Hemoglobin & the CBC

The most common blood disorder in the world is one most people never think to check. Anemia, defined simply as low hemoglobin, affects roughly a quarter of the entire global population, on the order of two billion people ^[1]. It is a quiet thief. It steals energy, makes the stairs feel steeper, leaves you pale and cold-handed and short of breath, and almost all of those symptoms are easy to file under stress, poor sleep, or just getting older.

The number that catches it sits on the cheapest and most frequently ordered blood test there is. Hemoglobin is the headline figure on a complete blood count, the CBC, a panel so routine it is easy to overlook. But the CBC does something better than simply telling you whether you are anemic. Read properly, the pattern of the numbers usually tells you why, which is the difference between knowing you have a problem and knowing what to do about it.

Hemoglobin is an iron-containing protein packed into red blood cells, and its job is to bind oxygen where it is plentiful, in the lungs, and release it where it is scarce, in the working tissues. Red cells are made in the bone marrow, live for about 120 days, and are then recycled. Building them well requires several things at once: iron to form the oxygen-binding heart of hemoglobin, vitamin B12 and folate to make the DNA that lets the precursor cells divide, and a hormone called erythropoietin, released by the kidneys, as the signal to ramp up production.

Anemia appears whenever one of those inputs fails, or when red cells are lost through bleeding or destroyed faster than they can be replaced. The CBC captures the result from several angles: hemoglobin (the oxygen-carrying capacity), hematocrit (the fraction of blood made up of red cells), the red cell count, and the red cell indices, of which the most useful single number is the MCV, the average size of your red cells.

The MCV is the decoder. A low MCV, meaning small red cells, points most often to iron deficiency, which is by far the leading cause of anemia worldwide ^{[1] [2]}. Iron is the raw material of hemoglobin, and when it runs short the marrow turns out small, pale cells. A high MCV, meaning large red cells, points instead to a shortage of vitamin B12 or folate, the two vitamins the marrow needs to build the DNA of dividing cells; without them the precursors cannot divide properly and emerge oversized and immature ^{[3] [4]}. A normal MCV, finally, fits anemia of chronic disease and inflammation, kidney disease with its lost erythropoietin signal, or recent blood loss.

This is why iron deficiency hits some groups so hard: menstruating women, endurance athletes, people eating little or no meat, and anyone with slow gastrointestinal blood loss all run their iron reserves low, and a low MCV with a low ferritin makes the diagnosis almost on its own. At the other end of the range, a high hemoglobin is usually far less ominous than it looks. Most often it simply reflects dehydration concentrating the blood, or the higher red cell mass of a smoker or someone living at altitude. Only a persistently high value, which can thicken the blood and raise clot risk, needs a closer look.