White spots on nails: what they usually mean

White spots on the nails are called punctate leukonychia, from the Greek for white nail. DermNet describes white nails generally as the most common nail discolouration and says the nail loses its pink undertone and appears white. The small flecks that most people notice are the commonest pattern. They are typically scattered over a few nails, and DermNet attributes them mainly to minor damage to the nail plate or its growing area, the matrix.

Because the nail grows slowly, the mark appears some time after the knock, which is why most people cannot remember what caused it. As the nail grows forward the spot moves towards the free edge and is trimmed away.

DermNet says true leukonychia from trauma happens when injury to the nail plate or matrix disrupts keratin, trapping air within the plate, which reflects light and makes the nail look white. Punctate leukonychia, it says, occurs after nail biting, manicuring, knocks and bangs and tight footwear.

The NHS nail problems page says most nail problems are caused by injuries or nail biting, staining, not trimming regularly or cutting at an angle, hands that are often in water or cleaning products, and fungal infection. Its advice for looking after nails applies equally to white spots: wear rubber gloves for wet work, trim straight across, do not bite or pick at nails or the skin around them, and avoid shoes that pinch the toes.

The Primary Care Dermatology Society adds that minor trauma is a common cause of leukonychia and tends to produce focal areas of white.

The shape and extent of the white area help distinguish ordinary spots from patterns that DermNet and the Primary Care Dermatology Society link with other problems. DermNet separates true leukonychia, which does not disappear when you press the nail onto the nail bed, from apparent leukonychia, where the problem lies in the nail bed and the colour fades under pressure.

DermNet lists onychomycosis (fungal infection), onycholysis, nail psoriasis, twenty-nail dystrophy and vitiligo of the nail as differentials for a white nail. The NHS describes fungal nail infection as often spreading to all of the nail, making it thicker and turning it white or yellow, and the nail may become brittle and crumbly. It also describes small dents or pits as a possible sign of nail psoriasis, eczema or alopecia, which can be mistaken for spots.

The NHS adds that nail problems can sometimes be a symptom of a more serious or long-term condition, naming psoriasis, iron deficiency anaemia, underactive or overactive thyroid, diabetes, and heart, lung or liver disease, and that some medicines can cause nail problems.

A few white flecks on one or two nails, with no other symptoms, are not usually a reason to seek care. The NHS advises seeing a GP if a nail has changed shape, changed colour or fallen off and you do not know why, or if the skin around the nails has become sore, red, swollen and warm, which can be a sign of infection. Self-referral to a podiatrist is also possible in some areas.

A fuller assessment is more reasonable if many nails are affected, if a band or whitening spans the whole width of the nail, if several nails are white at once, if you have recently started a new medicine, or if you also have symptoms such as fatigue, swelling or changes in your skin. DermNet says that when the cause is unclear, tests may include nail clippings to exclude fungal infection, a nail biopsy and blood tests, particularly renal and liver function.