Sampling for Coagulation Tests
Blood samples should preferably reach the laboratory within two hours. Samples greater than twenty-four hours old will not be tested.
Coagulation tests can only be performed on samples taken by clean venesection and filled to the correct level (indicated by the black arrow on the vacutainer).
Samples that are underfilled, overfilled, haemolysed or have been drawn slowly may give false results, potentially masking significant abnormalities. As such, these samples will not be processed.
Samples must not be taken from indwelling lines that have been flushed with Heparin as contamination frequently occurs giving erroneous results.
Please state clearly on the request form if the patient is receiving anticoagulation.
Investigation of possible bleeding disorders
The most important indicator of bleeding risk is the patient history.
If a bleeding disorder is suspected please initially request a Clotting Screen and a Full Blood Count. These will detect the vast majority of acquired bleeding disorders and most severe hereditary bleeding disorders. A few disorders, most notably von Willebrand’s Disease, cannot be excluded with these tests.
If the clotting screen is abnormal, or if there is a strong history of abnormal bleeding episodes despite a normal clotting screen result, the laboratory or a Consultant Haematologist should be contacted for advice about further investigation.
The Coagulation Screen
The prothrombin time (PT) and activated partial thromboplastin time (APTT) provide a basic screen of coagulation potential. This test forms the basis of any further investigations into any coagulation problems, and should be requested prior to a patient starting anti-coagulant therapy.
In accordance with the Trust policy, a Coagulation Screen will only be performed in the following instances:
- Investigation of a patient with a significant history of bleeding or bruising
- Monitoring coagulopathy associated with massive transfusion.
- As part of an investigation into Disseminated Intravascular Coagulation (DIC)
- In patients with liver or gall bladder disease
- In patients with Systemic Lupus Erythematosis (SLE)
- In cases of Intra-Uterine Death (IUD)
- Patients having liver or renal biopsies, Endoscopic Retrograde Cholangio-Pancreatography (ERCP), insertion of central venous lines, or insertion of permanent pacemakers.
- For baseline screening prior to starting anticoagulation
- Patients on intensive care and high dependancy units (DCC, ITU and HDU)
- in patients with pre-eclampsic toxaemia (PET)
- In patients with Cystic Fibrosis
- In oncology patients with thrombocytopenia
- In drug overdoses
- In patients due to undergo radiological procedures
- In patients <12 years old
Monitoring Anticoagulant Therapy
Warfarin (INR) and Heparin (APTT Ratio)
The two most common anticoagulants Warfarin and Unfractionated Heparin are monitored via the INR and APTT Ratio respectively. A baseline coagulation screen should always be requested prior to starting a patient on anticoagulants.
Heparin in Pregnancy
The monitoring of subcutaneous Heparin therapy in pregnancy is performed using a Heparin assay. The Consultant Haematologist should always be notified when a patient is placed on this therapy as special requirements are required for monitoring.
Disseminated Intravascular Coagulation (DIC)
A DIC screen should always be considered when there is an unexpected fall in platelet count, particularly with systemic infection. In addition to a coagulation screen; fibrinogen and D-Dimer levels should be requested. A prolonged clotting screen, reduced Fibrinogen and raised D-Dimer levels are indicative of DIC; however, these results may also be found post-operatively and in advanced liver disease. Raised D-Dimer levels are also found in DVT.
D-Dimer Requests for Suspected DVT and PE
D-Dimers should only be requested in the following instances:
- LOW or INTERMEDIATE PROBABILITY (Well’s score) of DVT
- LOW PROBABILITY (BTS scoring) of PE
Relevant clinical details, and the probability score, must be recorded on the request form or the test will not be performed.
Factor Assays (Prolonged Clotting Screen)
Prolongation of the coagulation screen that can be corrected with normal pooled plasma may be the result of a specific factor deficiency. Specific assays of each of the coagulation factors can be performed to determine the nature of this deficiency.
Factor Assays (Known Haemophilia A or B)
These patients should always be discussed with a Consultant Haematologist. Factor assays are required prior to surgery and to monitor replacement therapy.
Occasionally, prolongation of the coagulation screen cannot be corrected with normal pooled plasma. The most common reason for this is the presence of a Lupus inhibitor. This antiphospholipid antibody interferes with the phospholipid-containing reagents used in coagulation tests, giving high results. Despite the prolonged clotting time, bleeding problems are rare in these patients; and more commonly, the patients are prone to thrombotic events. Complex investigations including a Dilute Russels’ Viper Venom Time (DRVVT) are required to confirm the presence of a Lupus inhibitor.
A complete thrombophilia screen includes screening for deficiencies of the coagulation cascade inhibitors Antithrombin (formerly Antithrombin III), Protein C and Protein S: Activated Protein C resistance (Factor V Leiden mutation); Prothrombin gene mutation; and screening for a Lupus inhibitor (DRVVT).
Thrombophilia should be considered in:
- Unexpected thrombosis under the age of 45.
- Strong family history of thrombosis under the age of 45.
- Thrombosis in an unusual site.
- Patients well controlled on oral anticoagulants who continue to thrombose.
Patients should not be tested in the acute phase during suspected thrombosis (ie. at least one month post event). They should also stop anticoagulant therapy prior to testing.