Overview

• Evaluates entire secondary blood clotting system (common and intrinsic pathway and platelet).
• Test is independent of platelets (unless the platelet count is <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.

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).

Predictive value

• Prolonged ACT: presence of severe coagulopathy or platelet <10 x 10*9/l.

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 (<10 x 10*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 bleeding dog or cat. Vet Med May, 450-459.

Other sources of information

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