Every test, explained.

Haematocrit is the share of your blood that consists of red blood cells: at 0.45 almost half your blood is cells and the rest plasma. It tells the same story as haemoglobin from another angle, and it is the value that gives away soonest whether your blood is thickened.

Haemoglobin is the protein in your red blood cells that carries oxygen from your lungs to your muscles and brain. The value says how much oxygen your blood can carry, and with that whether you have anaemia; why, ferritin and the cell size tell you. Have blood drawn well hydrated and not after hard exertion.

MCH is the average amount of haemoglobin per red blood cell: how well each cell is filled. It almost always moves with the cell size, low in iron deficiency and high in a deficiency of B12 or folate, and it is one of the first cell values to respond when you top up iron.

MCHC is how densely the haemoglobin is packed in your red blood cells: the filling corrected for cell size. It is the slowest and most stable of the cell values, falls in an iron deficiency that has lasted a while, and a high value is more often a measurement artefact than a finding.

MCV is the average size of your red blood cells. It is the first fork in anaemia: small cells point at iron, large ones at B12, folate or alcohol, and because a red cell lasts four months, MCV is the report of the last few months, not of today.

RDW measures how much your red blood cells differ from each other in size. A low RDW is naturally low and healthy; a high RDW is an early signal that your bone marrow has started making cells of a different size, usually through iron deficiency or a deficiency of B12 or folate.

Erythrocytes are your red blood cells, and this value counts how many circulate per litre of blood. The count does not yet say how much oxygen your blood carries, that depends on how much haemoglobin sits in each cell; together with haemoglobin and the cell size it points at the kind of anaemia. Have blood drawn normally hydrated and not after hard exertion.

Ferritin is the protein your body stores iron in, and the value in your blood is the measure of how large that store is. It is the first value to know when you are tired, because the store runs empty long before you have anaemia.

Transferrin is the protein that carries iron through your blood, the taxi between gut, store and bone marrow. Your liver makes more of it when iron is scarce and less with inflammation, so a high transferrin is a body looking for iron and a low transferrin usually a body that is inflamed or undernourished.

Transferrin saturation is the share of your iron transport that is occupied right now: how many of the taxis have a passenger. It is the best measure of how much iron your cells can take now, low in a deficiency, high in overload, and the value that shows a deficiency that ferritin hides in inflammation.

Serum iron is the iron currently on its way through your blood, bound to its transport protein. It is a snapshot that sits highest in the morning and shoots up after an iron tablet or a piece of meat; on its own it says little, as the building block of transferrin saturation a lot.

Folate, or folic acid, is the B vitamin from green vegetables and legumes that your cells need to make DNA and divide. Too few vegetables, alcohol and a pregnancy are the most common reasons for a deficiency, and supplements the most common for a high value.

You need vitamin B12 for your nerves, your DNA and making red blood cells. It comes almost only from animal food and has a complicated uptake, so a deficiency comes as often from the stomach as from the plate.