Fingernails can reveal an amazing amount about a person's health, medical experts say, with a surprising number of conditions manifesting themselves with changes in the shape, colour or overall state of the nails.

"It may be the first sign, it may be the herald sign of ... an internal disease," says Dr. Yves Poulin, a Quebec City dermatologist and president-elect of the Canadian Dermatology Association.

Lung disorders, nasal polyps, anemia, inflammatory bowel syndrome and liver diseases can provoke changes in the fingernails.

In some cases those alterations can prompt people to seek medical attention, in the process bringing to light previously undiagnosed conditions. In others, the state of a patient's nails will help a physician clarify what is at play.

"For us, it helps to make the correct diagnosis to look at the nail," Poulin says.

The bed of the fingernails of healthy individuals should be a light pink. Nail beds that are white may suggest anemia - a red blood cell deficiency which itself can be a symptom of other, sometimes serious, diseases. When the nails themselves grow opaque and white, it can be a sign of liver disease.

White nails with a dark band at the tip - a condition called Terry's nails - can be a sign of aging but could also signal congestive heart failure, diabetes or liver disease, according to a photo slide show on fingernail conditions on the Mayo Clinic website. (http://www.mayoclinic.com/health/nails/WO00055)

Kidney problems are suspected with a condition known as half-and-half nail, in which the lower part of the nail bed is white but a portion towards the tip of the nail is pink.

Bluish nails can signal a lack of oxygen, a sign a person might be suffering from one of a number of lung conditions. Green nails can be caused by infection with Pseudomonas aeruginosa, a bacterium which is common in the environment. Antibiotics can clear up this condition.

Poulin says respiratory tract problems - such as nasal polyps and chronic sinusitis - can trigger yellow nail syndrome, which he describes as rare. It can be corrected in some cases, depending on the cause.

"I had a guy in recently, he was an attorney, he was 40 and he had yellow nails on all his nails. And he had a nose surgery and it all went away," Poulin says.

Strangely shaped or marked nails are also indicative of a variety of conditions.

Thickened, misshapen and cloudy nails - sometimes on the fingers, but more often on the toes - are generally a sign of infection with a fungus. Called onychomycosis, the condition is unsightly and makes the nails difficult to trim and maintain.

Onychomycosis can and should be treated, Poulin says, and the earlier the better. The longer the problem festers, the harder it is to treat, he says.

And while thickened toe nails may be merely an esthetic problem for a 60-year-old, when that person is 80 and diabetic, toenails that can't be trimmed can trigger infections in the skin around the nail bed, erode foot health and threaten mobility.

"It may be an open door for cellulitis, for infections of the skin, in diabetic people," Poulin says. "(But) this is often neglected. People don't look too much at their toenails."

A brown or black streak or dot under a nail that persists can be skin cancer - melanoma, which can be deadly if it isn't caught early. And if there is no evident reason for the change in pigmentation, it should be checked out, says Dr. Mark Davis, a dermatologist at the Mayo Clinic's Rochester, Minn., campus.

"If patients can remember some trauma to their nail - that they actually have a reason for getting blood under their nail and it's usually painful - then it's nothing to be worried about," Davis says.

"But if somebody develops a new pigmentation on their nail, just like a new mole on your skin, it's best to have a dermatologist look at it and make the judgment as to whether it could be a melanoma or whether it's just a mole. And sometimes that can be quite difficult even for the dermatologist to decide."

Melanomas under the nail aren't common, but they do occur. But because people don't necessarily know of the possibility, such melanomas can go undetected, threatening chances of survival.

"People come very late with melanoma of the nail plate," says Poulin. "They have a black streak in the nail for years."

Someone who has horizontal groves across all their fingernails has experienced an illness that has interrupted the growth of the nails. The condition, called Beau's lines, is associated with uncontrolled diabetes, circulatory diseases or illnesses associated with high fever, the Mayo Clinic says.

While nail changes can signal something is going on with a person's health, sometimes the message they send isn't specific to a particular disease.

"For example, when you see clubbing of the nails, there's like 20 different things that can be associated with that," Davis says. He adds the warning, though, that "if that happens and it's new, it can be a sign of lung cancer."

The term clubbing is used to describe the swelling or enlarging of the tips of the fingers, with the nails curving downwards over the tip. While some people are born with clubbing, if it develops later on it can be a symptom of lung disease, congenital heart disorders, inflammatory bowel disease or liver problems.

Spoon nails, on the other hand, come about when the fingernails soften and curl inward from the sides, creating a concave surface. Also known as koilonychia, spoon nails can be a sign of iron-deficiency anemia.

Davis suggests paying attention to, but not fretting unduly, over changes to fingernails.

"If they notice a change in their nails, I think it's reasonable to check on it, but not to get overly alarmed about it. Because there's lots of things that happen to the nails themselves that have nothing to do with any underlying conditions."

Copyright The Canadian Press