In people without a detectable thrombophilia, the cumulative risk of developing thrombosis by the age of 60 is about 12%. About 60% of people who are deficient in antithrombin will have experienced thrombosis at least once by age 60, as will about 50% of people with protein C deficiency and about a third of those with protein S deficiency. People with activated protein C resistance (usually resulting from factor V Leiden), in contrast, have a slightly raised absolute risk of thrombosis, with 15% having had at least one thrombotic event by the age of sixty. In general, men are more likely than women to experience repeated episodes of venous thrombosis.

People with factor V Leiden are at a relatively low risk of thrombosis, but may develop thrombosis in the presence of an additional risk factor, such as immobilization. Most people with the prothrombin mutation (G20210A) never develop thrombosis.

A number of acquired conditions augment the risk of thrombosis. A prominent example is antiphospholipid syndrome, which is caused by antibodies against constituents of the cell membrane, particularly lupus anticoagulant (first found in people with the disease systemic lupus erythematosus but often detected in people without the disease), anti-cardiolipin antibodies, and anti-β-glycoprotein 1 antibodies; it is therefore regarded as an autoimmune disease. In some cases antiphospholipid syndrome can cause arterial as well as venous thrombosis. It is also more strongly associated with miscarriage, and can cause a number of other symptoms (such as livedo reticularis of the skin and migraine).

Heparin-induced thrombocytopenia (HIT) is due to an immune system reaction against the anticoagulant drug heparin (or its derivatives). Though it is named for associated low platelet counts, HIT is strongly associated with risk of venous and arterial thrombosis. Paroxysmal nocturnal hemoglobinuria (PNH) is a rare condition resulting from acquired alterations in the "PIGA" gene, which plays a role in the protection of blood cells from the complement system. PNH increases the risk of venous thrombosis but is also associated with hemolytic anemia (anemia resulting from destruction of red blood cells). Both HIT and PNH require particular treatment.

Hematologic conditions associated with sluggish blood flow can increase risk for thrombosis. For example, sickle-cell disease (caused by mutations of hemoglobin) is regarded as a mild prothrombotic state induced by impaired flow. Similarly, myeloproliferative disorders, in which the bone marrow produces too many blood cells, predispose to thrombosis, particularly in polycythemia vera (excess red blood cells) and essential thrombocytosis (excess platelets). Again, these conditions usually warrant specific treatment when identified.

Cancer, particularly when metastatic (spread to other places in the body), is a recognised risk factor for thrombosis. A number of mechanisms have been proposed, such as activation of the coagulation system by cancer cells or secretion of procoagulant substances. Furthermore, particular cancer treatments (such as the use of central venous catheters for chemotherapy) may increase the risk of thrombosis further.

Nephrotic syndrome, in which protein from the bloodstream is released into the urine due to kidney diseases, can predispose to thrombosis; this is particularly the case in more severe cases (as indicated by blood levels of albumin below 25 g/l) and if the syndrome is caused by the condition membranous nephropathy. Inflammatory bowel disease (ulcerative colitis and Crohn's disease) predispose to thrombosis, particularly when the disease is active. Various mechanisms have been proposed.

Pregnancy is associated with an increased risk of thrombosis. This probably results from a physiological hypercoagulability in pregnancy that protects against postpartum hemorrhage.

The female hormone estrogen, when used in the combined oral contraceptive pill and in perimenopausal hormone replacement therapy, has been associated with a two- to sixfold increased risk of venous thrombosis. The risk depends on the type of hormones used, the dose of estrogen, and the presence of other thrombophilic risk factors. Various mechanisms, such as deficiency of protein S and tissue factor pathway inhibitor, are said to be responsible.

Obesity has long been regarded as a risk factor for venous thrombosis. It more than doubles the risk in numerous studies, particularly in combination with the use of oral contraceptives or in the period after surgery. Various coagulation abnormalities have been described in the obese. Plasminogen activator inhibitor-1, an inhibitor of fibrinolysis, is present in higher levels in people with obesity. Obese people also have larger numbers of circulating microvesicles (fragments of damaged cells) that bear tissue factor. Platelet aggregation may be increased, and there are higher levels of coagulation proteins such as von Willebrand factor, fibrinogen, factor VII and factor VIII. Obesity also increases the risk of recurrence after an initial episode of thrombosis.

