Test BackgroundThe coagulation screen comprises PT, APTT and fibrinogen.
Prothrombin time (PT)
• used as a measure of the extrinsic pathway of coagulation and determines the clotting tendency of blood. PT measures factors I, II, V, VII and X
• an abnormal PT suggests either a congenital or an acquired deficiency of a clotting factor in the extrinsic or common pathway
• deficiencies can be acquired in liver disease, vitamin K deficiency or treatment with vitamin K antagonist drugs, ie. warfarin, sinthrome, dindevan or phenindione
• if there is no suggestion of any of these deficiency states, a congenital deficiency should be considered
• International Normalised Ratio (INR) is derived from the PT; it is used to monitor warfarin dosage and will only be reported where a patient is taking warfarin or other dicoumarol drug (see INR)
Activated partial thromboplastin time (APTT)
• a global screening procedure used primarily to evaluate coagulation abnormalities in the intrinsic pathway but will also detect severe functional deficiencies in factors II, V, X or fibrinogen
• also widely advocated as a means to monitor the effectiveness of unfractionated heparin therapy, where the clotting time is prolonged in proportion to the level of heparin; this is normally reported as a ratio to the midpoint of the APTT reference range (see APTTR)
• APTT and APTTR are inappropriate for monitoring low molecular weight heparin (LMWH) dosage; the anti Xa assay is normally used for monitoring LMWH (see HEPAXA)
• in patients receiving oral anticoagulants, the circulating levels of factors II, VII, IX and X are reduced, therefore the APTT can be expected to be prolonged
• the presence of non-specific inhibitors, such as lupus-like anticoagulant, may prolong the APTT, but this effect is variable and specific lupus anticoagulant testing should be requested where suspected (see lupus)
• deficiencies of factors VIII, IX, XI and XII, and rarely von Willebrand factor (if causing a low factor VIII level), may lead to a prolonged APTT which will correct following mixing studies with normal plasma
• deficiencies of fibrinogen can indicate disseminated intravascular coagulation (DIC) or can be an indicator of bleeding problems or liver and renal dysfunction
• reduced fibrinogen can also be seen after massive transfusion and is used as an indicator for the requirement of fresh frozen plasma
• abnormal clotting findings may be followed by specific factor assays to enable differential diagnosis of acquired vs. inherited causes
A thrombin time (TT) may be reported in a small number of cases where other coagulation screen parameters are abnormal, if deemed appropriate.
|PT||:|| 9-12 s
|APPT||:|| 23-31 s
|Fibrinogen||:|| 1.8-4.0 g/L
Sample Required 4.5 mL sodium citrate (pale blue top) adults
1.8 mL sodium citrate (pale blue top) paediatrics
Turnaround Time 4 hours
NotesSpecial handling: avoid prolonged stasis during venepuncture.
Sample must be received by lab within 4 hours of collection.
Please note: samples will be rejected if underfilled or overfilled.