Blood Clotting (PTT) Test

What it is

Clotting is not one event but a cascade: a chain of proteins in the blood that activate one another in order until a clot forms. The PTT measures how long that chain takes to finish when it is started in a particular way in the laboratory, and the answer is a number of seconds. If one of the proteins in that arm of the cascade is missing, faulty, or being suppressed by a drug, the count runs long. It is a different test from the prothrombin time, which starts the cascade at a different point and is the one used for warfarin — the two are often drawn together but they are separate results and separate codes.

Why it is done

• To look into bruising or bleeding that has no obvious explanation
• To help diagnose an inherited bleeding disorder such as hemophilia
• To monitor someone being given heparin, where the dose is adjusted against the result
• To check clotting before a procedure in a patient with a history of bleeding, or to investigate clotting problems linked to autoimmune conditions

What to expect

A health professional draws blood from a vein in the arm with a small needle. It takes under five minutes and needs no fasting or other preparation. The usual brief sting at the puncture site, and sometimes a small bruise, are the whole of it. Results are numerical and are compared against the performing laboratory's own reference range, which is why a value from one lab is not directly interchangeable with one from another. When the test is being used to steer a heparin drip in hospital, it may be repeated several times in a day.

Questions worth asking

• Is this being billed on its own or as part of a coagulation panel? A panel bundles several codes and costs more than the one test.
• If it was drawn in hospital and marked urgent, is there a STAT surcharge on the line?
• Is the specimen going to the hospital's own laboratory or to an outside reference lab that will bill me separately, and is that lab in network?
• For repeat monitoring, is there a per-draw phlebotomy or venipuncture fee charged in addition to the test itself each time?