According to a 2014 review in the New England Journal of Medicine stated that a ferritin level below 30 ng/mL indicates iron deficiency, while a level below 10 ng/mL indicates iron-deficiency anemia. A 2020 World Health Organization guideline states that ferritin indicates iron deficiency below 12 ng/mL in apparently-healthy children under 5 and 15 ng/mL in apparently-healthy individuals of 5 and over.
Some studies suggest that women with fatigue and ferritin below 50 ng/mL see reduced fatigue after iron supplementation.
In the setting of anemia, low serum ferritin is the most specific lab finding for iron-deficiency anemia. However it is less sensitive, since its levels are increased in the blood by infection or any type of chronic inflammation, and these conditions may convert what would otherwise be a low level of ferritin from lack of iron, into a value in the normal range. For this reason, low ferritin levels carry more information than those in the normal range. A falsely low blood ferritin (equivalent to a false positive test) is very uncommon, but can result from a hook effect of the measuring tools in extreme cases.
Low ferritin may also indicate hypothyroidism, vitamin C deficiency or celiac disease.
Low serum ferritin levels are seen in some patients with restless legs syndrome, not necessarily related to anemia, but perhaps due to low iron stores short of anemia.
Vegetarianism is not a cause of low serum ferritin levels, according to the American Dietetic Association's position in 2009: "Incidence of iron-deficiency anemia among vegetarians is similar to that of non-vegetarians. Although vegetarian adults have lower iron stores than non-vegetarians, their serum ferritin levels are usually within the normal range."