Splinter hemorrhages are tiny streaks of blood trapped beneath a fingernail or toenail, running lengthwise like a sliver of wood embedded under the nail. They form when blood leaks from the small vessels in the nail bed, gets caught against the underside of the nail plate, and rides forward as the nail grows out. Most of the time, the cause is nothing more than everyday trauma to the fingers or toes, but in certain patterns and circumstances, these little lines can signal a serious underlying condition that deserves medical attention.
What They Look Like and How They Form
Splinter hemorrhages appear as thin, dark lines oriented in the same direction the nail grows. They can be black, brown, red, or purple, and they typically measure only a millimeter or two in width. The nail bed sits directly beneath the hard nail plate, and the tiny blood vessels running through it are arranged lengthwise. When one of those vessels leaks, blood seeps into the surrounding tissue and sticks to the underside of the nail. Because the nail keeps growing, that streak of blood slowly migrates toward the free edge of the nail over the course of weeks, eventually reaching the tip where it can be scraped or trimmed away.
They show up more often on fingernails than toenails. A single splinter hemorrhage on one nail, especially on a hand you use heavily, is almost always the result of a bump, a jam, or repetitive low-grade pressure. Multiple hemorrhages across several nails, or hemorrhages that appear without any obvious injury, are the patterns that prompt clinicians to look deeper.
Trauma Is the Leading Cause
The most common reason anyone develops a splinter hemorrhage is mechanical injury to the nail. You do not need a dramatic blow to trigger one. Gardening, typing on a hard keyboard, playing a stringed instrument, doing manual labor, or wearing shoes that press on the big toenail can all produce enough shear force to rupture a tiny nail-bed capillary. People who work with their hands for a living often have splinter hemorrhages and never think twice about them.
A study of toenail splinter hemorrhages in Chinese adults found that the great toenail was involved in about 94% of cases, and roughly two-thirds of patients had hemorrhages on just one toenail, consistent with localized mechanical stress rather than a body-wide process.1PubMed Central. Clinical Observation and Follow-Up of Subungual Splinter Hemorrhages of Toenails in Chinese Adults That pattern makes sense: the big toe absorbs the most pressure in tight footwear, and a single affected nail points toward a local cause rather than a systemic disease.
In older adults, trauma remains the dominant explanation as well. A study of elderly hospital patients found splinter hemorrhages in about 16% of them, and the strongest association was with the use of walking aids rather than any specific medical disorder.2PubMed. Splinter haemorrhages in the elderly The researchers concluded that splinter hemorrhages were an unhelpful physical sign in that age group because trauma explained nearly all of them.
When Splinter Hemorrhages Point to Infective Endocarditis
The medical reason splinter hemorrhages get so much attention is their long association with infective endocarditis, a serious infection of the heart valves. When bacteria settle on a heart valve, they form clumps of infected material that can break off and travel through the bloodstream. Some of those tiny fragments lodge in the nail-bed capillaries, blocking flow and causing the characteristic streaks. The connection was first described about a century ago. Sir Thomas Horder, writing in 1920, noted “a minute petechia in the form of a vivid linear splash of red at the side of the bed of a fingernail” and called it a frequent early sign that more obvious symptoms would follow.3The American Journal of Cardiology. Extracardiac Manifestations of Infective Endocarditis and Their Historical Descriptions
The reality, though, is that splinter hemorrhages are not a very reliable screening tool for endocarditis. A study of patients who had already been referred for suspected endocarditis found that splinter hemorrhages had a sensitivity of only about 26% and a specificity of roughly 83%.4PubMed. Diagnostic accuracy of splinter haemorrhages in patients referred for suspected infective endocarditis In plain terms, roughly three out of four people who actually had endocarditis did not have splinter hemorrhages, and about one in six people without endocarditis had them anyway. They are a piece of the puzzle rather than a definitive signal. The same study estimated that formally including splinter hemorrhages as a minor diagnostic criterion would reclassify about 12% of initially rejected cases upward to “possible” endocarditis, so they do carry some diagnostic weight in the right clinical context.
The key distinction is context. A splinter hemorrhage in someone with unexplained fever, a new heart murmur, night sweats, or a history of intravenous drug use is a very different finding from one in a person who just spent the weekend installing tile. When endocarditis is genuinely on the table, the hemorrhages tend to appear on multiple nails and are often accompanied by other small-vessel signs like tiny red spots on the palms or soles.
