You can be low on iron even when your haemoglobin looks fine

Of these, 38.6% had ferritin below 25 µg/L, the study's line for iron deficiency. Only 6.3% had iron-deficiency anaemia. Of those low on iron, 83.6% had no anaemia, so a haemoglobin test alone would have missed them. Periods were a risk factor, yet more than a quarter of girls who hadn't started them were already low on iron.

How many count as deficient depends on where you draw the line. Call ferritin below 15 µg/L deficient and you get 17%; below 50, and it's 77.5%. Which cut-off is right, the authors say, still needs working out. These are American figures. For anaemia, the kind haemoglobin does show, the WHO estimates that in 2019 it affected 30% of non-pregnant women aged 15 to 49 worldwide, 539 million, and names iron deficiency as its most common nutritional cause.

Weyand and colleagues write that current screening guidance may miss many people with iron deficiency, and that it's worth finding even without anaemia, since iron supplementation reduces fatigue and improves exercise performance. Their study covered girls and women up to 21. Still, if you're tired all the time and your haemoglobin is fine, ferritin is the question to ask.