Overview

• Evaluates entire secondary blood clotting system.
• Test is independent of platelets (unless the platelet count Hematology: platelet count <10 x 10*9)/l).

Sampling

Tests

Methodologies

Capillary tube

• Blood collected in fine capillary tube; time noted.
• Break off end of tube every 30 seconds until appearance of fibrin is noted.
• Measure time interval.

Activated clotting time

• Time taken for whole blood to clot inside glass tube containing contact activator (diatomaceous/earth).
• Less variable and more sensitive than capillary tube method.
• Performed at room temperature or 37°C (tube must be prewarmed).
• Special Vacutainer tube required (also allows clot retraction time to be measured).
• ACT: time taken to notice obvious clotting following addition of blood.

Whole blood clotting time

• Sample collected into warmed glass tube (37°C) without contact activator.
• Tube tipped every 30 seconds and kept at 37°C between tipping.
• Record time until blood clots.
• Less reliable and accurate than ACT.

Availability

• In veterinary practice.

Validity

Sensitivity

• Relatively insensitive but acceptable to help differentiate/evaluate presentations with bleeding.
• Evaluates same pathways as activated partial thromboplastin time (APTT) Hematology: activated partial thromboplastin time.

Predictive value

• Prolonged ACT → presence of severe coagulopathy.

Technique (intrinsic) limitations

• Deficiency of factor VII has no effect (extrinsic system of less importance).
• Does not evaluate extrinsic pathway.
• Be sure to concurrently evaluate platelets (automated and/or manual count) and correlate with examination of blood smear. Marked thrombocytopenia (<10x10*9/ml) may result in prolongation of ACT or WBCT.

Result Data

Further Reading

Publications

Refereed papers

• Recent references from VetMedResource and PubMed.
• Couto C J (1999) Clinical approach to the bleeding dog or cat. Vet Med May, 450-459.
• Sheafer S E & Couto C J (1999) Clinical approach to a dog with anticoagulant rodenticide poisoning. Vet Med May, 466-471.

Other sources of information

• Kaneko J J, Harvey J W & Brass M L (1997) Eds. Clinical Biochemistry of Domestic Animals. 5th edn. Academic Press, USA.
• Jain N C (1993) Essentials of Veterinary Hematology. Lea and Febiger, USA.