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]
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Abstract This test has been replaced by newer tests for platelet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
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
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
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,
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.