How to read an iron panel blood test

Two different questions, two different lines

Ferritin describes your reserve: the iron put away for later, like a strongbox. Serum iron describes what is circulating right now, and transferrin saturation describes how full the transport system is.

A reserve and a flow answer different questions, which is why a panel that reports only serum iron is hard to interpret. Serum iron also varies through the day, which is why iron studies are usually asked for in the morning.

Ferritin is the line that matters most for the common question, which is whether you are short of iron.

The threshold is genuinely disputed

This is the part worth knowing before you compare your figure with anything. A systematic review of 29 guidelines from professional associations across ten countries found that recommendations for diagnosing iron deficiency vary widely, and concluded that a serum ferritin cutoff of 100 µg/L should be considered in most conditions, with transferrin saturation at 20%, except in particular situations such as young healthy women with heavy periods.

A Swiss expert consensus, working on people without systemic inflammation, found that a threshold of 30 µg/L gives good accuracy for diagnosing iron deficiency without anaemia, and that a ferritin between 30 and 50 µg/L with transferrin saturation below 20% can also indicate it.

Read those two together and the practical conclusion is clear: a ferritin of 25 with a laboratory floor of 15 is not automatically fine, and the figure printed on your report as the lower limit is one convention among several. This is a question for your doctor with your symptoms in hand, not a line to settle from a table.

Inflammation can hide a deficiency

Ferritin rises with inflammation, independently of how much iron you have stored. An infection, a chronic inflammatory condition or a recent injury can therefore lift a ferritin into the normal range while the reserve underneath is empty.

This is why an iron panel is read next to an inflammation marker such as CRP, and why transferrin saturation is useful: it is less disturbed by inflammation than ferritin is.

The reverse case is simpler: a low ferritin is reliable. Ferritin does not fall for reasons other than a low iron store.

Before the test, and after it

An iron supplement taken on the morning of the draw distorts serum iron, which is why supplements are usually paused for a few days before iron studies. Ask your laboratory how long, because the answer depends on what is being measured.

And if a deficiency is found, the question of why matters more than the number. Iron deficiency is a finding that asks for an explanation, and the explanation is what the consultation is for.