Protein C

Some laboratories use 65 to 135 or 70 to 150 %; it is lower in newborns and rises a little with age; use the range your lab printed.

Protein C is a vitamin K dependent protein made by the liver that, once activated, switches off two of the clotting factors (V and VIII) and so acts as a natural brake on clotting. Too little of it makes clots more likely. The test measures how much is present or how well it works, as a percentage of 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 rarely when an unusual skin reaction occurs in the first days of warfarin.

• Oestrogen-containing medicines, slightly
• Older age, slightly
• Diabetes and a high body weight, slightly
• There are no medical conditions of importance where a high protein C is the concern

• Warfarin and other vitamin K blocking medicines, because protein C depends on vitamin K
• Inherited protein C deficiency, which is uncommon
• Liver disease
• Low vitamin K
• A recent clot, surgery or severe illness
• A severe illness that uses up clotting factors
• Some direct oral anticoagulants interfere with some methods

• A level measured on warfarin reads low and cannot be interpreted; the test is done off warfarin
• A level measured during or just after a clot or operation reads low and is repeated once recovered
• Different laboratory methods (activity versus antigen) give different numbers

No fasting needed. Citrate tube. Measured off warfarin, when well, and at least a few weeks after any clot.

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