Coagulation
Activated clotting time
Activated clotting time (ACT) is a whole-blood, point-of-care test that measures how long blood takes to clot after it is mixed with a contact activator such as celite, kaolin or glass particles. It is used mainly to monitor high-dose heparin anticoagulation at the bedside during procedures, and to confirm reversal of that effect.
Why it is measured
ACT lets clinicians titrate and confirm anticoagulation in real time during cardiopulmonary bypass, percutaneous coronary intervention, ECMO and haemodialysis, where standard laboratory clotting tests are too slow or insensitive at very high heparin doses. It also confirms safe return towards baseline after protamine reversal.
| Typical range | Indicative adult baseline (off heparin) is roughly 70 to 180 seconds, often around 100 to 140 seconds; values are method and device dependent and not interchangeable between systems. Procedural targets are deliberately higher and protocol specific, for example commonly over 400 to 480 seconds for cardiopulmonary bypass and lower targets (for instance about 200 to 300 seconds) for percutaneous coronary intervention. Ranges vary by method, activator and instrument, so each device should be interpreted against its own reference values. |
|---|---|
| Sample | Fresh native whole blood with no anticoagulant added, taken from a syringe, line or fingerstick depending on the device and dispensed immediately into the test cuvette, cartridge or tube. The sample must be analysed at once because clotting begins on contact with the activator. |
| Turnaround | Results are available at the bedside within minutes, typically about 1 to 10 minutes, because the device reports as soon as the clot endpoint is detected; more heavily anticoagulated samples take longer to reach that endpoint. |
Point of care devices that report it
- Abbott i-STAT 1 (Celite ACT and Kaolin ACT cartridges)
- Werfen / Accriva Hemochron Signature Elite
- Werfen / Accriva Hemochron Response
- Werfen / Accriva Hemochron Junior II
- Medtronic ACT Plus
- Medtronic HMS Plus (Hepcon) Haemostasis Management System
- Helena Actalyke MAX-ACT
Questions, answered
Why do two ACT analysers give different numbers on the same patient?
Different systems use different activators (celite, kaolin or glass particles) and different clot-detection methods, so they produce different absolute values and are not interchangeable. Best practice is to follow trends on the same device and method throughout a case rather than comparing readings across instruments.
What is ACT used to monitor?
It is used chiefly to monitor high-dose unfractionated heparin during procedures such as cardiopulmonary bypass, percutaneous coronary intervention, ECMO and some dialysis circuits, and to confirm that anticoagulation has returned towards baseline after protamine reversal. It is an operational monitoring tool, not a diagnostic test for bleeding disorders.
What can prolong ACT apart from heparin?
Hypothermia, haemodilution, low fibrinogen, low platelet count or impaired platelet function, and some other anticoagulants can all prolong ACT independently of heparin, which is why results are interpreted alongside the clinical context and the local protocol. This is general educational information and not advice for any individual patient.