Low MCHC on a Blood Test: What It Means

A full blood count reports several numbers about red blood cells. Some describe how many there are, some describe how big they are, and some describe how much haemoglobin they contain. MCHC belongs to the last group. Lab Tests Online UK explains that it measures the concentration of haemoglobin within red blood cells and that it helps a doctor interpret the MCH result and diagnose specific red cell disorders.

The distinction between MCH and MCHC is worth holding on to. MCH is the average amount of haemoglobin in each cell. MCHC is how concentrated that haemoglobin is, which takes the size of the cell into account. A cell can be small and still be densely packed, or large and thinly filled, so the two numbers are not interchangeable.

Because the result is a ratio calculated from other measurements, a low MCHC is rarely the first thing a clinician reacts to. It is a supporting detail that confirms a pattern already visible in the haemoglobin, the MCV and the MCH.

Lab Tests Online UK lists three situations in which a low MCHC is seen: iron deficiency anaemia, long-standing inflammation and thalassaemia. The same page links low MCV and low MCH to the same three groups, which is why these results often travel together on one report.

The NHS describes iron deficiency anaemia as the most common type of anaemia and says it is usually checked with a full blood count. Its stated causes include heavy periods, pregnancy and bleeding in the stomach or intestines, for example from stomach ulcers or from taking anti-inflammatory medicines such as ibuprofen or aspirin.

NICE Clinical Knowledge Summaries explains that a serum ferritin below 30 micrograms/L confirms iron deficiency, and that ferritin is harder to interpret when infection or inflammation is present because it can be high even in someone who is iron deficient. This is one reason a low MCHC and a normal-looking ferritin do not automatically cancel each other out.

The same NICE summary stresses that the cause of iron deficiency should be found and treated, and that men and postmenopausal women with iron deficiency anaemia are normally referred to gastroenterology unless there is obvious bleeding from somewhere other than the gut. The NHS notes that a faecal immunochemical test (FIT) may be used when the reason is not clear.

The NHS describes thalassaemia as a group of inherited conditions affecting haemoglobin, mainly in people of Mediterranean, south Asian, southeast Asian and Middle Eastern origin. It also explains that being a carrier, sometimes called having the trait, will not generally cause health problems for beta thalassaemia carriers, although it can matter when planning a family.

A reading that has been low and stable for years, with no tiredness and a normal ferritin, is the kind of picture that makes a clinician think of a carrier state rather than iron deficiency. The NHS says blood tests can be carried out at any age to check for thalassaemia or carrier status, and that screening is offered to all pregnant women in England.

This matters because iron tablets will not correct a result that comes from an inherited haemoglobin difference, so the distinction is worth making before treatment starts.

Lab Tests Online UK says a doctor will look at all the results together rather than in isolation. If the pattern suggests anaemia, further tests such as iron studies, vitamin B12 and folate may be requested, and a blood film, where the cells are examined under a microscope, is often used alongside the count.

It also states that many results slightly outside the normal range do not require treatment, and that the test may simply be repeated after a short period to see whether the result has changed. Reference ranges differ by age and sex, and each laboratory prints its own, so a result flagged on one report may not be flagged on another.

• Check the haemoglobin and MCV on the same report. If both are low, iron deficiency or an inherited pattern becomes more likely.

• Think about recent blood loss, heavy periods, pregnancy, or medicines such as anti-inflammatory painkillers, and mention them.

• Ask whether iron studies or a ferritin are planned, and whether the result should be repeated.

• Report new tiredness, breathlessness, palpitations or black stools promptly rather than waiting for the next scheduled test.

The NHS lists tiredness, shortness of breath, noticeable heartbeats, paler skin and headaches among the symptoms of iron deficiency anaemia and advises seeing a GP if you think you might have it. A low MCHC without symptoms is still worth raising, because the cause, particularly bleeding, can be present before it is felt.

Do not start iron supplements to correct a number without knowing the cause. NICE advises against using a trial of iron treatment in men and postmenopausal women because possible sources of gastrointestinal bleeding need to be excluded first.