MCV blood value in athletes: read the trend, not one result

Red blood cells are the cells that use their haemoglobin to carry oxygen from your lungs to your muscles. MCV, in full mean corpuscular volume, is the average volume of one such cell, expressed in femtolitres: a millionth of a billionth of a litre. The modern cell counter measures the size of each red cell individually as it streams past a sensor, and reports the average of all those measurements. The value comes as standard with the complete blood count and is rarely requested on its own. For adults 80 to 100 fl applies, and that normal range is deliberately not split by sex.

MCV is the classic classifier of anaemia. If haemoglobin is low, MCV splits that into three tracks: microcytic below 80 fl, with small cells, normocytic between 80 and 100 fl, and macrocytic above 100 fl, with large cells. Without MCV you know oxygen transport is short, but not where the investigation should look next. Small cells send it towards iron, with ferritin as the next value. Large cells point towards vitamin B12 and folate, or towards causes without a vitamin deficiency such as chronic alcohol use, liver disease, an underactive thyroid, a strong rise in young red cell production, or drugs such as methotrexate, hydroxyurea and zidovudine.

What matters most is what MCV is not: it is an average and no description of spread. A normal MCV does not exclude a deficiency. With a combined deficiency, iron plus B12 or folate at the same time, the small and the large cells cancel each other out in the average, and MCV can be entirely normal while two deficiencies sit side by side. The RDW, the measure of variation in cell size, is usually abnormal in that case. So never read MCV on its own; always read it alongside RDW and haemoglobin.

If you train seriously, MCV is most useful as a trend line. An endurance athlete can lose iron season after season without haemoglobin responding straight away, because the body protects haemoglobin for as long as it can and empties the stores first. What you see in that phase is no result outside the normal range, but an MCV drifting slowly downwards inside 80 to 100 fl, often with a rising RDW beside it as smaller new cells join the older, normal ones. Two measurements six months apart show that; one measurement never does.

Iron leaves an athlete by several routes at once: red cells damaged in the sole of the foot at every landing, losses through sweat at high training volumes, and losses through the gastrointestinal tract during hard efforts. In women, monthly blood loss is added to that. None of these is dramatic on its own, but they run continuously and they stack. Ferritin measures the stores directly; MCV shows whether the shortage has started to show up in the cells you are building.

There is a trap in the opposite direction too. Young red cells, reticulocytes, are larger than mature ones. In the first weeks after someone starts iron or B12, and also after bleeding or with accelerated red cell breakdown, the share of young cells rises sharply and lifts MCV with it. An MCV that climbs just after supplementation begins does not automatically mean something new is wrong.

Finally, alcohol, stated purely as fact. Alcohol raises MCV directly, even with no B12 deficiency in play, and the value falls back only slowly afterwards, roughly on the timescale over which red blood cells are replaced, up to about three months. Anyone who drinks regularly at the weekend and tracks their own blood work is looking at a number driven by two things at once. A mildly higher MCV in an otherwise normal blood count is in that situation first a question about habits and about the sample, and only then a clinical one.

MCV almost always arrives with the complete blood count, so you rarely order it separately. There are still sensible moments: at the start of a build phase so you have a baseline, and after a heavy block or a training camp. If you train at altitude or have just come back from it, measure at a fixed point relative to that camp, so that a temporary change in red cell production is not read as a trend. After a blood donation, a measurement only becomes informative once the stores have had time to recover.

Measure too when training stops giving back what it used to: less recovery between sessions, a higher heart rate at the same pace, or a flatness that sleep does not fix. Then order MCV together with ferritin, vitamin B12 and folate, so a combined deficiency does not disappear behind a normal average.

One technical point: red cells swell in an EDTA tube during longer storage. A sample analysed only after 24 hours gives an artificially raised MCV. An isolated, mildly abnormal MCV with nothing else out of place is therefore first a question about the sample, and only then a clinical one.