Red lines running lengthwise in your fingernails are most often splinter hemorrhages, tiny streaks of blood from damaged capillaries in the nail bed. The most common cause is minor trauma to the nail, like catching it on something or repetitive tapping. But when these lines appear on multiple nails without any obvious injury, they can signal something going on elsewhere in the body, from autoimmune conditions to heart valve infections.
Why the Lines Are Shaped Like Splinters
The blood vessels underneath your nails run lengthwise, parallel to the direction your nails grow. When one of these tiny capillaries breaks, the leaked blood follows that same channel, creating a thin vertical line that looks like a splinter stuck under the nail. When they first appear, these lines are reddish or purple. Within a few days, they typically darken to brown or black as the trapped blood ages.
Because the blood is embedded in the nail bed, the line moves outward as your nail grows. Fingernails grow roughly 3 to 4 millimeters per month, so you can roughly estimate when the damage happened based on how far the line is from the base of your nail. If an injury caused it, the line will eventually reach the tip and disappear as you trim your nails.
Trauma Is the Most Common Cause
Bumping your hand, jamming a finger, or even doing repetitive work with your hands (gardening, typing on hard keys, sports) can rupture those delicate capillaries. A single line on one nail, especially if you remember knocking it, is almost always harmless. These lines resolve on their own as the nail grows out, usually within a few weeks to a couple of months depending on where on the nail the line appeared.
When Multiple Nails Are Affected
The picture changes when red lines show up on several nails at the same time with no history of injury. This pattern points toward a systemic cause, meaning something in the bloodstream is damaging capillaries throughout the body rather than just at one nail.
Heart Valve Infection (Endocarditis)
Splinter hemorrhages have long been associated with infective endocarditis, a bacterial infection of the heart’s inner lining or valves. In endocarditis, tiny blood clots called microemboli break off from infected valve tissue and travel through the bloodstream. When they lodge in the small capillaries under the nail, they block blood flow and cause the characteristic red or brown lines. These lines tend to appear in the far end of the nail, closer to the fingertip. Endocarditis typically comes with other symptoms: persistent fever, fatigue, joint pain, unexplained weight loss, and sometimes painless red spots on the palms or soles.
Vasculitis and Autoimmune Conditions
Vasculitis, or inflammation of blood vessels, can damage the capillaries in the nail bed the same way microemboli do. Conditions like lupus and rheumatoid arthritis sometimes trigger this kind of small-vessel inflammation. In lupus, the immune system attacks blood vessels the way it attacks the kidneys, skin, or brain in other patients. Nail fold infarcts (tiny dark spots at the edges of the nail) and splinter hemorrhages are both recognized signs of small-vessel vasculitis. People with vasculitis often also notice small red or purple dots on the legs, rashes, or open sores around the ankles.
Psoriasis
Nail psoriasis causes focal bleeding in the tissue of the nail bed, which shows up as red or brown lines that shift outward as the nail grows. Psoriasis-related nail changes usually come alongside other signs: pitting (small dents in the nail surface), thickening, crumbling edges, or separation of the nail from the nail bed. If you already have psoriasis on your skin, nail involvement is common and affects up to half of people with the condition at some point.
Red Lines vs. a Red Half-Moon
Some readers searching for “red lines in fingernails” may actually be noticing a reddish discoloration of the lunula, the pale half-moon shape at the base of each nail. A red lunula is a different finding with its own set of associations: heart failure, liver cirrhosis, rheumatoid arthritis, lupus, psoriasis, and alopecia areata (patchy hair loss) have all been linked to it. If the redness you’re seeing is concentrated in that crescent-shaped area rather than running as a thin line along the length of the nail, a red lunula is the more likely explanation.
Where the Line Sits Matters
Location on the nail carries diagnostic weight. Lines in the outer third of the nail, near the fingertip, are more commonly linked to trauma or to the microemboli seen in endocarditis. Lines closer to the base of the nail, near the cuticle, tend to raise more concern for systemic conditions like vasculitis because they suggest the damage happened recently and from the inside. A healthcare provider evaluating your nails will note this distinction along with how many nails are involved and whether you have any other symptoms.
What to Watch For
A single red line on one nail after you’ve bumped your hand is not a reason to worry. But certain patterns deserve attention. Lines appearing on multiple fingers without trauma, especially alongside fever, fatigue, joint pain, unexplained weight loss, or new skin changes, suggest your body is dealing with something beyond a bumped nail. The combination of splinter hemorrhages with a persistent low-grade fever is a classic prompt for doctors to investigate endocarditis.
If the lines keep recurring, or if you notice other nail changes like pitting, thickening, or the nail pulling away from the skin underneath, that pattern is worth bringing up with your doctor. Nail changes are often an early visible clue to conditions that are otherwise hard to spot until they progress further.