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Activated Clotting Time, Whole Blood

Published: August 9, 2026Updated: August 16, 2026

A whole-blood test that measures how many seconds blood takes to form a clot after an activating agent is added.

Quick summary

["The activated clotting time helps assess blood clotting and is commonly monitored during surgery, dialysis, and certain medical treatments."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
89841-1
Short code
CTA UE ves-Bl WO+W contr IV
Synonyms
CTA UE ves-Bl WO+W contr IV
Category
liver
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
ct-angio
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What is this test?

The activated clotting time, or ACT, measures how long a whole-blood sample takes to form a clot after a clotting agent is added. It is different from a standard blood-clotting test performed in a laboratory.

Why might I need this test?

Clinicians may order ACT monitoring during cardiopulmonary bypass, some surgeries, dialysis, or while giving medicines that affect blood clotting. It can help guide the safe use of blood thinners or clot-reversing medicines.

How is the test performed?

A blood sample is placed in a testing device that activates the clotting process and measures the time needed for a clot to form. Some ACT results are available quickly at the bedside or operating room.

How do I prepare for the test?

No special preparation is usually required when ACT is checked with a point-of-care device. Tell your care team about blood thinners, bleeding disorders, recent bleeding, or any blood products you have received.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

Activated clotting time (ACT)

Frequently asked questions

Is the activated clotting time the same as aPTT?
No. They both assess blood clotting, but they use different samples, reagents, and testing methods, so their results are not interchangeable.
Does an abnormal ACT always mean I have a bleeding disorder?
No. ACT is often affected by blood thinners, heparin, blood products, and other treatments, so an abnormal result does not by itself establish a bleeding disorder.
Does the test require fasting?
Usually not. Follow any specific instructions from your clinician, especially if the test is being monitored alongside a procedure or medication.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy