From: Elizabeth M. Van Cott, M.D., and Michael Laposata, M.D., Ph.D., "Coagulation." In: Jacobs DS et al, ed. The Laboratory Test Handbook, 5th Edition. Lexi-Comp, Cleveland, 2001; 327-358.

Index of Tests

Clot Retraction [CO001000]

Related Information

Test Includes Test may include description of clot retraction, clot size and firmness, RBC fallout, serum "drip-out".

Abstract This test has been replaced by newer tests for platelet function and for Glanzmann thrombasthenia in most coagulation laboratories.

Specimen Whole blood

Container Red top tube

Collection Routine venipuncture; transport specimen to the laboratory immediately. (Note: Contact the laboratory prior to collecting the specimen, as the laboratory may not offer the test.)

Turnaround Time 24 hours

Reference Interval Clot retraction occurs within 4 hours.1

Optional considerations:2 With normal clots and normal hematocrits, the clot in a red top tube occupies 40% to 60% of the original volume. The remaining 40% to 60% consists of serum as well as red cells that fall out of the clot and settle to the bottom of the tube ("red cell fall-out"). Red cell fall-out is usually <5% of the original blood sample volume (centrifuged, after removing the clot). When normal clots are removed from the tube, serum drips from the clot at a rate of two drops or less in 2 minutes.

Use Currently, it is an infrequently used clinical test. In the past, it was a test for Glanzmann thrombasthenia and platelet function.

Limitations Platelet counts <100,000/microL, aspirin and related medications, monoclonal gammopathy (paraproteinemia), and polycythemia reduce the amount of clot retraction. Anemia increases clot retraction. With polycythemia, the increased number of red blood cells within the clot limits the extent to which the clot can retract.

Methodology The red top tube is kept at 37degrees C and the clot is examined at 1, 2, 4, and 24 hours for clot retraction. When the clot retracts, it pulls away from the walls of the tube. Normally, a few red blood cells fall out of the clot, and they can be seen at the bottom of the tube.

Optional approach:2 The initial blood specimen can be placed in a graduated tube such that volumes can be approximated. A wooden stick can be placed in the tube prior to clot formation, so that the clot can be removed from the tube for examination. A normal clot is firm and tightly attached to the stick.

Additional Information During clot formation, platelets aggregate as fibrinogen binds to platelet glycoprotein IIb/IIIa, linking platelets to each other. Normally, clot retraction occurs subsequently, as platelets within the clot contract. Glycoprotein IIb/IIIa is necessary for platelet aggregation as well as for clot retraction. In Glanzmann thrombasthenia, clot retraction and platelet aggregation are reduced because glycoprotein IIb/IIIa is deficient. With dysfibrinogenemia, hypofibrinogenemia, or disseminated intravascular coagulation (DIC), the clot can be small and an increased number of red blood cells fall out of the clot.


1. Brown BA, Hematology: Principles and Procedures, 6th ed, Philadelphia, PA: Lea and Febiger, 1993, 271.

2. Sirridge MS and Shannon R, Laboratory Evaluation of Hemostasis and Thrombosis, 3rd ed, Philadelphia, PA: Lea and Febiger, 1983, 83-90.


Hantgan RR and Mousa SA, "Inhibition of Platelet-Mediated Clot Retraction by Integrin Antagonists,"Thromb Res, 1998, 89(6):271-9.

Rooney MM, Farrell DH, van Hemel BM, et al, "The Contribution of the Three Hypothesized Integrin-Binding Sites in Fibrinogen to Platelet-Mediated Clot Retraction,"Blood, 1998, 92(7):2374-81.


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