Hypercoagulability in pregnancy, particularly due to inheritable thrombophilia, can lead to placental vascular thrombosis. This can in turn lead to complications like early-onset hypertensive disorders of pregnancy, pre-eclampsia and small for gestational age infants (SGA). Among other causes of hypercoagulability, Antiphospholipid syndrome has been associated with adverse pregnancy outcomes including recurrent miscarriage. Deep vein thrombosis has an incidence of one in 1,000 to 2,000 pregnancies in the United States, and is the second most common cause of maternal death in developed countries after bleeding.

The risk of VTE is increased in pregnancy by about five times because of a more hypercoagulable state, a likely adaptation against fatal postpartum hemorrhage. Additionally, pregnant women with genetic risk factors are subject to a roughly three to 30 times increased risk for VTE. Preventative treatments for pregnancy-related VTE in hypercoagulable women were suggested by the ACCP. Homozygous carriers of factor V Leiden or prothrombin G20210A with a family history of VTE were suggested for antepartum LMWH and either LMWH or a vitamin K antagonist (VKA) for the six weeks following childbirth. Those with another thrombophilia and a family history but no previous VTE were suggested for watchful waiting during pregnancy and LMWH or—for those without protein C or S deficiency—a VKA. Homozygous carriers of factor V Leiden or prothrombin G20210A with no personal or family history of VTE were suggested for watchful waiting during pregnancy and LMWH or a VKA for six weeks after childbirth. Those with another thrombophilia but no family or personal history of VTE were suggested for watchful waiting only. Warfarin, a common VKA, can cause harm to the fetus and is not used for VTE prevention during pregnancy.

The exact number of cases of HIT in the general population is unknown. What is known is that women receiving heparin after a recent surgical procedure, particularly cardiothoracic surgery, have a higher risk, while the risk is very low in women just before and after giving birth. Some studies have shown that HIT is less common in those receiving low molecular weight heparin.

Warfarin necrosis usually occurs three to five days after drug therapy is begun, and a high initial dose increases the risk of its development. Heparin-induced necrosis can develop both at sites of local injection and - when infused intravenously - in a widespread pattern.

In warfarin's initial stages of action, inhibition of protein C and Factor VII is stronger than inhibition of the other vitamin K-dependent coagulation factors II, IX and X. This results from the fact that these proteins have different half-lives: 1.5 to six hours for factor VII and eight hours for protein C, versus one day for factor IX, two days for factor X and two to five days for factor II. The larger the initial dose of vitamin K-antagonist, the more pronounced these differences are. This coagulation factor imbalance leads to paradoxical activation of coagulation, resulting in a hypercoagulable state and thrombosis. The blood clots interrupt the blood supply to the skin, causing necrosis. Protein C is an innate anticoagulant, and as warfarin further decreases protein C levels, it can lead to massive thrombosis with necrosis and gangrene of limbs.

Notably, the prothrombin time (or international normalized ratio, INR) used to test the effect of warfarin is highly dependent on factor VII, which explains why patients can have a therapeutic INR (indicating good anticoagulant effect) but still be in a hypercoagulable state.

In one third of cases, warfarin necrosis occurs in patients with an underlying, innate and previously unknown deficiency of protein C. The condition is related to purpura fulminans, a complication in infants with sepsis (blood stream infection) which also involves skin necrosis. These infants often have protein C deficiency as well. There have also been cases in patients with other deficiency, including protein S deficiency, activated protein C resistance (Factor V Leiden) and antithrombin III deficiency.

Although the above theory is the most commonly accepted theory, others believe that it is a hypersensitivity reaction or a direct toxic effect.

Prognosis varies depending on the underlying disorder, and the extent of the intravascular thrombosis (clotting). The prognosis for those with DIC, regardless of cause, is often grim: Between 20% and 50% of patients will die. DIC with sepsis (infection) has a significantly higher rate of death than DIC associated with trauma.

