Anti thrombin III Test

An antithrombin III test is recommended in the following conditions: 

• Repeated blood clot formation 
• Abnormal clot formation despite the intake of blood-thinning medicines
• A family history of clotting problems
• Presence of a clot near the brain or in the abdomen
• Venous thromboembolism (a clot forms in a vein and then moves to another part of the body), especially before the age of 50 years 
• Presence of blood clots in lungs

Your doctor may not order this test if there is clot formation in arteries instead of veins. This is because antithrombin III deficiency is usually not associated with clots in arteries.

 No special preparations are needed before this test. However, you must share all the relevant information about your health and medications with your doctor. Several medicines such as heparin, warfarin and chemotherapy with the medicine asparaginase can interfere with the results of this test. Also, let the doctor know about any herbal medicines, vitamins and other supplements that you may be taking. If you have any concerns regarding the test, please speak to your doctor beforehand.

A laboratory technician or nurse will collect a blood sample from a vein in your arm using a sterile needle. About 7 mL of blood will be collected in case of adults and about 2 mL from babies. You may feel a temporary stinging sensation when the needle is inserted. Some people experience bruising on the injection site. In most cases, it subsides on its own. In case the bruising persists, please inform your doctor.

Normal results:

Results of an antithrombin III test are usually reported as a percentage. If the value is within the standard percentage range for the corresponding age group, it is considered normal. The reference range may vary slightly from one testing facility to another. Table 1 shows a general reference range of values for people of different ages. Your doctor will best be able to explain what your result signifies for you. 

Table 1: Reference range of antithrombin III for different age groups

In the case of adults, a value between 80% and 120% is generally considered normal.

Abnormal results:

A value of around 40-60% is usually seen in people with deficiency of antithrombin III. Lower than normal values may be seen in the following conditions:

• Liver transplant
• Liver cirrhosis
• Malnutrition
• Deficiency of antithrombin III from birth
• Deep vein thrombosis
• Nephrotic syndrome
• Disseminated intravascular coagulation
• Late pregnancy/early postpartum period
• Pulmonary embolism
• Hypercoagulation state
• Septicaemia
• Recovery period after surgery

Your doctor may order further tests to confirm the exact reason for the clotting problem or may go ahead with the treatment plan. 

Higher than normal levels of antithrombin are not seen to cause any health problems or bleeding tendencies. However, it may be due to:

• Low levels of vitamin K
• Haemophilia
• Kidney transplant

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.