MCHC

Of the three cell values the blood count calculates, MCHC is the most refined: the haemoglobin divided by the share of red cells, so how much oxygen carrier sits per litre of cell. It corrects MCH for cell size. A large cell with plenty of haemoglobin and a small cell with little can have the same MCHC; they are both equally full for their size. Only when the cell has too little haemoglobin for its size does the MCHC fall, and that happens in an iron deficiency and in thalassaemia. That makes MCHC the slowest and least sensitive of the cell values. In an iron deficiency MCV and MCH fall together, and the division of the two stays put for a long time; only in a deficiency that has lasted a while do the cells enter the blood so pale that the concentration falls too. In a B12 or folate deficiency MCHC does not change at all, because the oversized cells are proportionally just as full. Your report shows MCHC in mmol/L; internationally in g/dL. Normal is about 19 to 22 mmol/L, the same for men and women and at every age. There is not always a band beside it. A cell can only be filled up to a certain limit, and that is why a high MCHC is rare and usually says something about the measurement.

MCHC is rarely the value you learn something new from, but it is the value that confirms a picture. In an iron deficiency already working through into production, MCV and MCH sit low; if MCHC falls with them, the deficiency is clear and no longer early. In a B12 or folate deficiency MCHC actually stays normal while MCH and MCV sit high, and that difference helps tell the two kinds of anaemia apart. For anyone topping up iron, MCHC is the last of the cell values to recover, and so a sign that the marrow is making completely normal cells again. The high side is different from the other cell values. A red cell can only be filled with haemoglobin up to a certain limit, so a genuinely high MCHC hardly ever occurs. If it is there anyway, nine times out of ten it is a measurement artefact: cold antibodies that make the cells clump in the tube, plenty of fat in the blood after a meal, or a tube that has haemolysed. That is usually recognised at the measurement itself, and a repeat settles it. The one real reason for a high MCHC is worth knowing: hereditary spherocytosis, in which the cells are ball-shaped and therefore more densely filled. It goes together with a mild anaemia, occasional slight jaundice and an enlarged spleen, often runs in families, and is very manageable once it is known.

A low MCHC means cells that hold too little haemoglobin for their size. From common to rare: an iron deficiency that has lasted a while, and thalassaemia trait. In both cases MCV and MCH sit low too; ferritin says which of the two it is. A low MCHC with normal MCV and MCH is usually a matter of rounding and says nothing. A high MCHC is usually a measurement artefact: cold antibodies, fat in the blood or a haemolysed tube. Have blood drawn again fasting, and the value usually sits back within the band. If it stays high, with a mild anaemia, occasional jaundice and many reticulocytes, that fits hereditary spherocytosis, and then investigation of the cell shape belongs with it. Set MCHC beside MCV, MCH and haemoglobin, and beyond that beside ferritin with a low value. A single slightly abnormal MCHC with an otherwise normal blood count asks for a repeat, not for an explanation.