MCV

A red blood cell is made in the bone marrow in a series of divisions, and how large it ends up depends on what was available during those divisions. If there is too little iron to make haemoglobin, the cell divides once more before it is full enough, and enters the blood small and pale. If there is too little vitamin B12 or folate to make DNA, the division falters while the cell keeps growing, and it enters the blood too large. MCV, the average volume, therefore tells which of the two is going on. It is an average over cells that already exist, and a red cell lasts about four months. So MCV changes slowly: an iron deficiency only makes the cells smaller once the old, normal cells have been broken down and replaced. For an early sign you look at RDW, the spread in cell size, which rises as soon as the new cells differ from the old. And with a deficiency of iron and B12 at the same time the effects can cancel out, with a normal MCV and a high RDW. Your report shows MCV in fL. Normal is 80 to 100 fL, and that range is the same everywhere. MCV is calculated from haematocrit and the red cell count, and does not move with your fluid balance.

MCV is the value that points the way straight away with a low haemoglobin. Small cells: iron, and then ferritin is the next question, plus where the blood is going. Large cells: B12 or folate, and then those two are the next question, plus alcohol and the thyroid. Normal cells with anaemia: blood loss, a chronic inflammation or the kidneys. That three-way split saves a detour, and it sits in every blood count, even when nothing seems wrong yet. For anyone without anaemia, MCV is mainly interesting at the high end. Alcohol is the most common reason for oversized red cells in someone otherwise healthy: it disturbs production in the marrow directly, and an MCV at the top of the band or above it, with a normal B12 and folate, is often the first measurable trace of drinking too much, earlier than the liver values. It falls again in the months after you cut down, and that makes it an honest measure for anyone who wants to know whether the switch has really been flipped. A B12 deficiency from plant-based eating, antacids or metformin also lets MCV rise, often years before haemoglobin falls. At the low end without anaemia, thalassaemia trait is the most common explanation: small cells, a high count, a normal ferritin. That is not a disease and asks for nothing, except that you know it, because it looks like iron deficiency and iron does not help then.

A low MCV means small cells. The most common explanation first: an iron deficiency, thalassaemia trait, and rarely a chronic disease. Ferritin tells the difference: low in iron deficiency, normal in thalassaemia. In an iron deficiency the MCV is only low once the deficiency has existed for months; the symptoms and the ferritin come before it. A high MCV means large cells. In order: a deficiency of vitamin B12 or folate, alcohol, an underactive thyroid, medicines such as methotrexate and some drugs against epilepsy and HIV, and rarely a bone marrow problem. B12 and folate you settle with a draw, alcohol you settle yourself, and the thyroid with TSH. A high MCV with a normal B12 and folate is nine times out of ten alcohol. Read MCV beside haemoglobin, MCH, MCHC and RDW, and beyond that beside ferritin with a low value and B12, folate and TSH with a high one. Compare over time, because the trend says more than one number.