Koilonychia, also known as spoon nails,[1]: 782 is a nail disease that can be a sign of hypochromicanemia, especially iron-deficiency anemia.[2]: 656 [3] It refers to abnormally thin nails (usually of the hand) which have lost their convexity, becoming flat or even concave in shape. In a sense, koilonychia is the opposite of nail clubbing. In early stages nails may be brittle and chip or break
easily.
Even though koilonychia has been associated with iron deficiency in case reports, it is more likely seen as an occupational change in nails and may be idiopathic; ruling out iron deficiency anemia in these patients is the only work-up necessary in this condition.[4]