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Plasma or Serum Clotting Time (Activated Clotting Time, ACT)

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

This test measures how long it takes for blood to clot after activation, often used to monitor anticoagulant therapy.

Quick summary

["ACT helps track blood thinning medications and ensures safe dosing."]

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

Key facts at a glance

LOINC
71515-1
Short code
CLOSINGTIME
Synonyms
—
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
esrd
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 ACT Used For?

ACT is primarily used in medical settings to evaluate blood clotting function, especially when managing anticoagulant medications like heparin. It helps doctors determine if the medication is working properly or if adjustments are needed to prevent excessive bleeding or clotting.

How Is ACT Performed?

A small sample of your blood is taken and placed in a special tube. The test measures the time it takes for the blood to form a clot after a chemical activator is added. This process is typically done in a lab using a specialized machine.

Why Might You Need an ACT?

You may need an ACT if you’re undergoing procedures that require blood thinning (like a heart catheterization) or if you’re on anticoagulant medications. It’s also used to monitor how well these medications are working in your body.

How Long Does It Take to Get Results?

Results are usually available within a few hours after the blood sample is collected, depending on lab processing times.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

ACT

Frequently asked questions

Why was I asked to fast before this test?
Fasting may be recommended to ensure accurate results, especially if your blood sample is collected after meals. However, some tests like ACT do not require fasting, so follow your healthcare provider’s specific instructions.
What does a high or low ACT mean?
A high ACT may suggest your blood is clotting too slowly, which could be due to insufficient anticoagulant levels or other clotting disorders. A low ACT might indicate your blood is clotting too quickly, possibly from overmedication or a clotting disorder. Your doctor will interpret results in the context of your medical history and other tests.
Can I take my usual medications before this test?
Always check with your healthcare provider before taking any medications, including over-the-counter drugs or supplements. Some medications can affect blood clotting, so they may need to be adjusted or temporarily paused before the test.
  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