Coagulation tests
Coagulation tests are laboratory tests used to monitor blood clotting activity, i.e. the process that stops bleeding triggered by a more or less serious wound. Clotting is a natural process that takes place in everyone’s blood whenever an injury causes blood to escape from a wound. Prothrombin, a protein present in the blood, is activated and leads to the formation of a clot, which stops or limits blood loss. If the injury is minor, the clotting proteins are not always needed: the activity of the platelets, which block the loss of blood straight away, is enough. After a wound that damages the wall of a blood vessel, the platelets – the first to arrive at the injury to stem the bleeding – form what is known as the “haemostatic plug”. At the same time, if needed because the wound is larger, the clotting factors are activated, i.e. proteins arrive to reinforce the “haemostatic plug”, until fibrinogen (another clotting protein) is converted into fibrin by prothrombin. Fibrin stabilises the “haemostatic plug”, permanently consolidating the clot formed in the injured area. Fibrinolysis is then activated: it dissolves the clot, which is reabsorbed, and at the same time starts the wound-repair process, at the end of which the vessel wall is restored to its normal structure.
The main laboratory tests for studying blood clotting activity are therefore:
- The prothrombin time (or prothrombin activity), also called PT
- The partial thromboplastin time, also called PTT
Both these tests have replaced the now obsolete “clotting time” test, which was less precise and sensitive.
• Fibrinogen.
In addition, although they are not part of blood clotting activity as such, platelets are also involved in stopping bleeding, so two further tests help the doctor get an overall picture of this process:
- The platelet count
- The bleeding time.
What they mean
Prothrombin time (PT): in people who are not taking anticoagulants, the INR (International Normalised Ratio) is normally around 1 (about 0.8-1.2); in patients on oral anticoagulants, the target range is usually 2-3 (up to 3.5 in some cases).
If the INR is above the normal range, this may indicate insufficient dietary vitamin K (essential for the production of prothrombin), reduced production of prothrombin by the liver or liver disease, or that the person is taking anticoagulant medicines.
If the INR is above the target range in a patient taking anticoagulants, the blood is too “thin” and the doctor will review the anticoagulant therapy.
The partial thromboplastin time (PTT) can be considered normal when the time needed for a clot to form is up to 40 seconds.
• If the time needed for a blood clot to form is longer than 40 seconds, the clotting process
carried out by the clotting proteins, particularly fibrin, is not working properly.
Fibrinogen can be considered normal when its value is between 0.200 and 0.450 grams per decilitre of blood.
If the fibrinogen value is lower than normal, it means that the body is using up clotting factors even in the absence of bleeding (so-called “disseminated intravascular coagulation”). This process can be triggered, for example, by leukaemia.
If the fibrinogen value rises above normal, it indicates inflammation that the doctor should investigate.
The platelet count is normal when the value is about 150,000-450,000 platelets per microlitre of blood.
If the value is increased (so-called thrombocytosis), the person is producing too many platelets, and the doctor may consider thinning the blood with aspirin-based medicines, especially if the platelet count exceeds one million.
If the value is lower than normal (so-called thrombocytopenia), possible causes include leukaemia, and spontaneous bleeding may occur.
The bleeding time can be considered normal when bleeding stops within 5 minutes; otherwise there may be thrombocytopenia or a platelet function disorder (thrombocytopathy).
Why they are done
Coagulation tests are recommended by the doctor when a person is due to have surgery or simply wants to know their state of health; to monitor anticoagulant therapy when a person has had conditions linked to abnormal blood clotting, such as a heart attack or stroke; or to diagnose disorders of the platelets or clotting factors when a person has, for example, frequent bruising. In particular, if a person is due to have surgery or wants to check their state of health, it is advisable to carry out all the tests described. When a person is taking coumarin-type anticoagulants – which make the blood much “thinner” to prevent clots from forming – because they have had, for example, a heart attack or stroke, the PT (prothrombin time) must be monitored. Finally, if a person is taking heparin anticoagulants, which work differently from coumarins, a PTT (partial thromboplastin time) is advisable.
How they are done
This is a simple blood test. The tests can be carried out together or separately, depending on what the doctor requests, and fasting is not required. Medicines do not affect the result, except for aspirin-based medicines and anti-inflammatory drugs in general, because they reduce platelet aggregation and make it harder to stop bleeding. For this reason, you should tell the person taking the blood sample that you are taking these medicines, as they will need to press on the puncture site for longer to stop the bleeding caused by the needle.
Results
NORMAL VALUES
Prothrombin time (PT), 70-120%
on anticoagulant therapy 18-38%
INR 0.8-1.2 (on anticoagulant therapy, usually 2-3)
Partial thromboplastin time (PTT) up to 40 seconds
Bleeding time up to 5 minutes
Fibrinogen 0.200-0.450 g/dl
Platelets: under 5 years 221,000-561,000/mm³
6-15 years 190,000-493,000/mm³
over 16 years 150,000-450,000/mm³