Purpura fulminans is rare and most commonly occurs in babies and small children but can also be a rare manifestation in adults when it is associated with severe infections. For example, Meningococcal septicaemia is complicated by purpura fulminans in 10–20% of cases among children. Purpura fulminans associated with congenital (inherited) protein C deficiency occurs in 1:500,000–1,000,000 live births.

It is known that diabetes causes changes to factors associated with coagulation and clotting, however not much is known of the risk of thromboembolism, or clots, in diabetic patients. There are some studies that show that diabetes increases the risk of thromboembolism; other studies show that diabetes does not increase the risk of thromboembolism. A study conducted in the Umea University Hospital, in Sweden, observed patients that were hospitalized due to an thromboembolism from 1997 to 1999. The researchers had access to patient information including age, sex, vein thromboembolism diagnosis, diagnostic methods, diabetes type and medical history. This study concluded that there is, in fact, an increased risk of thromboembolism development in diabetic patients, possibly due to factors associated with diabetes or diabetes itself. Diabetic patients are twice as likely to develop a thromboembolism than are non-diabetic patient. The exact mechanism of how diabetes increases the risk of clot formation remains unclear and could possibly be a future direction for study.

From previous studies, it is known that long distance air travel is associated with high risk of venous thrombosis. Long periods of inactivity in a limited amount of space may be a reason for the increased risk of blood clot formation. In addition, bent knees compresses the vein behind the knee (the popliteal vein) and the low humidity, low oxygen, high cabin pressure and consumption of alcohol concentrate the blood. A recent study, published in the British Journal of Haematology in 2014, determined which groups of people, are most at risk for developing a clot during or after a long flight. The study focused on 8755 frequent flying employees from international companies and organizations. It found that travelers who have recently undergone a surgical procedure or who have a malignant disease such as cancer or who are pregnant are most at risk. Preventative measures before flying may be taken in these at-risk groups as a solution.

Patients who have undergone kidney transplant have a high risk of developing RVT (about 0.4% to 6%). RVT is known to account for a large proportion of transplanted kidney failures due to technical problems (damage to the renal vein), clotting disorders, diabetes, consumption of ciclosporin or an unknown problem. Patients who have undergone a kidney transplant are commonly prescribed ciclosporin, an immunosuppressant drug which is known to reduce renal blood flow, increase platelet aggregation in the blood and cause damage to the endothelial tissue of the veins. In a clinical study conducted by the Nuffield Department of Surgery at the Oxford Transplant Centre, UK, transplant patients were given low doses of aspirin, which has a some anti-platelet activity. There is risk of bleeding in transplant patients when using anticoagulants like warfarin and herapin. Low dosage of aspirin was used as an alternative. The study concluded that a routine low-dose of aspirin in kidney transplant patients who are also taking ciclosporin significantly reduces the risk of RVT development.

The three factors of Virchow's triad—venous stasis, hypercoagulability, and changes in the endothelial blood vessel lining (such as physical damage or endothelial activation)—contribute to DVT and are used to explain its formation. Other related causes include activation of immune system components, the state of microparticles in the blood, the concentration of oxygen, and possible platelet activation. Various risk factors contribute to DVT, though many at high risk never develop it.

Acquired risk factors include the strong risk factor of older age, which alters blood composition to favor clotting. Other important acquired risk factors include major surgery and trauma, both of which may increase the risk because of tissue factor from outside the vascular system entering the blood. In orthopedic surgery, venous stasis may be temporarily provoked by a cessation of blood flow as part of the procedure. Cancer can grow in and around veins, causing venous stasis, and can also stimulate increased levels of tissue factor. Pregnancy causes blood to favor clotting, and in the postpartum, placental tearing releases substances that favor clotting. Oral contraceptives and hormonal replacement therapy increase the risk through a variety of mechanisms, including altered blood coagulation protein levels and reduced fibrinolysis.

