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General device and laboratory information only, not medical advice. It does not describe how to perform a test or how to interpret an individual patient’s result.
Short answer
Activated clotting time (ACT) is a whole-blood coagulation test in which blood is added to a tube or cartridge containing an activator, such as celite, kaolin or glass particles, and the time to clot formation is measured by an instrument. In the United States, FDA regulates the test system as an “activated whole blood clotting time test” (product code JBP, 21 CFR 864.7140, Class II). Results from different devices are not interchangeable.
Definition and FDA classification
The federal regulation defines the device by its purpose: an activated whole blood clotting time test is “a device, used to monitor heparin therapy for the treatment of venous thrombosis or pulmonary embolism by measuring the coagulation time of whole blood.” It sits in Subpart H (Hematology Kits and Packages) of 21 CFR Part 864 and is Class II, subject to performance standards (Cornell LII copy of the eCFR).
The openFDA device classification database lists the same device as product code JBP, “Activated Whole Blood Clotting Time”, Class 2, regulation number 864.7140, review panel Hematology (openFDA). For supply and equipment records, JBP and 864.7140 are the identifiers that connect a clotting-time analyzer or test tube to its regulatory category.
How the test works in practice
The Massachusetts General Hospital laboratory handbook describes the specimen as whole blood and the consumable as one tube containing an activator of coagulation, such as celite (diatomaceous earth), kaolin or glass particles. The instrument then detects clot formation. Two detection approaches are described (MGH laboratory test handbook):
- Tube-based analyzers such as Hemochron and Actalyke detect the clot by the movement of a magnet in the tube.
- Cartridge-based systems such as HemoTec, i-STAT and GEM PCL detect clotting through plunger resistance, cleavage of a thrombin substrate, or flow through a light window, depending on the system.
Practical-Haemostasis, a clinical laboratory reference, describes ACT as essentially a point-of-care test of coagulation, with the activator triggering the intrinsic pathway and the test depending on the patient’s own platelets as a source of phospholipid.
Activators compared
The activator in the tube changes the baseline the instrument reports, so each activator has its own reference range and no single number applies across tubes. Practical-Haemostasis names celite as the original activator, with kaolin and glass particles in common use, and notes that some systems run without an added activator.
| Activator in the tube | What the references say |
|---|---|
| Celite (diatomaceous earth) | The original activator; its result is prolonged by aprotinin more than a kaolin-based result (MGH). |
| Kaolin | Commonly used; generally not prolonged by aprotinin (MGH). |
| Glass particles | Listed as an activator alongside celite and kaolin (MGH, Practical-Haemostasis). |
The same Practical-Haemostasis page states that ACT devices should not be used interchangeably and that cut-off values should be determined for each device. The MGH handbook says the same thing about methods: they are not standardized, so results from different methods are not interchangeable.
What can make two ACT results differ
Both references list factors outside the nominal clotting process that change a result. This list is for understanding why results differ between devices and settings, not for judging a patient.
- Activator type and instrument (above).
- Platelet count and platelet function, including platelet glycoprotein IIb/IIIa inhibitors (both references).
- Haemodilution (both references) and temperature (MGH).
- Lupus anticoagulants and factor deficiencies (MGH).
- Aprotinin, which prolongs celite-based results but generally not kaolin-based ones (MGH).
Related device: the bleeding time device
A different hematology device is sometimes searched alongside ACT. The regulation at 21 CFR 864.6100 defines a bleeding time device as “a device, usually employing two spring-loaded blades, that produces two small incisions in the patient’s skin,” and states that the time required for bleeding to stop is a measure of the effectiveness of the coagulation system, primarily the platelets. It is Class II (special controls) and exempt from premarket notification subject to 21 CFR 864.9; the openFDA product code is JCA, “Device, Bleeding Time” (Cornell LII and openFDA). It belongs to Subpart G (Manual Hematology Devices), whereas ACT tests sit in Subpart H. The two are separate device types with separate product codes.
Where it matters for labs, clinics and supply records
- Classification. ACT tests are Class II device records under JBP and 864.7140; bleeding time devices are Class II under JCA and 864.6100. Use those codes, not a generic “coagulation” label, when cataloguing.
- Tube and instrument pairing. Because tubes and cartridges differ in activator and detection method, and results are not interchangeable between methods, a tube belongs to the analyzer system it was made for.
- Device-specific ranges. Any cut-off belongs to a specific device, as Practical-Haemostasis states, so a range from one system should not be carried across to another in a procedure or record.
ACT tubes are not the same as the evacuated draw tubes covered in blood draw tube colors. For the additives used in citrate and clot-activator draw tubes, see blue top tube additive and red top tube additive. Background on the specialty is in what is hematology.
Common misunderstandings
ACT is the same as aPTT
It is not. Practical-Haemostasis describes ACT as less precise than the aPTT, without a high correlation with aPTT or with heparin anti-Xa levels.
Any ACT tube works in any analyzer
Not according to the references above: activators differ, detection principles differ, and the methods are not standardized.
A bleeding time device and an ACT test measure the same thing
No. The ACT is a whole-blood test of clotting time on an instrument and is regulated as a heparin-monitoring test (864.7140). The bleeding time device is defined by the time bleeding takes to stop and is regulated separately (864.6100).
Frequently asked questions
What is activated clotting time?
A whole-blood coagulation test in which blood is added to a tube or cartridge containing an activator such as celite, kaolin or glass particles, and an instrument measures the time to clot formation.
How is an ACT test regulated in the United States?
As an activated whole blood clotting time test: product code JBP, 21 CFR 864.7140, Class II.
Are ACT results the same from every analyzer?
No. The MGH handbook states that the methods are not standardized and results from different methods are not interchangeable.
Is ACT the same as a bleeding time device?
No. They are separate device types: JBP and 864.7140 for ACT, JCA and 864.6100 for the bleeding time device.
Sources
- 21 CFR 864.7140, Activated whole blood clotting time tests: https://www.law.cornell.edu/cfr/text/21/864.7140 (checked 2026-10-30; 45 FR 60611, Sept. 12, 1980).
- 21 CFR 864.6100, Bleeding time device: https://www.law.cornell.edu/cfr/text/21/864.6100 (checked 2026-10-30; 45 FR 60604, Sept. 12, 1980, amended 63 FR 59225, Nov. 3, 1998).
- openFDA device classification API, product codes JBP and JCA: https://open.fda.gov/apis/device/classification/ (checked 2026-10-30).
- Practical-Haemostasis, Activated clotting time (ACT): https://practical-haemostasis.com/Miscellaneous/act (checked 2026-10-30).
- Massachusetts General Hospital laboratory test handbook, activated clotting time: https://mghlabtest.partners.org/?p=1070 (checked 2026-10-30).
