ACTIVATED CLOTTING TIME (ACT) - blood

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Specimen: Freshly drawn venous blood without anticoagulant; volume as specified for instrument in use. Blood added immediately to instrument cartridge.

Method: Modified APTT. Several instruments are marketed as suitable for “bedside” monitoring of heparin therapy.

Reference Interval: As specified by manufacturer. See Interpretation.

Application: Limited application in the acute monitoring of heparin therapy during cardiopulmonary surgery and in intensive care and dialysis units. Available methods have variable and limited correlation with the APTT and may be influenced by other variables (eg PCV and fibrinogen). A specific instrument should only be selected for use if its performance has been validated against the methods used in the pathology laboratory; correlation with the APTT and plasma heparin levels should be checked at regular intervals.

Interpretation: The prolongation of the ACT above baseline is used as a rough guide to the degree of heparinisation and, at times, the appropriate dose of protamine sulphate. A more precise correlation would require prior characterisation of performance by the pathology laboratory, including testing samples with known levels of plasma heparin.

Reference: O'Neill AI et al. Anaesth Intensive Care 1991; 19: 592-601.

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