Autoimmune and Vascular Diseases
Beyond infection, a number of autoimmune and inflammatory conditions can produce splinter hemorrhages. The mechanism is different from endocarditis: instead of infected clumps blocking vessels, the immune system itself attacks the walls of small blood vessels, leading to inflammation and leakage. Splinter hemorrhages have been reported in lupus, antiphospholipid syndrome, systemic sclerosis, Sjögren’s syndrome, and dermatomyositis.5Archives of General Internal Medicine. Splinter Hemorrhage as a Telltale of Active Extra-Cranial Giant Cell Arteritis A Case Report and Review of the Literature They also appear in various types of vasculitis, including granulomatosis with polyangiitis and polyarteritis nodosa.
In patients with antiphospholipid syndrome, a condition that makes the blood prone to abnormal clotting, splinter hemorrhages tend to show up alongside clotting events in arteries elsewhere in the body. In a report of five such patients, the hemorrhages appeared at the same time as arterial thrombosis in various sites, suggesting the nail-bed streaks were caused by tiny clots forming in the nail vessels rather than by vessel-wall inflammation.6PubMed. Multiple subungual splinter hemorrhages in the antiphospholipid syndrome: a report of five cases and review of the literature
What ties these conditions together is that the splinter hemorrhages rarely show up alone. A person with active lupus or vasculitis will usually have other symptoms: joint pain, skin rashes, fatigue, kidney problems, or general malaise. Finding unexplained splinter hemorrhages may occasionally be an early clue to one of these conditions, but it is a supporting detail rather than a standalone alarm.
Skin Diseases That Affect the Nails
Several common skin conditions involve the nails directly and can produce splinter hemorrhages as part of a broader nail problem. Psoriasis is one of the most frequent culprits. When psoriasis affects the nail bed, it can cause a whole cluster of changes including separation of the nail from the bed, discolored “oil-drop” patches, thickening of the skin beneath the nail, and splinter hemorrhages.7PubMed Central. Nail psoriasis – what a rheumatologist should know about If you already know you have psoriasis and your nails look rough, splinter hemorrhages are part of the package and not a reason to panic about your heart.
Lichen planus, another inflammatory skin condition, can also involve the nails. In a review of nail lichen planus cases, splinter hemorrhages showed up in about 14% of patients with nail-bed involvement.8PubMed Central. Clinical features and treatment outcomes of nail lichen planus: A retrospective study Fungal nail infections and a less common condition called Darier disease round out the dermatologic causes.9PubMed Central. Nitrofurantoin-induced splinter hemorrhages: A forgotten culprit In all these cases, the hemorrhages are a secondary feature of a disease that is already affecting nail structure, not a sign of something cardiovascular.
Medications That Can Trigger Them
Certain drugs cause splinter hemorrhages as a side effect, sometimes in a large proportion of people taking them. This is especially well documented with a class of cancer drugs called multikinase inhibitors, which are used to treat kidney cancer and certain other tumors. These drugs work partly by blocking the growth of new blood vessels, a mechanism that is useful against tumors but also interferes with the normal repair of tiny capillaries in the nail bed. Between roughly 40% and 65% of patients taking these drugs develop painless splinter hemorrhages, mostly in the fingernails.10Journal of Clinical Oncology. Cutaneous Side Effects of Multikinase Inhibitors Used in Renal Cell Cancer Researchers have even suggested that the appearance of these hemorrhages might serve as a visible indicator that the drug is effectively blocking blood vessel growth, though that idea is still being studied.
Other medications linked to splinter hemorrhages include certain antibiotics, sedatives, and drugs that target epidermal growth factor receptors.9PubMed Central. Nitrofurantoin-induced splinter hemorrhages: A forgotten culprit The antibiotic nitrofurantoin, commonly prescribed for urinary tract infections, has been identified as one such trigger. If you notice splinter hemorrhages appearing shortly after starting a new medication, that timing is worth mentioning to your doctor. Drug-induced hemorrhages typically resolve after the medication is stopped or changed.
Kidney Disease and Dialysis
People with chronic kidney failure, particularly those on hemodialysis, tend to develop a range of nail changes. Among the most commonly observed are “half-and-half nails” (where the lower half of the nail turns white and the upper half turns brown), disappearance of the lunula (the pale crescent at the nail base), and splinter hemorrhages.11PubMed. Nail disorders in patients with chronic renal failure undergoing hemodialysis The combination of uremia, altered clotting, and frequent vascular access makes these patients more vulnerable to tiny bleeds in the nail bed. For a dialysis patient, splinter hemorrhages are part of a recognizable pattern rather than a mysterious finding, and their treating team is generally already aware of it.
