Medical Matters > Symptoms – Abnormal red blood cells

ME Essential Autumn 2023


After feeling more fatigued than usual, my GP arranged some blood tests – all of which were normal apart an abnormality with my red blood cells called macrocytosis. Can this can be caused by ME? I understand that there has been some research from New Zealand that found abnormally shaped red blood cells that impaired the blood flow in tiny capillaries.


Macrocytosis is the medical term that is used to describe red blood cells that are larger than normal. It may cause no signs or symptoms and can be detected on a routine blood test.

ME/CFS does not cause macrocytosis and this abnormality is not the same as the deformed red blood cells that were found in some research carried out many years ago in New Zealand by Dr Les Simpson.

Macrocytosis isn’t a specific disease, but it may indicate an underlying medical problem.

Common causes of macrocytosis include:

  • Vitamin B-12 deficiency
  • Folate deficiency
  • Liver disease
  • Alcoholism
  • Hypothyroidism
  • A side-effect of certain medications – such as those used to treat cancer, seizures and autoimmune disorders
  • Increased red blood cell production by the bone marrow (regeneration) to correct anaemia, for example, after blood loss.
  • Some of these conditions have symptoms, especially fatigue, that overlap with ME/CFS. So if this abnormality is found it should always be properly investigated to find the cause.

If you have macrocytosis, further blood tests can help determine its cause. But in some cases, it may be necessary to remove a sample of the bone marrow — the spongy tissue inside your bones — for testing (a bone marrow biopsy and aspiration).

Treatment of macrocytosis, if needed, is directed at the underlying cause. In the case of vitamin B-12 or folate deficiency, treatment may include dietary modification and dietary supplements or injections.

If macrocytosis is caused by more severe anaemia, a blood transfusion might be necessary.


Information provided by The ME Association should not be construed as medical advice. Don't assume any new or worsened symptoms are simply the result of having ME/CFS or Long Covid. We recommend that any information you deem relevant is discussed with your NHS GP as soon as possible. It is important that you seek personalised medical advice from the GP who is in charge of your care and who knows you well.

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