Blood clots become significantly more common with age due to a combination of biological changes, chronic health conditions, reduced mobility, and dehydration. In the general population, venous blood clots occur at a rate of roughly 1 to 2 per 1,000 people per year. For adults over 80, that rate climbs to about 8 per 1,000, and in elderly patients in long-term care settings, the incidence reaches nearly 3 per 100 person-years, making it one of the most pressing cardiovascular risks in later life.
How Aging Changes the Blood Itself
Even in perfectly healthy older adults, the blood becomes more prone to clotting with each passing decade. The body gradually produces higher levels of several proteins involved in coagulation. Fibrinogen, one of the key building blocks of a clot, increases by about 10 mg/dL per decade. A person in their 20s typically has fibrinogen levels around 250 mg/dL, while someone in their early 60s averages closer to 300 mg/dL. Factor VIII, another clotting protein, rises steadily and can exceed 200 units/dL in healthy people over 60, roughly double the level considered normal in younger adults.
At the same time, the body’s ability to break down clots (a process called fibrinolysis) becomes less efficient. This creates a lopsided equation: more clot-building material circulating in the blood and fewer mechanisms to dissolve clots that do form. Rising levels of the inflammatory signal interleukin-6 drive some of these changes, linking the low-grade chronic inflammation common in aging directly to a higher clotting tendency.
Blood Vessel Walls Lose Protective Function
Healthy blood vessels are lined with a thin layer of cells called the endothelium. This lining actively prevents clots by repelling platelets and releasing substances that keep blood flowing smoothly. With age, these endothelial cells accumulate damage and enter a state of senescence, meaning they stop functioning properly but remain in place. Senescent endothelial cells lose their ability to prevent platelet clumping and no longer produce adequate amounts of clot-dissolving factors.
The body’s repair system for damaged vessel walls also weakens. In younger people, progenitor cells can migrate to injured spots and restore the endothelial lining. In older adults, this repair capacity is largely exhausted. The result is that small injuries to blood vessel walls, which would be quickly patched in a younger person, can become sites where clots begin to form. This is especially relevant in arteries stiffened by years of high blood pressure or cholesterol buildup.
Atrial Fibrillation and Stroke Risk
Atrial fibrillation (AFib), an irregular heart rhythm that becomes increasingly common after age 65, is one of the most dangerous causes of blood clots in older adults. When the heart’s upper chambers quiver instead of contracting fully, blood pools in a small pouch called the left atrial appendage. That stagnant blood can form clots that travel to the brain and cause a stroke.
The numbers are striking. In people aged 50 to 59, AFib accounts for about 1.5% of all strokes. By ages 80 to 89, that figure jumps to 23.5%, meaning nearly one in four strokes in that age group is caused by AFib. Many older adults with AFib don’t feel obvious palpitations, so the condition can go undetected for years while clot risk quietly builds.
Cancer and Blood Clot Risk
Cancer at any age raises the risk of blood clots, but since most cancers are diagnosed in people over 65, this connection hits older adults hardest. Tumors release substances that activate the clotting system, including cytokines and other molecular signals that push the blood toward a hypercoagulable state. Certain cancers carry especially high risk, including cancers of the kidney, ovary, pancreas, and blood cancers like lymphoma and myeloma.
What makes this particularly tricky is that even precancerous conditions can trigger clotting. A common precursor to blood cancer called MGUS (monoclonal gammopathy of undetermined significance) has been linked to elevated rates of both venous and arterial clots that persist for up to ten years after diagnosis. In some cases, a blood clot is the first sign that leads doctors to discover an underlying malignancy.
Immobility and Bed Rest
Reduced physical activity is one of the most straightforward triggers for blood clots in older adults. When the legs stay still for extended periods, blood flow through the deep veins slows dramatically. This stagnation, known as venous stasis, is one of the three classic conditions that promote clot formation (alongside blood vessel injury and hypercoagulability).
The physiological response to immobility kicks in faster than many people realize. Within the first 24 hours of bed rest, the body’s clot-dissolving activity drops measurably. Blood viscosity increases. Patients confined to bed for more than a week show significantly higher rates of clot formation in autopsy studies. For older adults recovering from illness, injury, or surgery, even a few days of limited mobility can be enough to tip the balance. Long periods of sitting, whether during travel, in a wheelchair, or simply due to frailty, carry a similar though less acute risk.
Surgery on Hips and Knees
Orthopedic surgery is a particularly high-risk scenario. In one study of patients aged 60 and older who underwent surgery for a hip fracture, 42.8% developed a deep vein thrombosis (DVT) when no preventive blood thinners were used. Broader estimates for hip and knee replacement surgeries without preventive treatment range from 17% to 58%.
The risk is so well established that blood clot prevention, usually with medication and compression devices, is now standard after any major joint surgery. Still, the combination of tissue damage from the procedure itself, general anesthesia, and days of limited movement afterward creates a perfect storm for clot formation in older surgical patients.
Dehydration and Blood Viscosity
Older adults are particularly vulnerable to dehydration, and even mild fluid deficits make the blood thicker and more prone to clotting. The kidneys become less efficient at concentrating urine with age, and the sense of thirst often diminishes. Many older people also limit fluids to avoid frequent trips to the bathroom, especially at night.
Thicker blood doesn’t just flow more slowly. It also increases platelet aggregation and raises levels of clot-promoting factors. In younger people, the body compensates for temporary changes in blood viscosity fairly effectively. In older adults, these homeostatic mechanisms are less responsive, meaning a night of poor hydration can have a meaningful effect on clot risk. This is one reason strokes that occur during sleep (called wake-up strokes) are more common in older, dehydrated individuals.
Why Symptoms Are Easy to Miss
One of the most dangerous aspects of blood clots in older adults is how easily they go unrecognized. A pulmonary embolism, which is a clot that travels to the lungs, presents with shortness of breath in 59% to 91% of elderly patients, rapid breathing in 46% to 74%, and chest pain in 26% to 59%. The problem is that every one of these symptoms overlaps with conditions already common in older people: heart failure, pneumonia, COPD, or simple deconditioning.
Compared to younger patients, older adults with a pulmonary embolism are less likely to report chest pain and more likely to experience a rapid heart rate or fainting. Syncope (a sudden loss of consciousness) can be the only obvious sign. Because these presentations are so nonspecific, pulmonary embolism is frequently missed or diagnosed late in elderly patients. Any unexplained shortness of breath, sudden rapid heartbeat, or fainting episode in an older adult with known risk factors warrants urgent evaluation.
Deep vein thrombosis in the legs can also present subtly. Swelling in one leg, warmth, or a dull ache may be attributed to arthritis, fluid retention, or poor circulation rather than a clot. In frail or immobile patients who can’t easily describe their symptoms, a DVT may go unnoticed until it breaks loose and reaches the lungs.