Blood clots when collected in a glass tube without any anticoagulant. The clot begins to retract (after blood has clotted) within 30 seconed, and is about 50 percent at the end of 1 hour. At the end of 18-24 hours the clot should have retracted completely.

Clot retraction refers to the tendency of a blood clot to draw in from the sides of a glass test tube.

A blood clot is a mass formed within a blood vessel to stop bleeding by blockading the wound, preventing the escape of blood from the blood vessel. Formation of a blood clot is a multistep process that is tightly regulated

Blood clot formation normally starts with injury to a blood vessel, which causes it to constrict. Called the vascular phase, this is the first reaction of a blood vessel to damage. It reduces the flow of blood to the site of injury, minimizing blood loss.

Next, the circulating platelets clump along the site of blood vessel injury. The platelets form a foundation for a blood clot and release chemicals that stimulate clotting.

The coagulation phase then causes a blood clot to form. Clotting occurs when an enzyme called thrombin converts a soluble protein, fibrinogen, into its insoluble form, fibrin. Fibrin proteins make up the bulk of a blood clot.

Thrombin is activated by the merging of two pathways, the intrinsic and extrinsic pathways, into the common pathway. These are initiated by different parts of the body after blood vessel damage:

The intrinsic pathway begins in blood with the activation of circulating proteins; The extrinsic pathway begins in the blood vessel with the release of protein factors by damaged cells lining the vessel. The extrinsic pathway is the first to activate. The intrinsic pathway then reinforces the extrinsic pathway and provides longer-lasting clotting effects.

Coagulation factors are central to the action of these pathways. Each factor activates the next in a stepwise fashion. Once a coagulation factor is activated, it remains active. Thus, with each step in the pathway, more and more factors are activated. This results in a cascade of events similar to the snowball effect.

A counter pathway ensures that the size of the growing blood clot stays in check. Problems with this regulatory pathway may lead to a dangerous condition where a blood clot forms within blood vessels (thrombosis)

The Clot Retraction Blood Test helps determine the time needed for a blood clot to form in a glass test tube and for its edges to draw in from the sides of the glass. It was once regularly used to diagnose clotting disorders; but presently, the test is rarely used. 

A high value for the Clot Retraction Blood Test may indicate: 

Thrombasthenia
Bernard-Soulier syndrome

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.