Splinter Hemorrhage Versus Nail Melanoma
One fear people have when they spot a dark streak under a nail is melanoma, the most dangerous form of skin cancer. Melanoma can grow beneath the nail, producing a dark longitudinal band that might superficially resemble a splinter hemorrhage. The good news is that the two look quite different under close inspection. Subungual melanoma typically produces irregular bands of pigment, sometimes with a triangular shape, and often causes the Hutchinson sign, where dark pigment spreads from under the nail onto the surrounding skin fold. In a study comparing confirmed melanomas with subungual hemorrhages using dermatoscopy, the Hutchinson sign, irregular bands, and ulceration were found in a high proportion of melanoma cases but were absent in hemorrhages.12PubMed. Dermoscopy of subungual haemorrhage: its usefulness in differential diagnosis from nail-unit melanoma
A practical test you can do at home is to watch whether the dark mark grows out with the nail. Splinter hemorrhages are fixed to the nail plate and move toward the tip as the nail grows over several weeks. Melanoma pigment originates from cells in the nail matrix and stays in place or extends, rather than migrating distally. If a dark line under your nail does not budge over a month or two, or if it widens, darkens, or produces discoloration on the skin next to the nail, see a dermatologist. If it clearly tracks forward with nail growth, it is almost certainly blood, not pigment.
Where on the Nail the Streak Appears
Clinicians pay attention to whether a splinter hemorrhage sits near the tip of the nail (the distal end) or near the base (the proximal end). Distal hemorrhages, the ones closer to the free edge of your nail, are overwhelmingly associated with trauma and minor mechanical insults. This makes intuitive sense: the part of the nail closest to the tip absorbs the most direct impact from day-to-day use of your hands.
Proximal hemorrhages, closer to the cuticle, raise more suspicion for systemic causes. Emboli from infected heart valves and clots from hypercoagulable states tend to lodge in the deeper part of the nail bed, where the vessels are more protected from external force. A splinter hemorrhage that appears near the cuticle without any history of trauma is a more meaningful clinical finding than one sitting near the tip. If you are trying to self-assess, this is one of the most useful pieces of information: location along the length of the nail matters.
When You Should Actually Worry
One case report summarized the clinical approach well: finding splinter hemorrhages should prompt a detailed evaluation, but if no evidence of endocarditis or another systemic condition turns up on examination and lab tests, the hemorrhages typically carry no clinical significance and resolve on their own without treatment.13PubMed Central. Nitrofurantoin-induced splinter hemorrhages: A forgotten culprit – Section: Discussion The hemorrhages themselves are harmless. What matters is whether they are a surface marker of something going on inside.
There are several patterns that warrant a visit to your doctor:
- Multiple nails affected: Hemorrhages appearing across several fingers or toes simultaneously, without a clear traumatic explanation, suggest a systemic cause.
- Accompanying symptoms: Fever, unexplained weight loss, fatigue, joint pain, new skin rashes, or a new heart murmur alongside splinter hemorrhages shifts the picture significantly.
- Proximal location: Streaks near the cuticle rather than near the nail tip deserve more attention.
- Recent medication changes: New hemorrhages appearing shortly after starting a drug, especially antibiotics or cancer therapies, should be reported to the prescribing physician.
- Risk factors for endocarditis: If you have a prosthetic heart valve, a history of intravenous drug use, recent dental procedures, or a prior episode of endocarditis, any new splinter hemorrhage is worth flagging.
A single hemorrhage on a fingernail that you use every day, with no fever and no other symptoms, is overwhelmingly likely to be from bumping into something. You can safely watch it migrate toward the nail tip over the coming weeks and trim it away.
Why They Seem More Common Than They Used To Be
If it feels like splinter hemorrhages get mentioned more often now in medical settings, there is a practical reason. Improved cancer therapies, particularly drugs that block blood vessel growth, produce splinter hemorrhages at remarkably high rates. As these drugs have become standard treatment for kidney cancer, thyroid cancer, and other malignancies, oncologists see them routinely.10Journal of Clinical Oncology. Cutaneous Side Effects of Multikinase Inhibitors Used in Renal Cell Cancer In that context, the hemorrhages are considered a pharmacological side effect rather than a diagnostic clue, and they often appear alongside other nail and skin changes caused by the same drug class.
Meanwhile, the growing awareness of nail examination as a window into overall health has led more clinicians to look at nails carefully during routine physicals. For most people, this increased attention means discovering harmless trauma-related streaks that would have gone unnoticed a generation ago. It does not mean the condition itself is becoming more common in the general population.