The water-drop test for the diagnosis of koilonychia

Koilonychia is a common concave nail dystrophy in which the nail plate is depressed centrally and elevated laterally.1 The term has Greek origins, with koilos meaning spoon. The pathophysiology of koilonychia, or spoon nails, is poorly understood. It has been associated with dermatologic disorders such as alopecia areata, psoriasis, and lichen planus; endocrinopathies such as hypothyroidism, hyperthyroidism, and diabetes mellitus; nutritional deficiencies, notably iron deficiency anemia; genodermatoses; and traumatic and occupational exposures.