Lupus Anticoagulant (LAC) Test

LAC test is performed to determine the cause of the following:

• An unexplained clot in an intact artery or vein
• Recurrent miscarriages
• Stroke or heart attack
• Unexplained prolonged partial thromboplastin time (PTT)

The LAC test might also be performed when the symptoms of DVT are suspected, which include the following:

• Leg pain or tenderness in one leg (affected leg)
• Severe pain in the affected area
• Swelling of the affected leg (oedema)
• A feeling of warmth in the area of the clot
• Discolouration of the affected leg

Furthermore, this test can be performed when the symptoms of pulmonary embolism are suspected, which include the following:

• Sudden dyspnoea (breathlessness)
• Cough
• Haemoptysis (blood in the sputum)
• Chest pain
• Increased pulse rate (tachycardia)
• Sudden collapse

LAC test is also suggested along with other tests, such as cardiolipin antibodies and anti-beta-2-glycoprotein I to diagnose antiphospholipid syndrome; and factor V Leiden, antithrombin, protein C or protein S for diagnosis of thrombophilia.

No special preparations are needed for the LAC test.

It is a simple procedure that takes less than 5 minutes. An experienced laboratory technician will insert a small needle into a vein in your arm to withdraw a small quantity of blood in a sterile vial or test tube. A momentary pinprick pain would be felt when the needle goes into the vein.

Also, there is a minimum risk of pain, light-headedness and bruising at the site of injection. However, most of the times, these indications disappear quickly. Rarely, there an infection occurs at the site of withdrawal of blood.

LAC test is usually performed in conjunction or after evaluating PTT levels. Normal PTT usually indicates an absence of LAC, while prolonged PTT needs LAC evaluation.

Normal results: The absence of LACs indicates normal results, which mean elevation of PTT is not due to autoantibodies, but it may be due to some other causes like deficiency of coagulation factors.

Abnormal results: The presence of LAC indicates abnormal results, which mean that the signs and symptoms of the current episode of DVT or venous thromboembolism are due to LACs.
LAC test is not performed solitarily or is not diagnosed just on a single reporting of LAC, but it is performed at successive occasions, usually 12 weeks apart, and need more than two positive readings to indicate the presence of LAC.

In cases of prolonged PTT, certain other tests are performed to confirm the diagnosis of LACs. These tests include complete blood count, which indicates low platelet levels (thrombocytopenia) and is seen in association with LACs; coagulation factor assay (like fibrinogen) to rule out coagulation factor deficiency that may cause elevation of PTT and thrombin time, which helps in excluding heparin therapy as a cause of elevation of PTT.

LAC can be falsely positive in certain cases, such as in people receiving heparin therapy. It might be positive in patients having autoimmune diseases, HIV infection, certain cancers, and with certain medical preparations, such as penicillin, hydralazine, quinidine and phenothiazines.

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.