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Hematology: activated clotting time
(whole blood clotting time WBCT, clotting time, activated clotting time, ACT) 		Feedback

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

Uses

In combination


With bleeding time, clot retraction and platelet count
  • Investigating disorders of hemostasis.
  • Screening for deficiency of any one of following factors: VIII, IX, X, XI, XII, I (fibrinogen) and II (prothrombin).
Other points
  • Test gives similar results to APTT Hematology: activated partial thromboplastin time , but prolonged WBCT requires activity of clotting factor to be less than 5%.
  • APTT is prolonged if activity of clotting factor is less than 30% normal.
Sampling Top

Source of test material
  • Standard venipuncture Jugular venipuncture.

Quantity of test material
  • 1 ml whole blood.

Sample collection technique
  • For ACT discard first 2 ml; further 2 ml then collected into special tube.

Quality control

Precautions
  • Difficult sampling/tissue trauma during blood collection right_arrow increased tissue thromboplastin in sample right_arrow shorter (false) clotting times.

Sample storage
  • Perform test immediately.
  • Test should be done in veterinary practice (not submitted to a commercial laboratory).
Test(s) Top

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 right_arrow 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 Top

Normal (reference) values
  • Whole blood clotting time <13 minutes.
  • ACT <130 seconds (room temperature), <90 seconds (37°C).

Abnormal values


Increased clotting time
  • Deficiency of any one of factors: VIII, IX, X, XI, XII, I (fibrinogen) and II (prothrombin) (most commonly), or marked thrombocytopenia (rare).

Errors and Artifacts
  • Traumatic sample collection may reduce clotting time.
  • Barbiturates.
  • Prior administration of anticoagulants or salicylates.
  • Failure to recognize clot.
  • Inadequate mixing.
  • Failure to prewarm tube to 37°C.
Sources Top

Publications

Refereed papers
  • 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 (eds) (1997) Clinical Biochemistry of Domestic Animals. 5th edn. Boston: Academic Press.
  • Jain N C (1993) Essentials of Veterinary Hematology. Philadelphia: Lea and Febiger.

Vetstream contributor(s)
  • Dr Kathy Freeman DVM MS PhD , Animal Health Trust, Clinical Pathology and Diagnostic Lab Services, Lanwades Park, Kentford, Newmarket, Suffolk CB8 7UU, UK.
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Hematology: activated partial thromboplastin time
Hematology: platelet count
Jugular venipuncture
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