Clue 8: Depressions running across the nail horizontally
Look for: White ridges running across the width of the nail bed. These so-called "Beau's lines" (after the French physician who described them) can occur in all or just one nail; if in all nails, they're at about the same place on all of them. They're actual ridges in the nail plate itself.
What it might mean: Diabetes, psoriasis, Raynaud's disease -- or just a trauma to the nail. Beta-blockers and drugs used in chemotherapy can also produce Beau's lines. Some people develop them simply as a result of aging.
Next steps: Consider this effect just one piece of a puzzle. Nails grow about 1 mm every six to ten days, so doctors use this measurement to estimate when the problem might have begun.
Clue 9: White bands running across the nail horizontally
Look for: The white-colored bands, known as "Mees' lines," run transverse (parallel to the white tips of the nails). They may affect one nail or several, occurring at about the same spot on each nail. Because the problem is in the nail itself, the line moves forward as the nail ages -- allowing doctors to date the time the problem began.
What it might mean: Arsenic poisoning! Hair and tissue samples should be tested to verify. It's pretty rare these days, Anderson says, but worth knowing about.
Next steps: Make an appointment to see a doctor -- and avoid eating anything you don't prepare yourself!