The disease term venous thromboembolism (VTE) includes the development of either DVT or pulmonary embolism (PE). Genetic factors that increase the risk of VTE include deficiencies of three proteins that normally prevent blood from clotting—protein C, protein S, and antithrombin—in addition to non-O blood type and mutations in the factor V and prothrombin genes. Deficiencies in antithrombin, protein C, and protein S are rare but strong, or moderately strong, risk factors. These three thrombophilia increase the risk of VTE by about 10 times. Factor V Leiden, which makes factor V resistant to inactivation by activated protein C, and the genetic variant prothrombin G20210A, which causes increased prothrombin levels, are predominantly expressed in Caucasians. They moderately increase risk for VTE, by three to eight times for factor V Leiden and two to three times for prothrombin G20210A. Having a non-O blood type roughly doubles VTE risk. Non-O blood type is common in all races, making it an important risk factor. Individuals without O blood type have higher blood levels of von Willebrand factor and factor VIII than those with O blood type, increasing the likelihood of clotting.

Some risk factors influence the location of DVT within the body. In isolated distal DVT, the profile of risk factors appears distinct from proximal DVT. Transient factors, such as surgery and immobilization, appear to dominate, whereas thrombophilias and age do not seem to increase risk. In upper-extremity DVT, the most important risk factor is having a central venous catheter, and thoracic outlet syndrome also increases risk.

Unfractionated heparin, low molecular weight heparin, warfarin (not to be used during pregnancy) and aspirin remain the basis of antithrombotic treatment and prophylaxis both before and during pregnancy.

While the consensus among physicians is the safety of the mother supersedes the safety of the developing fetus, changes in the anticoagulation regimen during pregnancy can be performed to minimize the risks to the developing fetus while maintaining therapeutic levels of anticoagulants in the mother.

The main issue with anticoagulation in pregnancy is that warfarin, the most commonly used anticoagulant in chronic administration, is known to have teratogenic effects on the fetus if administered in early pregnancy. Still, there seems to be no teratogenic effect of warfarin before six weeks of gestation. However, unfractionated heparin and low molecular weight heparin do not cross the placenta.

Thrombosis prevention is initiated with assessing the risk for its development. Some people have a higher risk of developing thrombosis and its possible development into thromboembolism. Some of these risk factors are related to inflammation. "Virchow's triad" has been suggested to describe the three factors necessary for the formation of thrombosis: stasis of blood, vessel wall injury, and altered blood coagulation. Some risk factors predispose for venous thrombosis while others increase the risk of arterial thrombosis.

DIC is observed in approximately 1% of academic hospital admissions. DIC occurs at higher rates in people with bacterial sepsis (83%), severe trauma (31%), and cancer (6.8%).

Due to the rarity of Purpura fulminans and its occurrence in vulnerable patient groups like children research on the condition is very limited and evidence based knowledge is scarce. Currently, there is only one Purpura fulminans related clinical research project, http://www.sapfire-registry.org/, which is registered with clinicaltrials.gov.

Warfarin-induced skin necrosis (or, more generally, Anticoagulant-induced skin necrosis) is a condition in which skin and subcutaneous tissue necrosis (tissue death) occurs due to acquired protein C deficiency following treatment with anti-vitamin K anticoagulants (4-hydroxycoumarins, such as warfarin).

Warfarin necrosis is a rare but severe complication of treatment with warfarin or related anticoagulants. The typical patient appears to be an obese, middle aged woman (median age 54 years, male to female ratio 1:3). This drug eruption usually occurs between the third and tenth days of therapy with warfarin derivatives. The first symptoms are pain and redness in the affected area. As they progress, lesions develop a sharp border and become petechial, then hard and purpuric. They may then resolve or progress to form large, irregular, bloody bullae with eventual necrosis and slow-healing eschar formation. Favored sites are breasts, thighs, buttocks and penis, all areas with subcutaneous fat. In rare cases, the fascia and muscle are involved.

Development of the syndrome is associated with the use of large loading doses at the start of treatment.

