High MCH on a blood test: causes and what it means

MCH tells you how much haemoglobin, the oxygen-carrying protein, sits inside an average red blood cell. When red cells are larger than normal, each one tends to hold more haemoglobin, so the MCH rises with them. For that reason a high MCH is most often found alongside a high MCV, the measure of red cell size, and the question for the doctor is why the red cells are large.

Lab Tests Online UK, a public information service on laboratory tests, puts it plainly: a high MCH is usually associated with macrocytic anaemia, meaning anaemia with large red blood cells. It is a clue that leads to further tests, not a diagnosis, and a slightly high result in someone who feels well is often repeated.

The red cell indices on a full blood count describe different aspects of the same cells: MCV is the average size, MCH the average haemoglobin per cell, MCHC the concentration of haemoglobin within cells, and RDW how much the cells vary in size. A review in the Journal of General and Family Medicine defines macrocytic anaemia as an MCV above 100 fL, and uses MCV to classify the type of anaemia.

MCH on its own is rarely the deciding value. Your doctor will look at the MCV, haemoglobin and the other values together, and the range printed on your report is the one to compare against.

The review divides causes of macrocytic anaemia into megaloblastic, caused by deficiency or impaired use of vitamin B12 or folate, and nonmegaloblastic, caused by liver dysfunction, alcoholism, myelodysplastic syndrome or hypothyroidism. Lab Tests Online UK gives a similar list of causes of a high MCV: B12 deficiency, folate deficiency, liver disease, an underactive thyroid, alcohol excess and some bone marrow disorders.

• Vitamin B12 deficiency, which the review calls the most common cause of megaloblastic anaemia.

• Folate deficiency.

• Alcohol excess and liver disease.

• An underactive thyroid.

• Medicines; the review mentions that in one hospital series, medications accounted for a large share of macrocytosis, including antiretroviral therapy.

• Bone marrow disorders such as myelodysplastic syndrome, which is more common in older people.

Because B12 and folate deficiency are the best known and most treatable causes, they are usually checked first. The NHS says pernicious anaemia, where the immune system attacks the stomach cells needed to absorb B12, is the most common cause of B12 deficiency in the UK. Poor diet is an uncommon cause, but can happen with a vegan diet without supplements or fortified foods, and certain medicines, including some anticonvulsants and proton pump inhibitors, can affect how much of these vitamins the body absorbs. The NHS adds that B12 and folate deficiency are more common in older people, affecting around 1 in 10 people aged 75 or over and 1 in 20 aged 65 to 74.

The NHS says that although many symptoms improve with treatment, some problems caused by the deficiency can be irreversible, so it is important to be diagnosed and treated as soon as possible, and most cases are treated with injections or tablets. The review adds that B12 deficiency does not necessarily lead to anaemia or macrocytosis, so a normal MCH does not rule it out if symptoms such as tingling or numbness are present.

Lab Tests Online UK says that depending on the results a doctor may request vitamin B12 and folate levels, a blood film in which the cells are examined under a microscope, iron studies, and a reticulocyte count that shows how quickly the marrow is producing new red cells. Bone marrow biopsy is rarely needed.

The review suggests that when myelodysplastic syndrome is suspected, for example when low white cells or platelets accompany anaemia, a haematology consultation may be appropriate. Many of the causes in the lists above are vitamin, alcohol, liver, thyroid or medicine explanations that can be addressed.

The NHS advises seeing a GP if you think you may have a vitamin B12 or folate deficiency. Symptoms it lists include extreme tiredness, a lack of energy, pins and needles, a sore and red tongue, mouth ulcers, muscle weakness, problems with vision and psychological changes such as low mood or confusion. Bring up any such symptoms alongside the blood result.