What is red line in fingernails?

If the red lines run in the direction of the growth of the nail, they are called splinter hemorrhages as they look like a red splinter under the nail. The hemorrhages may be caused by tiny clots that damage the small capillaries under the nails.

They can be a sign of a number of diseases: infection of the heart valves called endocarditis, damage from swelling of the blood vessels (vasculitis) or tiny clots that damage the small capillaries (microemboli).