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Coagulation laboratory tests

Activated clotting time

Reviewer: Jeremy Parsons, M.D. (see Reviewers page)
Revised: 16 June 2012, last major update June 2012
Copyright: (c) 2002-2012, PathologyOutlines.com, Inc.


● Whole blood clotting assay, performed at point of case, including operating room or catheterization lab (Am J Clin Pathol 2011;135:741), to monitor high-dose heparin anticoagulation (cardiopulmonary bypass surgery) or to immediately measure heparin (ECMO, hemodialysis, cardiac catheterization)
Note: test is necessary for high-dose heparin monitoring because PTT is often unclottable at very high heparin levels
● Whole blood is collected into tube with celite (diatomaceous earth), kaolin, glass particles or other activator of intrinsic pathway
● Tube may need to be shaken to disperse the activator
● Tube is monitored by instrument that records time until clot is formed
● Do not collect blood from line containing heparin
● Target reference range depends on the method: usually 70-180 seconds, 400+ seconds for cardiopulmonary bypass operations
● Does not correlate well with PTT but heparin level can be measured using an anti-Xa assay
● Affected by platelet count and function, lupus anticoagulant, factor deficiencies, patient and ambient temperature, hemodilution, aprotinin (reversible platelet inhibitor that prolongs celite-based tests)

Additional references

Lab Report for Physicians 1982;4:17

End of Coagulation > Coagulation laboratory tests > Activated clotting time

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