Clotting Screen

Routine clotting tests comprise of the prothrombin time (PT), fibrinogen, activated partial thromboplastin time (APTT), thrombin time and D-Dimer.  Abnormal clotting results that cannot be explained in the clinical context are investigated by Specialist Laboratory Haematology: Thrombosis and Haemostasis laboratory at St James Hospital and at Bradford Royal Infirmary.

The clotting screen is a bundled group of tests used pre-operatively to assess bleeding risk and used to monitor bleeding conditions and some therapies.  It tests the PT (and INR), APTT and fibrinogen.

View the current BCSH guidelines for assessing bleeding risk prior to surgery.

Prothrombin Time (PT)

The PT measures the vitamin K dependant clotting pathways (extrinsic pathway) and is therefore of particular use in measuring the effect of warfarin therapy (warfarin is a vitamin K antagonist).  Normal ranges for the PT vary up and down the country and worldwide depending on the sensitivity of the reagent and the technology used to determine the clotting times.  The INR (international normalized ratio) is a calculation which is used that takes into account the normal clotting times and the sensitivity of the reagent used to provide a ratio which is the same worldwide.  For example, a PT of 16seconds in Leeds and a PT of 33 seconds in London can produce the same INR.  The prothrombin time is also used to determine if there are any deficiency in extrinsic clotting factors and is a useful liver function test.  Raised PTs without cause should be investigated further. It is useful to view the guidelines for the management of patients on oral anticoagulants requiring dental surgery. and the guidelines for Oral Anticoagulant therapy (WARFARIN).

Activated Partial Thromboplastin Time (APTT)

Measures the intrinsic clotting pathway.  Is particularly useful in monitoring heparin therapy.  The APTT ratio provides the ratio of APPT : Normal Clotting time and is the primary calculation used to monitor heparin therapy.  The APTT is also useful in detecting clotting factor deficiencies of the intrinsic pathway and can be raised in the presence of factor deficiencies and lupus anticoagulants.  Raised APTTs without cause should be investigated further.  The APTT ratio calculation may change from time to time and you should contact the laboratory for advice if you are calculating your own ratio.

View the guidelines on the use and monitoring of heparin.

Thrombin Time

Is primarily requested by the Liver Disease Units and measures the time it takes for fibrinogen to form fibrin (one of the later stages of the common clotting pathway).  It is also requested by the laboratories to confirm the presence of heparin contamination of a sample in the event of an unexplained raised APTT.

Fibrinogen

The fibrinogen reported routinely is derived from the PT reaction as it occurs.  It is primarily a screen.  Any low fibrinogen detected by this method is substituted for the Clauss Fibrinogen test which measures fibrinogen directly.  In DIC (Disseminated Intravascular Coagulation) the derived fibrinogen may be misleading.  Supplying relevant clinical details will ensure the biomedical scientists report the most appropriate fibrinogen for the clinical situation.  The Clauss Fibrinogen can be measured routinely if asked for on the request card and if DIC is suspected; the Clauss fibrinogen should always be measured.

D-Dimers

During the formation of a stable clot, amino acid are excised by various enzyme cascade reactions.  The D-Dimer peptide is excised from the D portion of fibrin as the clot hardens.  It is therefore a useful predictor of recent clot formation.  It is not specific however and can be affected by many other conditions such as rheumatoid arthritis.  It should only be used as a negative predictor for VTE (venous thromboembolism)  i.e. a raised D-Dimer is not diagnostic of a clot formation, but a normal D-Dimer can be a used as a negative predictor of venous thrombosis.  In DIC the D-Dimer levels are often very high.

View the guidelines on the diagnosis and management of DIC.

View the guidelines for Diagnosis of deep vein thrombosis in symptomatic outpatients and the potential for clinical assessment and D-dimer assays to reduce the need for diagnostic imaging.

The INR and APTT ratio do not have normal ranges but therapeutic ranges.  These are based on the condition being treated and are decided by the clinical teams.

Links

British Committee for Standards in Haematology (BCSH) - Guidelines Pages


Page updated: 21/02/17 | Updated by: Michael O'Sullivan