Haemoglobin

Each red cell is packed with about 270 million haemoglobin molecules, and each of those carries four atoms of iron, each able to hold one molecule of oxygen. The colour change everyone has noticed — bright red in an artery, darker in a vein — is haemoglobin loaded and unloaded. It also carries part of the carbon dioxide back and helps buffer the blood's acidity.

What the numbers mean

Reported in grams per decilitre on a complete blood countComplete blood countA complete blood count measures the three families of cell in blood: red cells that carry oxygen, white cells that fight infection, and platelets that clot. It is the commonest blood test ordered anywhere and the usual first step in tiredness, fever, bruising or bleeding.Read the full definition, typically about 13.0 to 17.0 g/dL in adult men and 12.0 to 15.0 g/dL in adult women, with lower ranges in pregnancy and in children. Every laboratory prints its own range and it is the one to read your result against. Below the threshold is anaemiaAnaemiaAnaemia means the blood carries less haemoglobin than it should, so less oxygen reaches the tissues. It usually shows itself as tiredness, breathlessness on exertion and pallor, and in Pakistan the commonest cause by a wide margin is a shortage of iron.Read the full definition; the level alone does not say why, and the red cell size reported alongside it is what starts to answer that.

A high haemoglobin is less familiar but also meaningful. It occurs with prolonged dehydrationDehydrationDehydration is a shortage of water and salts in the body, from losing more than is taken in through diarrhoea, vomiting, fever, heat or simply not drinking. In children and older people it becomes dangerous quickly and is the main cause of death from ordinary diarrhoea.Read the full definition, which concentrates the blood, in heavy smokers, in people living at altitude, in chronic lung disease, and occasionally in a bone marrow disorder that needs investigating.

Two local points. Thalassaemia trait is common in Pakistan and produces a mildly low haemoglobin with unusually small cells; it is not iron deficiency, does not respond to iron, and matters most because two carriers can have a severely affected child — which is the argument for testing before marriage where there is family history. And a pulse oximeterPulse oximeterA pulse oximeter is the clip placed on a fingertip to measure how much oxygen the blood is carrying, shown as a percentage. A reading of 95% or above is normal for most healthy adults.Read the full definition measures the percentage of haemoglobin carrying oxygen, not how much haemoglobin there is, so a severely anaemic person can read a perfectly normal 98 per cent while starved of oxygen.