Nail discoloration, in which the nails appear white, yellow, or green, can result from different infections and conditions of the skin. In about 50% of cases, discolored nails are a result of infections with common fungi that can be found in the air, dust, and soil. There are many species of fungi that can affect nails. By far the most common, however, is called Trichophyton rubrum. This type of fungus has a tendency to infect the skin and is therefore known as a dermatophyte. Pseudomonas is a type of bacteria that infects the nail bed and results in a greenish color to the nails. Red or black (that may sometimes appear bruised) nails may result from a hematoma (a collection of blood) under the nail as a result of trauma (including ingrown toenails).
Chronic medical conditions also can affect the appearance of the nails. Specific color changes in the nails can be suggestive of diabetes or of liver, kidney, heart, or lung conditions. This is why doctors pay specific attention to nails during a routine physical examination.
Other, rare causes of discolored nails include the "yellow nail syndrome," an inherited condition that results in slow-growing, yellowing, discolored nails and is associated with lymphedema (swelling of tissues due to the accumulation of fluid) and lung diseases. Nails may also appear lightened to a whitish-yellow color if there has been separation of the nail from the nail bed, termed onycholysis.
The skin, mucous membranes, and nails may appear blue when there is inadequate oxygenation of blood (cyanosis), but this is not true discoloration of the nail itself.
Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.
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