Antithrombin

Some laboratories use 75 to 125 %; newborns have lower levels; use the range your lab printed.

Antithrombin is a protein made by the liver that switches off thrombin and several other activated clotting factors, acting as one of the body's natural brakes on clotting. It is also the protein through which heparin works. The test measures how active it is, reported as a percentage of the amount found in pooled normal plasma.

Doctors request it as part of a thrombophilia screen after an unexplained clot, particularly in a young person or with a strong family history, and occasionally when heparin appears to be working less well than expected.

• Warfarin, slightly
• There are no common medical causes of a high antithrombin and a high value is not of any importance

• Inherited antithrombin deficiency, which is uncommon
• Heparin, while it is being given
• Liver disease, because less is made
• Nephrotic syndrome, where it is lost in the urine
• A severe illness that uses up clotting factors
• A recent clot, surgery or major illness
• Oestrogen-containing medicines and pregnancy, slightly
• Asparaginase, a cancer medicine

• A level measured during or just after a clot, an operation or heparin use reads low and is repeated once recovered
• Oestrogen-containing contraception or HRT lowers it a little as an expected effect
• Pregnancy lowers it a little as an expected change

No fasting needed. Citrate tube. Best measured when well, off heparin, and at least a few weeks after any clot or operation.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.