Normal clot retraction time is 0-2 hours. If the weight of the clot or the percent of extruded serum is used as the end measure, the result depends on the volume of the specimen used, which varies from laboratory to laboratory. 
The coagulation cascade is complex. Platelets play a major role to initiate the process and regulate it through biochemical and mechanical interactions. The 3 steps of this process for platelets are adhesion, aggregation, and finally, retraction.  The clot retraction study measures the time taken for a platelet plug to undergo this last step, which indicates overall platelet function.
A normal value means that outside-in signaling through integrin αIIbΒ3 in the clotting process is normal. This generalizes that overall platelet function is normal, although it does not test every factor. Outside-in signaling refers to a cascade of reactions started by integrin ligand binding. 
A long clot retraction time is abnormal and suggests an abnormality somewhere in the coagulation cascade after integrin αIIbΒ3 interacts with fibrin. This suggests a prolonged bleeding time and abnormal platelet thrombus formation, such as can be seen with acquired or hereditary platelet disorders. Integrin αIIbΒ3 receptor dysfunction is specifically implicated in Glanzmann thrombasthenia.  Low platelets or fibrinogen as well as high RBC concentrations prolong clot retraction time. Anitplatelet medications can also prolong clot retraction time.
A low value, or short clot retraction time, may suggest tendencies toward thrombosis and other pathologies. This could also be due to nonstandardized conditions for the test.
Collection and Panels
The clot retraction test can be drawn under any circumstances.
The sample should be a one-time collection of whole blood drawn into a red top tube. No anticoagulant should be used because this invalidates the results. Specimens should be set up in test tubes and observed with a camera. Photographs of the process should be taken initially and then every 30 minutes thereafter. This could also be done with a continuous clock and a human observer.
Each specimen must be normalized to a standard platelet count by mixing platelet rich plasma with the patients platelet poor plasma.  An alternate method described involves placing the platelet rich plasma into a vial with a metal rod and agonist to initiate clot formation. After the clot forms, the rod and clot are removed and the amount of serum extruded is measured. 
The sample is collected into a red top tube. The specimen should not be stored. Also, the laboratory temperature should be standardized because this can alter the result. The specimen should be left undisturbed so as not to interfere with clotting.
There are various test to assess platelet function including swirling, hypotonic shock response, aggregometry, adhesion tests, flow cytometry, and thromboelastography. 
When endothelium of a blood vessel gets injured, exposed collagen triggers fibrinogen conversion to fibrin, trapping platelets and forming a thrombus. Integrin αIIbΒ3 bound to fibrin helps contract the platelet actin-myosin cytoskeleton, pulling the clot in and extruding excess fluid. The exact mechanism is still nebulous, but discoveries are being made into some of the surface domains and subsequent signaling.  In vivo, retraction shrinks the damaged edges and forms a more stable clot. In vitro, this can be measured and is the clot retraction study. In order for the aforementioned pathway to work normally, Integrin αIIbΒ3 and the factors in the final common coagulation cascade must function normally. Thus, this assay is a simple in vitro technique to assess platelet function. 
Platelet dysfunction manifests as excessive hemorrhage at mucocutaneous sites as well as ecchymoses and petechiae. However, this is true for both quantitative and qualitative abnormalities. After a platelet count rules out the quantitative problem, a prolonged bleeding time suggests a qualitative issue with the platelets. Different assays can try to highlight given issues in acquired or hereditary platelet dysfunction. 
This test is considered by many as dated and nonspecific; it has largely been replaced with more sophisticated methods in most labs.
Prolonged bleeding times or signs of platelet dysfunction (eg, ecchymoses, petechiae) warrant work-up. This test is one of a battery the clinician can use to investigate further. Ultimately, one may need the help of an expert such as a hematologist.