Clotting Screen Blood Test Explained

There is no single fixed definition of a clotting screen, and the exact tests differ between hospitals. Lab Tests Online UK describes the PT as commonly requested with the APTT as a routine coagulation screen. Fibrinogen and a platelet count are frequently added because they address different parts of the clotting process.

Together they cover separate steps. Clotting factors in the blood, most made in the liver, are measured by the PT and APTT. Fibrinogen is the protein that is converted into the fibrin net that holds a clot together. Platelets are the cells that first plug an injury. A normal clotting screen means these steps were working in the sample, which is reassuring but is not a guarantee that no bleeding problem exists.

Lab Tests Online UK says a routine coagulation screen may be performed if a person has unexplained bleeding or easy bruising, or before surgery to assess the efficiency of the clotting system. The APTT may also be requested along with PT when someone presents with bleeding, bruising or a thromboembolism, and in suspected disseminated intravascular coagulation.

The PT is also a sensitive way to monitor serious liver disease, and it may be used to check for vitamin K deficiency. People on warfarin have the INR measured regularly, which is a single part of the screen used on its own.

The screen may also be requested with a platelet count when investigating a bleeding disorder or abnormal clot formation, and Lab Tests Online UK notes that heparin therapy requires platelet monitoring to detect heparin-induced thrombocytopenia promptly.

Lab Tests Online UK publishes a simple comparison of the PT and APTT that is used by clinicians as a starting point. The pattern points toward a group of possibilities rather than a diagnosis, and further tests follow.

It is common for people to be told that blood tests, including clotting, are needed before a procedure. Lab Tests Online UK states that numerous studies show the PT and APTT are not useful as pre-surgical screens for bleeding tendency, and that asymptomatic patients are usually only screened if their doctor believes it is necessary to evaluate bleeding risk.

They are more useful when there is a story. The same page mentions surgery that carries an increased risk of blood loss and a history of nosebleeds and bruising that may suggest an inherited or acquired factor deficiency. In practice the bleeding history, family history and medicines often matter as much as the screen.

The blood is drawn from a vein into a sodium citrate tube, and the tube has to be filled to the marked line or the result can be inaccurate. Lab Tests Online UK also says that heparin contamination from flushed lines is the commonest reason for a falsely prolonged APTT, which is why a repeat is often requested rather than assumed to be abnormal.

Tell the person taking your blood about anticoagulants, antibiotics, painkillers and supplements, because Lab Tests Online UK says various foods, alcohol and many medicines can interfere with the PT and that antibiotics and painkillers can increase the INR.