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Thrombin Time (Activated Clotting Time, ACT) - Plasma

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

This test measures how long it takes for blood to clot after thrombin is added to your plasma sample. It helps assess blood clotting function, especially in patients on anticoagulant medications.

Quick summary

["ACT helps monitor blood clotting and medication effectiveness."]

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

Key facts at a glance

LOINC
73533-2
Short code
Amitrip ind plt IgM SerPl Ql FC
Synonyms
Amitrip ind plt IgM SerPl Ql FC
Category
hematology
Sample
serum
Container
Red-top tube (SST)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
flow-cytometry
CPT
82784
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 ACT Measures

ACT evaluates the intrinsic pathway of blood clotting, which involves factors like prothrombin and fibrinogen. It indirectly reflects the efficiency of thrombin formation, a critical enzyme in clot formation.

Why It’s Ordered

Doctors may order ACT before procedures requiring blood clotting (e.g., surgeries, catheter placements) or in patients on anticoagulants like heparin to ensure safe clotting times.

Sample Collection and Preparation

A blood sample is drawn into a special tube containing sodium citrate to prevent clotting. Plasma is separated and tested within 2 hours of collection for accurate results.

Common Uses and Clinical Contexts

ACT is often used in settings like hemodialysis, cardiac procedures, or monitoring heparin therapy. It helps balance anticoagulation to prevent excessive bleeding or clots.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

ACT (Plasma)

Frequently asked questions

What might cause an elevated ACT?
Elevated ACT can indicate impaired clotting (e.g., from liver disease, vitamin K deficiency, or anticoagulant use). Low ACT may suggest excessive anticoagulation or clotting disorders.
How often is ACT retested?
ACT is retested based on clinical needs—typically after dose adjustments of anticoagulants or before invasive procedures. Your healthcare provider will determine timing.
Can diet or medications affect ACT results?
Dietary factors (e.g., vitamin K-rich foods) or certain medications (e.g., warfarin) may influence clotting factors and ACT results. Always inform your provider of all medications.
  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