Risk factors for developing antiphospholipid syndrome include:

- Primary APS

- genetic marker HLA-DR7

- Secondary APS

- SLE or other autoimmune disorders

- Genetic markers: HLA-B8, HLA-DR2, HLA-DR3

- Race: Blacks, Hispanics, Asians, and Native Americans

There is an additional elevated risk of adrenal gland bleeds leading to Waterhouse–Friderichsen syndrome (Neisseria meningitidis caused primary adrenal insufficiency). This will require adrenal steroid replacement treatment for life.

The main causes of thrombosis are given in Virchow's triad which lists thrombophilia, endothelial cell injury, and disturbed blood flow.

Heparin-induced thrombocytopenia (HIT) is the development of thrombocytopenia (a low platelet count), due to the administration of various forms of heparin, an anticoagulant. HIT predisposes to thrombosis (the abnormal formation of blood clots inside a blood vessel) because platelets release microparticles that activate thrombin, thereby leading to thrombosis. When thrombosis is identified the condition is called heparin-induced thrombocytopenia and thrombosis (HITT). HIT is caused by the formation of abnormal antibodies that activate platelets. If someone receiving heparin develops new or worsening thrombosis, or if the platelet count falls, HIT can be confirmed with specific blood tests.

The treatment of HIT requires stopping heparin treatment, and both protection from thrombosis and choice of an agent that will not reduce the platelet count any further. Several alternatives are available for this purpose and mainly used are danaparoid, fondaparinux, argatroban and bivalirudin.

While heparin was discovered in the 1930s, HIT was not reported until the 1960s.

The overall absolute risk of venous thrombosis per 100,000 woman years in current use of combined oral contraceptives is approximately 60, compared to 30 in non-users. The risk of thromboembolism varies with different types of birth control pills; Compared with combined oral contraceptives containing levonorgestrel (LNG), and with the same dose of estrogen and duration of use, the rate ratio of deep venous thrombosis for combined oral contraceptives with norethisterone is 0.98, with norgestimate 1.19, with desogestrel (DSG) 1.82, with gestodene 1.86, with drospirenone (DRSP) 1.64, and with cyproterone acetate 1.88. Venous thromboembolism occurs in 100–200 per 100,000 pregnant women every year.

Regarding family history, age has substantial effect modification. For individuals with two or more affected siblings, the highest incidence rates is found among those ≥70 years of age (390 per 100,000 in male and 370 per 100,000 in female individuals), whereas the highest incidence ratios compared to those without affected siblings occurred at much younger ages (ratio of 4.3 among male individuals 20 to 29 years of age and 5.5 among female individuals 10 to 19 years of age).

Some malignancies, especially gliomas (25%), as well as adenocarcinomas of the pancreas and lung, are associated with hypercoagulability (the tendency to form blood clots) for reasons that are incompletely understood, but may be related to factors secreted by the tumors, in particular a circulating pool of cell-derived tissue factor-containing microvesicles. Some adenocarcinomas secrete mucin that can interact with selectin found on platelets, thereby causing small clots to form.

In patients with malignancy-associated hypercoagulable states, the blood may spontaneously form clots in the portal vessels, the deep veins of the extremities (such as the leg), or the superficial veins anywhere on the body. These clots present as visibly swollen blood vessels (thrombophlebitis), especially the veins, or as intermittent pain in the affected areas.

Evidence supports the use of heparin in people following surgery who have a high risk of thrombosis to reduce the risk of DVTs; however, the effect on PEs or overall mortality is not known. In hospitalized non-surgical patients, mortality decreased but not statistically significant. It does not appear however to decrease the rate of symptomatic DVTs. Using both heparin and compression stockings appears better than either one alone in reducing the rate of DVT.

In hospitalized people who have had a stroke and not had surgery, mechanical measures (compression stockings) resulted in skin damage and no clinical improvement. Data on the effectiveness of compression stockings among hospitalized non-surgical patients without stroke is scarce.

