MCH (mean cell haemoglobin): what the result tells you

Haemoglobin is the oxygen-carrying protein packed inside red blood cells. The haemoglobin result on a blood count tells you how much there is in a volume of blood, but it does not say how it is distributed. MCH answers a different question: on average, how much haemoglobin does one red cell carry? Lab Tests Online UK defines it as the average amount of haemoglobin in each red blood cell.

The analyser derives MCH by dividing the haemoglobin measurement by the red cell count. The result is expressed in picograms per cell, a tiny unit that you will see on the report but rarely need to think about. What matters is whether it sits inside the laboratory's printed interval.

It is one of several red cell indices that come automatically with a full blood count. Together with MCV, MCHC and RDW they describe the volume, size and haemoglobin content of red cells. Lab Tests Online UK notes that the indices help in diagnosing or monitoring conditions such as anaemia, iron or B12 deficiency and thalassaemia.

Taken alone MCH rarely changes management. It becomes useful when it agrees with, or contradicts, the other indices, helping to confirm a pattern such as small, pale cells that suggest iron deficiency.

No fasting or special preparation is needed; the sample is a routine venous blood draw. Reference intervals differ by age and sex, and the laboratory report shows the one that applies to you. Recent transfusion, heavy drinking, certain medicines and vitamin or mineral deficiencies can change red cell measurements, as the full blood count guide from Lab Tests Online UK explains.

Because MCH is derived from other measurements, an unusual sample, such as one with very high white cell counts or clumping, can make it look odd. Scientists check for this and may repeat the test or examine a blood film.

According to Lab Tests Online UK, a low MCH can indicate iron deficiency anaemia, inflammatory conditions or thalassaemia. Doctors describe red cells with too little haemoglobin as hypochromic, meaning pale. When iron is short the marrow cannot build enough haemoglobin, so each cell is smaller and paler.

The NHS lists tiredness, shortness of breath, palpitations, paler than usual skin and headaches as symptoms of iron deficiency anaemia, and notes that heavy periods, pregnancy, and bleeding in the stomach or intestines are common causes. Where iron is normal and MCH stays low, tests for thalassaemia or other haemoglobin variants may be considered.

A mildly low MCH with a normal haemoglobin and normal iron stores is fairly common and may simply reflect a thalassaemia trait or an early change. It still deserves a look at the other indices.

Lab Tests Online UK says a high MCH is usually associated with macrocytic anaemia, in which red cells are unusually large. Each large cell can carry more haemoglobin, so the average per cell increases even though the cells do not work well. The usual suspects are vitamin B12 or folate deficiency, liver disease, an underactive thyroid and alcohol excess.

The NHS describes B12 and folate deficiency anaemia as occurring when a lack of the vitamins causes abnormally large red cells that cannot function properly. Because the neurological effects can become permanent if deficiency continues, a raised MCH or MCV is worth following up even when anaemia is not obvious.

Follow-up depends on the pattern. A low MCH leads most often to iron studies and ferritin; a high one leads to B12, folate, thyroid and liver tests. Your doctor may also request a blood film or reticulocyte count, as Lab Tests Online UK suggests for abnormal full blood counts.

If you are told you have iron deficiency, the NHS says treatment usually involves iron tablets for about six months once the cause has been found, with repeat blood tests to check recovery. Always discuss supplements with a clinician first.