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Lab test update - MCV

Consultant haematologist Dr Mike Galloway uses a case history to discuss MCV testing

Consultant haematologist Dr Mike Galloway uses a case history to discuss MCV testing


What are the most common causes of a raised MCV and MCH?

As the size of the red cell increases, so will the amount of haemoglobin.

A high MCH is really a secondary reflection of the increase in the size of the cell seen in macrocytosis. Therefore the MCH itself is not particularly useful as a test.

In relation to the MCV, the level of macrocytosis can give a clue to the underlying diagnosis. For example, as the MCV increases above 100fl, the probability of B12 and folate deficiency also increases. This is particularly true in patients with an MCV of over 130fl except for patients who are on hydroxycarbamide, which inevitably causes macrocytosis.

MCVs in the range of 100-110fl are more likely to be related to other causes of macrocytosis such as alcohol misuse, liver disease, hypothyroidism, some anti-neoplastic drugs and HIV infection with the use of zidovudine. Haemolysis or a myelodysplastic syndrome may also present as a macrocytosis.

What further tests should be performed to establish the cause?

It is important to make sure the patient is not on hydroxycarbamide or other drugs such as azathioprine since these can cause macrocytosis. In this situation, no further investigations are required. Similarly, the history should also reveal any alcohol misuse or chronic liver disease.

If the MCV is above 110fl, B12 and folate levels should be requested.

If the MCV is below 110fl, and the patient is not anaemic, then B12 and folate deficiency are less likely and it is worth doing LFTs and excluding hypothyroidism.

A blood film examination is usually helpful as hypersegmented neutrophils and macro ovalocytes are associated with vitamin B12 deficiency, whereas a uniform macrocytosis is seen with alcohol misuse, target cells with liver disease and polychromasia with haemolysis. Similarly, other features of a myelodysplastic syndrome can be picked up on a blood film.

How often is no cause found?

A mild macrocytosis with MCV of 100-102fl is quite frequent, and no specific cause is found. However, as mentioned earlier, it is important to exclude treatable causes such as B12 deficiency or hypothyroidism.

Do these parameters vary with age?

The MCV does not change with age.

If standard tests are normal and there is no history of excess alcohol, how should the patient be followed up?

If all the blood tests are normal, it is reasonable not to perform any further follow-up. Very occasionally, a myelodysplastic syndrome may present with isolated macrocytosis. It may take many years for other abnormalities to appear such as anaemia or neutropaenia, but it is hard to justify a routine follow-up for all patients with macrocytosis.

Dr Mike Galloway is consultant haematologist at City Hospitals Sunderland NHS Foundation Trust and chair of the Council of the Association of Clinical Pathologists

Competing interests: none declared is a easily navigated website with detailed guidance on lab testing in primary care - including an a seciton on macrocytosis

Key messages Key messages

• MCH is a less useful test than MCV
• MCV above 100fl increases the probability of B12/folate deficiency. This is particularly true if MCV rises above 130fl – except if the patient is taking an anti-neoplastic drug
• MCVs between 100 and 110fl are likely to be due to:
– alcohol abuse
– liver disease
– hypothyroidism
– some anti-neoplastic drugs
– HIV treatment including zidovudine
• Mild macrocytosis (MCV of 100-102fl) is quite frequent with no specific cause found, but B12 deficiency or hypothyroidism must be excluded

case study

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