Transferrin Saturation

Iron travels through your blood in transferrin, the transport protein that can carry two iron atoms per molecule. Transferrin saturation is the percentage of those seats that is occupied right now: serum iron divided by the transport capacity. At a third occupied, plenty of iron is circulating for the marrow; at a sixth there is too little; at more than half the transport is full and free iron starts reaching tissue where it does not belong. That makes saturation a different question from ferritin. Ferritin says how full the warehouse is; saturation says whether it is delivering. Usually they run in step, but exactly where they diverge sits the information. With an inflammation your body locks iron away in the store: ferritin rises, saturation falls, and the deficiency that ferritin hides, saturation shows. In iron overload saturation rises first, years before ferritin becomes high and before there is damage. Your report shows saturation in percent. Normal is about 20 to 43% for a fasting morning draw, where the labs overlap; below 20% an iron deficiency is likely, above 45% fits overload. Because the value is calculated from serum iron, it swings with it over the day: highest in the morning, and after an iron tablet or a meat meal temporarily much higher.

The first reason to know your saturation is that it sees what ferritin misses. Anyone regularly inflamed, through sport, a chronic condition, excess weight or simply a cold in the week of the draw, has a ferritin that looks too good. Saturation then actually falls, and a low saturation beside a normal or high ferritin is the picture of an iron deficiency hiding behind inflammation. For athletes that is the ordinary situation: after hard training the body sits slightly in inflammation mode for days, and saturation is then the most honest of the iron values, provided you have blood drawn on a rest day in the morning. The second reason is the other side, and it is more important than it seems. Inherited iron overload, haemochromatosis, occurs in the Netherlands in about one in two hundred people of Northern European descent and often goes unnoticed for decades, because the symptoms are vague: tiredness, joint pain, abdominal complaints, less desire. Meanwhile the iron builds up in liver, heart, pancreas and joints. Saturation is the earliest value that gives it away, well before ferritin sits high, and the treatment is simple and complete: giving blood regularly. A persistent saturation above 45%, fasting and without a supplement, is therefore the one iron value on which you do not wait. The third reason is that saturation shows whether topping up works. After starting iron, saturation rises within days, long before ferritin moves; if it stays low, the iron is not being absorbed, and that is a clue that it lies with the gut or with an inflammation.

A low saturation means too little iron is available right now. From common to rare: an iron deficiency, an inflammation locking the iron away, drawn in the afternoon or after hard training, and a pregnancy, in which the transport grows and the baby asks for iron. Ferritin and CRP tell the difference: low saturation with low ferritin is an empty store, low saturation with high ferritin and high CRP is a store on lock. A high saturation is usually an iron tablet or an iron-rich meal in the hours before the draw, and that is settled with a fasting repeat. Fasting and without a supplement, and persistently above 45%, it is iron overload or a liver releasing its iron; ferritin and the liver values say which, and with overload investigation for haemochromatosis belongs, in family members too. Set saturation beside ferritin and CRP, and beyond that beside serum iron and transferrin, which it is calculated from. Have blood drawn in the morning, fasting, on a rest day, and only compare morning values with each other.