Clotting time Health Dictionary

The process of blood solidification. Clotting is important in stemming bleeding from damaged blood vessels. However, unwanted blood clotting can occur inside major blood vessels and cause a myocardial infarction (heart attack) or stroke (see thrombosis).

When a blood vessel is damaged, it constricts immediately to reduce blood flow to the area. The damage sets off a series of chemical reactions that lead to the formation of a clot to seal the injury. First, platelets around the injury site are activated, becoming sticky and adhering to the blood-vessel wall. Then, the activated platelets release chemicals, which, in turn, activate blood clotting factors. These factors, together with vitamin K, act on fibrinogen and convert it to fibrin. Strands of fibrin form a meshwork, which traps red blood cells to form a clot.

There are several anticlotting mechanisms to prevent the formation of unwanted clots. These include prostacyclin (a prostaglandin), which prevents platelet aggregation, and plasmin, which breaks down fibrin (see fibrinolysis). Blood flow washes away active coagulation factors; and the liver deactivates excess coagulation factors.

Defects in blood clotting may result in bleeding disorders.

Excessive clotting (thrombosis) may be due to an inherited increase or defect in a coagulation factor (see factor V), the use of oral contraceptives, a decrease in the level of enzymes that inhibit coagulation, or sluggish blood flow through a particular area.

Treatment is usually with anticoagulant drugs such as heparin or warfarin.... blood clotting

The period during which the biting cycle of a given mosquito species when the largest number of females take blood meals.... peak biting time

Tests to screen for and diagnose bleeding disorders, usually resulting from deficiencies or abnormalities of blood coagulation factors or of platelets (see blood clotting). Tests are also used to monitor treatment with anticoagulant drugs.... blood-clotting tests

a quickly performed test to assess the adequacy of circulation in an individual with poor cardiac output. An area of skin is pressed firmly by (say) a fingertip until it loses its colour; the number of seconds for the area to return to its original colour indicates capillary refill time. Normal capillary refill takes around 2 seconds. Slow capillary refill may occur globally in an individual with poor circulation or in a small area (e.g. a toe) in which local circulation is compromised (due, for example, to peripheral vascular disease). This test may not be very useful in people with dark skin.... capillary refill time

see coagulation factors.... clotting factors

(clotting time) the time taken for blood or blood plasma to coagulate (see blood coagulation). When measured under controlled conditions and using appropriate techniques, coagulation times may be used to test the function of the various stages of the blood coagulation process.... coagulation time

the time in minutes between a patient with S–T elevation *myocardial infarction reaching the hospital door and inflation of a balloon or other interventional device in the occluded coronary artery. It is a key indicator of the timeliness of an emergency *percutaneous coronary intervention service.... door to balloon time

(PT) the time taken for blood clotting to occur in a sample of blood to which calcium and thromboplastin have been added. A prolonged PT (compared with a control sample) indicates a deficiency of *coagulation factors, which – with calcium and thromboplastin – are required for the conversion of prothrombin to thrombin to occur in the final stages of blood coagulation. Measurement of PT is used to control anticoagulant therapy (e.g. with warfarin). See INR.... prothrombin time

the rapid acquisition and manipulation of ultrasound information from a scanning probe by electronic circuits to enable images to be produced on TV screens almost instantaneously. The operator can place the scanning probe accurately on the region of interest in order to observe its structure and appreciate moving structures within it (see Doppler ultrasound). Using similar techniques, the instantaneous display of other imaging modalities, such as *computerized tomography scanning and *magnetic resonance imaging, can now be achieved. Real-time imaging is useful in guiding *interventional radiology procedures, for example, allowing a needle to be guided accurately as it is passed into the body. It is also useful for observing dynamic physiological activity.... real-time imaging