Antithrombin activity

Antithrombin (AT) activity is measured in a functional chromogenic assay. Diluted test plasma is incubated with an excess of FIIa and unfractionated heparin whereby all of the AT forms a complex with FIIa and heparin. The amount of FIIa that enters this complex is determined by the amount of AT in the sample, and it is the residual FIIa that is then reacted with a chromogenic substrate. Cleavage of the substrate by FIIa generates a coloured product, p-nitroaniline, the intensity of which is inversely proportional to the AT activity.
Clinical details: 
The complex orchestration of cellular and molecular participants of haemostasis achieves a crucial yet fine balance of procoagulant and anticoagulant mechanisms. Deficiencies of natural anticoagulant regulators of secondary haemostasis, such as antithrombin, protein C and protein S, and gain of function mutations in genes for FII and FV, are heritable disorders that can shift this balance and increase the risk of venous thromboembolic disease. Antithrombin (AT) is a serine protease inhibitor whose main target enzymes are thrombin and FXa, although it also possesses activity against FIXa, FXIa and TF:FVIIa. It is more efficient at inhibiting free thrombin and FXa than when they are contained within activation assemblies on clot surfaces, thus acting as a scavenger of potentially lethal enzymes that would otherwise diffuse away and cause unnecessary fibrin formation elsewhere. This serves to localise clot formation to the site of injury and limit coagulation. Hereditary AT deficiency can manifest as a reduced concentration of normally functioning AT, (type I defect), or a deficiency of function, (type II defect).
Reference range: 

77 -110 iu/dL

Sample type and Volume required: 
External requests: Citrated platelet poor plasma
500µL x 1 aliquot
Internal requests: please refer to EPR label

Turnaround time: 
7 - 12 days
Special sample instructions: 

The sample should be analysed or processed & stored within 4 hours of venepuncture. Please ensure sample tubes are filled exactly to the fill-line as underfilling or overfilling creates a dilution error and leads to inaccurate results.  Note that direct thrombin inhibitors will interfere with this FIIa based method, please include any anticoagulant information with request.

Storage and transport: 
It is advised that citrated plasma is frozen prior to transport and sent to the laboratory on dry ice to maintain sample quality and integrity.
Diagnostic Haemostasis and Thrombosis Department
St Thomas': 020 7188 2797; Guy's: 020 7188 7188 ext. 53860
St Thomas' Hospital
North Wing - 4th and 5th Floors
Westminster Bridge Road
London SE1 7EH

Laboratory opening times

Guy's Hospital
Southwark Wing - 4th Floor
Great Maze Pond
London SE1 9RT

Outside core hours, contact Duty Haemostasis Biomedical Scientist
For clinical advice or interpretation of results, please contact the laboratory in the first instance.

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Last updated: 03/10/2022