X 100)/HCt

Normal: 33-37 g/L

Apply rules of 3 in the formula above: (Hb X 100)/(3 X Hb) = 33. Hence, MCHC is around 33g/L in lower limit. Normal range is 35±2 g/L.

## Interconversions

MCHC = (MCH X 100)/MCV

MCH = (MCHC X MCV)/100

MCV = (MCH X 100)/MCHC

## Red Cell Distribution Width (RDW)

Measure of anisocytosis (variability in RBC size) i.e. standard deviation of MCV

RDW = (Standard deviation of red cell volume X 100)/Mean cell volume

Normal range = 11 to 15%

## Classification of Anemia based on MCV

### MCV low (Microcytic)

**Normal RDW**

- Thalasemmia trait
- Anemia of chronic disease
- HbH

**High RDW**

- Iron deficiency
- Beta-thalasemmia
- Sickle/HbC trait

### MCV normal (Normocytic)

**Normal RDW**

- Normal
- Anemia of chronic disease
- Hemoglobinopathies
- Hereditary spherocytosis
- Transfusion
- Chemotherapy
- CLL, CML
- Hemorrhage

**High RDW**

- Mixed deficiency
- Early iron or folate deficiency
- Myelofibrosis
- Sideroblastic anemia

### MCV high (Macrocytic)

**Normal RDW**

- Aplastic anemia
- Pre-leukemia

**High RDW**

- Folate deficiency
- Vitamin B12 deficiency
- Immune hemoglobin
- Cold agglutinins
- CLL