MCH (Mean Corpuscular Haemoglobin)

MCH (Mean Corpuscular Haemoglobin) measures the average amount of haemoglobin inside a single red blood cell, expressed in picograms (pg). It is calculated automatically as part of a Full Blood Count (FBC) by dividing total haemoglobin by the red blood cell count. MCH works alongside MCV and MCHC to characterise the size and haemoglobin content of red blood cells, which is essential for diagnosing the type of anaemia present.

MCH is a key marker used by Australian pathology labs to classify anaemia. Low MCH (hypochromic) most commonly indicates iron-deficiency anaemia or thalassaemia, while high MCH (hyperchromic) suggests vitamin B12 or folate deficiency, alcohol use, or hypothyroidism. Because MCH is reported on every standard FBC, it is one of the first clues a GP uses to decide whether to order iron studies, B12, folate or further haematology testing.

High MCH (above ~32 pg) means each red blood cell carries more haemoglobin than usual, typically because the cells themselves are larger. This pattern is called macrocytic anaemia when haemoglobin is also low. The most common causes in Australia are vitamin B12 deficiency (often related to vegan/vegetarian diets, pernicious anaemia or long-term metformin use), folate deficiency, chronic alcohol use, hypothyroidism, certain medications (methotrexate, anticonvulsants, hydroxyurea), and liver disease. MCH is best interpreted alongside MCV — if both are high, B12 and folate testing is the standard next step. A raised MCH with a normal MCV is usually benign laboratory variation but is worth repeating with B12, folate, TSH and a reticulocyte count if it persists.

Low MCH (below ~27 pg) means each red blood cell contains less haemoglobin than usual, producing hypochromic, often microcytic red cells. The overwhelming cause in Australia is iron-deficiency anaemia — driven by inadequate dietary iron, heavy menstrual bleeding, pregnancy, gastrointestinal blood loss (ulcers, polyps, coeliac disease, colon cancer), or chronic inflammatory conditions. Thalassaemia trait (more common in patients of Mediterranean, Middle Eastern, South-East Asian or African heritage) also causes low MCH and is often distinguished from iron deficiency by ferritin and haemoglobin electrophoresis. Anaemia of chronic disease and lead poisoning are less common causes. A GP investigating low MCH will typically request iron studies (ferritin, transferrin, transferrin saturation), and if iron is normal, consider haemoglobin electrophoresis.

MCH is automatically reported on every Full Blood Count (FBC), so most Australians have it tested at least annually as part of routine health checks. Anyone with established iron deficiency, B12/folate deficiency, thalassaemia trait or chronic kidney disease should retest every 3–6 months until the anaemia is corrected and stable. Heavy menstruators, pregnant women, vegans, vegetarians, distance athletes and people post-bariatric surgery should have an FBC plus iron studies at least every 12 months.

Always interpreted alongside MCV (mean corpuscular volume), MCHC, RDW, haemoglobin, ferritin, transferrin saturation, vitamin B12 and folate. MCV + MCH together classify the anaemia: low MCV + low MCH = microcytic hypochromic (iron deficiency or thalassaemia); high MCV + high MCH = macrocytic (B12, folate, alcohol, hypothyroidism).