Method of activated clotting times

1. ACT

a) Mainstay of heparin monitoring during cardiac surgery

b) Introduced in 1966

c) Uses large surface area provided by diatomaceous earth (celite) or kaolin to initiate contact activation [are activants]

d) Temperature is critical for consistent results; devices warm blood to 37°C

i) ACT rises if tube is not prewarmed

2. Manual ACT method

a) Add 1 ml blood to 37°C tube containing activator celite

b) Start timer

c) Mix contents by inversion of tube

d) Place in 37°C water bath

e) At 1 min and at 5 sec intervals after, tube is withdrawn from bath and tilted while observing for visible clotting

f) Stop timer

3. Automated ACT methods

a) Hemochron

i) Celite or kaolin test tubes

ii) Requires 2 ml whole blood sample

iii) Timer started as the blood sample is placed in the test tube

iv) After vigorous mixing, the tube is inserted into within a heat block & rotated continuously

v) Endpoint is detected by displacement of a rotatory magnetic bar by the clot

b) Hemotec

i) Kaolin test cartridge

ii) Requires only 0.4 ml whole blood sample

a) Useful for paediatrics

b) More rapid sample rewarming

iii) Whole blood is injected into each of two plastic cartridges, which are the placed into the heat block

iv) Each cartridge contains a plunger movement that is engaged by a mechanical lifting device when the test is initiated

v) The initial plunger movement mixes the whole blood with a kaolin activator, and the timer is started

vi) A continuous rise & fall of the plunger agitates each sample

vii) Endpoint is detected by the decrease in the drop rate of the plunger

viii) The absence of plunger fall is detected photooptically hence stops the timer

ix) Gives slightly shorter ACT than Hemochron due to different sample volume, mode of clot detection & activators