MCH

A red blood cell is essentially a bag of haemoglobin, and MCH is how much sits on average in each bag: the total haemoglobin in your blood divided by the number of red cells. It is a calculated value that says something about production. If there was enough iron while the cell was being made, it is well filled; if there was too little, it enters the blood pale and light. If there was too little B12 or folate, the cell has grown too large and takes proportionally more haemoglobin with it. So MCH almost always runs in step with MCV, the cell size: small cells are also pale cells, large cells are also heavily filled cells. The two tell the same story, and MCH is often the more sensitive of the two, because in an iron deficiency the filling drops slightly earlier than the volume. Like MCV it is an average over cells that last four months, so it moves slowly. Your report shows MCH in fmol or in amol; internationally in picograms, which gives very different figures. Normal is about 1.70 to 2.10 fmol per cell. There is not always a band beside it, because MCV usually says the same already.

MCH is the value that shows whether your red cells are well filled, and with iron that is the heart of the matter. An iron deficiency makes cells that are paler, and MCH therefore often drops just slightly earlier than MCV and well before haemoglobin goes below the limit. An MCH at the low end with a ferritin at the low end is a deficiency already working through into production, even when the blood count is otherwise still called normal. For anyone who menstruates, eats plant-based or exercises a lot, that is an early signal. The second reason is that MCH shows whether topping up is working. After starting iron, the marrow makes better-filled cells within days, and in the weeks after MCH creeps up while the old, pale cells are broken down. That makes it a good measure to follow whether the iron tablet does what it should, together with the reticulocytes and the haemoglobin. At the high end MCH tells the same as MCV: a deficiency of B12 or folate, or alcohol. And with small, pale cells and a normal ferritin, together with a high cell count it points at thalassaemia trait, which is not a disease but good to know.

A low MCH means pale, poorly filled cells. In order of how often it occurs: an iron deficiency, thalassaemia trait, and rarely a chronic disease. Ferritin tells the difference: low in iron deficiency, normal in thalassaemia. A low MCH with a normal haemoglobin is a deficiency that already sits in production but not yet in the transport. A high MCH means large, heavily filled cells. In order: a deficiency of vitamin B12 or folate, alcohol, and an underactive thyroid. B12, folate and TSH you settle with a draw; alcohol you settle yourself. MCH and MCV should sit high together here; if they do not, the picture is mixed and RDW says more. Always read MCH beside MCV, MCHC, haemoglobin and RDW, and beyond that beside ferritin with a low value and B12 and folate with a high one. Mind the unit, because fmol, amol and picograms give very different figures.