Macrocytosis: a nice breakdown

When ever a complete blood count (CBC) is ordered, the haematology machines will dutifully spit out a mean corpuscular volume (MCV), measured in femtolitres (fL). (The 'femto' prefix is a bit like 'micro' as a prefix, but whereas the latter means 1 millionth-of-a-[whatever], femto- indicates one trillionth-of-a-[whatever]). The MCV is a measure of the red cells' average volume.

An MCV of over about 100 fL is usually considered to define a macrocytosis (red cells with an abnormally high volume - which basically equates to red cells with an abnormally high size under the microscope). The first thing to do when you find a macrocytosis is to differentiate between a megaloblastic and a non-megaloblastic picture. This is largely done by getting a haematologist to look at a smear of the peripheral blood. With megalobastosis, you get:

• Hypersegmented neutrophils: neutrophils usually have 2-5 lobes. Megaloblastosis is defined by more than 5% of the neutrophils having 5 lobes, or any having 6 or more lobes.
• Oval macrocytes: the red cells are obviously large, but in megaloblastosis they are oval in shape (as opposed to round).

If you are dealing with a megaloblastic macrocytosis, the two commonest causes are folate deficiency (check the red cell folate level) and B12 deficiency (check the serum B12 level). Start there, and only look for other causes if both levels are normal despite the megaloblastic picture. Other causes include myelodysplasia, HIV infection and certain drugs.

If you're dealing with a non-megaloblastic macrocytosis, then you can use this handy mnemonic that my haematology professor taught me - ALARM:

• Alcoholism
• Liver disease
• Aplastic anaemia
• Reticulocytosis
• Myelodysplasia, marrow infiltration, or myxoedema (i.e. hypothyroidism).

That should help!