Hemoglobin

Every red blood cell is full of haemoglobin, a protein with iron at its core that binds oxygen in your lungs and releases it again in your tissues. Your haemoglobin value is the amount of that protein per litre of blood, and with that the capacity of your oxygen transport: how much your muscles and brain get delivered per heartbeat. It is the value that defines anaemia, and the value that helps decide your endurance. Haemoglobin says how much capacity you have, not why it is too low or too high. For that you read it beside the other values from the same tube: the cell size, MCV, which becomes small in iron deficiency and large in a deficiency of B12 or folate, and beside ferritin, your iron store. That store runs empty months before haemoglobin falls; a normal haemoglobin therefore does not rule out an early iron deficiency. Your report shows haemoglobin in mmol/L. Normal is about 8.5 to 11.0 mmol/L for men and 7.5 to 10.0 mmol/L for women, and this is one of the few values the labs agree on exactly. It is a concentration, so your fluid balance counts too: thickened blood gives a higher number without there being more cells.

Haemoglobin is what you feel when you walk up the stairs. If it falls, every muscle gets less oxygen per heartbeat, and you notice that as tiredness that does not pass, breathlessness on exertion, a faster heart rate, dizziness on standing, paleness and slower recovery after sport. It is the last step of an iron deficiency, and usually the store has been empty for months by then. Anyone tired with a low haemoglobin almost always has a reason that can be found and reversed, and the most common is iron: heavy periods, too little iron from food, or blood loss you do not see. For anyone who does sport, haemoglobin is the value behind endurance, and at the same time a value with a pitfall. Endurance training enlarges your blood volume: more plasma is added than cells, and the haemoglobin per litre dips a fraction while your total oxygen transport is actually larger. That is an adaptation, not anaemia, and you tell the difference with ferritin. An athlete with a low-normal haemoglobin and a normal ferritin is fit; an athlete with a low-normal haemoglobin and an empty ferritin is running on an empty tank. The high side is usually not a problem. Drinking too little thickens the blood and lifts the number; altitude, smoking and sleep apnoea set the body to making more red cells because too little oxygen comes in at night or in thin air. Smoking is the lever you have yourself there: carbon monoxide occupies part of your haemoglobin, and your body makes more to compensate. Only a persistently high value without one of those explanations belongs to a rare bone marrow condition, and that stands out because the white cells and platelets rise too.

A low haemoglobin means anaemia, and the question is of which kind. From common to rare: an iron deficiency, blood loss, a larger plasma volume in endurance athletes and in pregnancy, a deficiency of vitamin B12 or folate, and a chronic inflammation or kidney disease. The cell size points the way: small cells are iron, large cells are B12 or folate, normal cells are blood loss, inflammation or the kidneys. Ferritin confirms the iron, and in a man or a woman after the menopause blood loss in the gut is the first place to look. A high haemoglobin is usually thickened blood: too little drunk, sweated, or drawn after a hard effort. Drink normally and have blood drawn again, and the value usually sits back within the band. If it stays high, smoking, sleep apnoea and time at altitude are the usual reasons; smoking you leave, sleep apnoea you have investigated. A high haemoglobin with plenty of white cells and platelets too is the rare picture that asks for further investigation of the bone marrow. Set haemoglobin beside MCV and ferritin, and beyond that beside haematocrit, the red cell count and the reticulocytes, which show whether the marrow responds. Have blood drawn well hydrated and not after hard exertion, and compare over time at the same time of day.