The American College of Physicians (ACP) gave three strong recommendations with moderate quality evidence on VTE prevention in non-surgical patients: that hospitalized patients be assessed for their risk of thromboembolism and bleeding before prophylaxis (prevention); that heparin or a related drug is used if potential benefits are thought to outweigh potential harms; and that graduated compression stockings not be used. As an ACP policy implication, the guideline stated a lack of support for any performance measures that incentivize physicians to apply universal prophylaxis without regard to the risks. Goldhaber recommends that people should be assessed at their hospital discharge for persistent high-risk of venous thrombosis, and that people who adopt a heart-healthy lifestyle might lower their risk of venous thrombosis.

In those with cancer who are still walking about yet receiving chemotherapy, LMWH decreases the risk of VTE. Due to potential concerns of bleeding its routine use is not recommended. For people who are having surgery for cancer, it is recommended that they receive anticoagulation therapy (preferably LMWH) in order to prevent a VTE. LMWH is recommended for at least 7–10 days following cancer surgery, and for one month following surgery for people who have a high risk of VTEs.

In adults who have had their lower leg casted or placed in a brace for more than a week, LMWH decreased the risk of VTEs. LMWH is recommended for adults not in hospital with an above-knee cast and a below-knee cast, and is safe for this indication.

Following the completion of warfarin in those with prior VTE, long term aspirin is beneficial.

Complications related to extravasation are possible with any medication.

Since Vesicants are blistering agents, extravasation may lead to irreversible tissue injury.

Extravasation is particularly serious during Chemotherapy, since chemotherapy medications are highly toxic.

In recent years, healthcare professionals are becoming more aware of this problem.

The presence of antiphospholipid antibodies (aPL) in the absence of blood clots or pregnancy-related complications does not indicate APS (see below for the diagnosis of APS).

Antiphospholipid syndrome can cause arterial or venous blood clots, in any organ system, or pregnancy-related complications. In APS patients, the most common venous event is deep vein thrombosis of the lower extremities, and the most common arterial event is stroke. In pregnant women affected by APS, there is an increased risk of recurrent miscarriage, intrauterine growth restriction, and preterm birth. A frequent cause of such complications is placental infarctions.

In some cases, APS seems to be the leading cause of mental and/or development retardation in the newborn, due to an aPL-induced inhibition of trophoblast differentiation. The antiphospholipid syndrome responsible for most of the miscarriages in later trimesters seen in concomitant systemic lupus erythematosus and pregnancy.

Other common findings, although not part of the APS classification criteria, are low platelet count, heart valve disease, and livedo reticularis. There are also associations between antiphospholipid antibodies and headaches, migraines, and oscillopsia. Some studies have shown the presence of antiphospholipid antibodies in the blood and spinal fluid of patients with psychological symptoms.

Very few patients with primary APS go on to develop SLE.

While the prognosis of cryofibrinoginemic disease varies greatly depending on its severity as well as the severity of its associated disorders, satisfactory clinical outcomes are reported in 50-80% of patients with primary or secondary disease treated with corticosteroid and/or immunosuppressive regimens. However, relapses occur within the first 6 months after stopping or decreasing therapy in 40-76% of cases. Sepsis resulting from infection of necrotic tissue is the most common threat to life in primary disease whereas the associated disorder is a critical determinant of prognosis in secondary disease.

In 2004 the first adequately large scale study on the natural history and long-term prognosis of this condition was reported; this showed that at 16 months follow-up 57.1% of patients had full recovery, 29.5%/2.9%/2.2% had respectively minor/moderate/severe symptoms or impairments, and 8.3% had died. Severe impairment or death were more likely in those aged over 37 years, male, affected by coma, mental status disorder, intracerebral hemorrhage, thrombosis of the deep cerebral venous system, central nervous system infection and cancer. A subsequent systematic review of nineteen studies in 2006 showed that mortality is about 5.6% during hospitalisation and 9.4% in total, while of the survivors 88% make a total or near-total recovery. After several months, two thirds of the cases has resolution ("recanalisation") of the clot. The rate of recurrence was low (2.8%).

In children with CVST the risk of death is high. Poor outcome is more likely if a child with CVST develops seizures or has evidence of venous infarction on imaging.