• The mean corpuscular volume or MCV represents the direct measurement of the size of the red blood cells. 
• It is a useful starting point in defining the type of anaemia. If the MCV is reduced in the presence of anaemia it is called microcystic anaemia, if increased macrocystic and if normal normocystic anaemia. 
• Microcystic anaemia is seen in iron deficiency anaemia and thalassaemia
• Macrocystic anemia occurs in folate and/or vitamin B12 deficiency. 
• Normocystic anaemia occurs in chronic disease, renal failure, bone marrow failure and acute blood loss.

b. 

• Haematocrit is also known as packed cell volume (PCV) it is a measurement of the volume of the red blood cells in a given volume of blood. 
• The blood is first centrifuged and the amount of sedimentation is expressed as percentage of the blood volume. 
• Haematocrit is usually decreased in anemia and increased in polycythaemia. However, many factors can affect it for example, it is reduced in pregnancy due to expansion of the blood volume and increased in dehydration due to contraction of the volume.

c. 

• Serum B12 can be used to detect vitamin B12 deficiency. 
• A significant decrease is seen in malnutrition and in patients with pernicious anaemia (caused by antibody against the intrinsic factors). 
• Its deficiency results in macrocystic anaemia with hypersegmentation of the neutrophils, this is because vitamin B12 is necessary for DNA synthesis. Severe anaemia can result in retinal flame hemorrhage. 
• Vitamin B12 is also important for myelination, its deficiency causes subacute combined degeneration of the cord and can give rise to optic neuropathy. Its may be the cause of alcohol-tobacco amblyopia.