When blood collected in a test tube first clots, the entire column of blood solidifies. As time passes, the clot diminishes in size. Serum (the fluid remaining after blood coagulates) is expressed, and only the red blood cells remain in the shrunken fibrin clot. Because platelets are necessary for this process, the speed and extent of clot retraction roughly reflect the adequacy of platelet function. Individuals with thrombocytopenia or platelet dysfunction, for example, have samples with scant serum and a soft, plump, poorly demarcated clot.

The results of the clot retraction test should be evaluated in relation to other hematologic, platelet, and coagulation studies. If the client has a low hematocrit, for example, the clot is small and the volume of serum is great. In contrast, individuals with polycythemia or hemoconcentration have poor clot retraction because the numerous red blood cells contained in the clot separate the fibrin strands and interfere with normal retraction.

If fibrinogen levels are low, the initial clot is so fragile that the fibrin strands rupture, and red blood cells spill into serum when retraction begins. If there is excessive fibrinolysis, as often happens with reduced fibrinogen levels, the incubated tube may contain only cells and fibrin with no fibrin clot at all. Low fibrinogen levels and excessive fibrinolysis are seen in disseminated intravascular coagulation (DIC).²⁹

The clot retraction test also can be modified to demonstrate the inhibitory effect of antiplatelet antibodies, especially those associated with drugs. Clot retraction is abolished if more than 90 percent of platelet activity is neutralized. Serum suspected of containing antibodies can be added to normal blood to see if retraction is inhibited.³⁰

• A normal clot, gently separated from the side of the test tube and incubated at 98.6°F (37°C), shrinks to about half its original size within 1 hour. The result is a firm, cylindrical fibrin clot that contains all of the red blood cells and is sharply demarcated from the clear serum.

• Rough handling of the sample alters clot formation.

• Evaluation of adequacy of platelet function
• Evaluation of thrombocytopenia of unknown etiology
• Suspected antiplatelet antibodies resulting from immune disorders or drug-antibody reactions
• Suspected abnormalities of fibrinogen or fibrinolytic activity
• Monitoring of response to conditions that predispose to DIC

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

A venipuncture is performed and approximately 5 mL of blood is collected in a red-topped tube. The sample is promptly sent to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are essentially the same as for any study involving the collection of a peripheral blood sample.

• Because the client may have platelet dysfunction or deficiency, maintain digital pressure directly on the puncture site for 3 to 5 minutes after the needle is withdrawn.
• Inspect the site for excessive bruising after the procedure.