Leukonychia is a nail change that shows as white spots, lines or a whole white nail plate, most often caused by a small knock to the nail matrix, and the mark grows out with the nail over several months.
Also known as: white spots on nails, punctate leukonychia, white marks on nails, milk spots
Leukonychia is the medical name for white marks in the nail. The commonest form is punctate leukonychia: one or more small white dots on one or two nails. The nail plate is made in the matrix, the tissue under the skin at the base of the nail (Medscape). A knock to that area, a manicure tool pushed too hard or a habit of picking at the cuticle disturbs the cells as they form, and the flaw appears weeks later as a white spot further up the nail.
The spot is part of the plate, so it cannot be scrubbed or moisturised away. It moves forward as the nail grows and is filed off at the free edge. Fingernails grow about 3 to 4 mm a month and a full plate takes around six months to replace, so a spot near the base needs several months to reach the tip (Medscape). Polish covers it at once; a base coat and a milky or sheer shade hide it well.
Many people connect white spots with a lack of calcium. Injury is the usual cause. Healthline notes that zinc and calcium deficiency are sometimes linked to white spots, but a single spot on one nail with a normal diet needs no supplement.
See a GP or dermatologist when the whole nail turns white, when white bands cross every nail at the same height, when spots keep appearing without any knock, or when a nail changes shape or colour and you do not know why (NHS). Those patterns can point to a fungal infection, a skin condition or an internal cause, and a doctor can check with a simple examination.