Coagulation tests
Activated partial thromboplastin time (APTT)
Also known as partial thromboplastin time with kaolin, PTTK, this tests the intrinsic and common pathways.
Interpretation:
- Normal APTT does not exclude mild, but clinically significant, coagulation factor deficiency (e.g. as in mild haemophilia, von Willebrand's disease) as many reagents give a prolonged APTT only at coagulation factor levels of 30%.
- Prolonged APTT with PT normal
- Heparin therapy
- Suggests deficiency of factor VIII, IX, XI or XII.
- Prolongation APTT with prolonged PT
- suggests factor X, V, II or I (fibrinogen) deficiency, all of which are rare. (The APTT is normal in factor VII deficiency (PT prolonged) and factor XIII deficiency).
- High dose heparin
- DIC
- A prolonged APTT which is not corrected by the in vitro addition of normal plasma
- coagulation factor (VIII or IX) inhibitor or a lupus inhibitor.
Artefactual prolongation of the APTT may be due to the presence of heparin in the sample, difficult or slow collection, addition of an incorrect volume of blood to the tube, delay in mixing blood with the citrate anticoagulant, suboptimal specimen storage or a prolonged interval between collection and testing.
This tests the extrinsic and common pathways.
More sensitive than the APTT for the detection of coagulation factor deficiencies due to vitamin K deficiency, liver disease.
The results are expressed as an international normalised ratio (INR) when the test is used to monitor oral anticoagulant therapy. The INR is not, however, valid for other patients, especially those with liver disease.
Interpretation:
An abnormal result is most often due to
- liver disease
- vitamin K deficiency
- oral anticoagulant therapy
- high dose heparin
Investigation of possible acquired or inherited disorders of haemostasis; occasionally indicated in the investigation of unexplained thrombosis.
Interpretation
Prolonged by
- afibrinogenaemia,
- hypofibrinogenaemia,
- dysfibrinogenaemia.
- heparin (corrects with protamine),
- fibrin degradation products,
- paraproteins (partial correction with protamine)
Reptilase Time
Reptilase is a thrombin-like enzyme that differs from thrombin in its specificity and extent of cleavage of the fibrinogen molecule. Reptilase is inhibited only slightly or not at all by Heparin and fibrin degradation product (FDP), thus making it useful in the differential diagnosis of a prolonged thrombin time (TT).
Interpretation
Prolonged by:
- lupus anticoagulant
- abnormal fibrinogen or low fibrinogen levels
A reptilase time is only needed in the presence of a prolonged thrombin time. In this case, if the reptilase time is normal, the presence of heparin is confirmed.
If it is prolonged, heparin can be excluded and other causes of prolonged thrombin time may be pursued such as: hypofibrinogenemia and dysfibrinogenemia.
Activated clotting time (ACT)
This is performed as a 'bedside' test. It is used to quantify the heparin effect during haemofiltration. The normal range is 100-140 seconds.
Clinical use of the ACT in assessing adequacy of heparinisation
- A linear heparin/dose response curve has been well documented
- Many reports have shown less blood loss with no increase in fibrin formation using ACT guided heparin dosing as opposed to protocol dosing in cardiopulmonary bypass surgery
- Serial ACT measurements to develop individual heparin/dose response curves should be used.
- "Target" ACT is very system and unit dependent
Interpretation
Prolonged by
- Heparin
- Hypothermia
- Platelet dysfunction
- Haemodilution
- Cardioplegic solutions
- Hypofibrinogenaemia
- Factor deficiencies
Blood products
Blood
10 - 15ml/kg
Keep Hb > 10g/dl in acute resuscitation phase - (generally first 24 hours)
Transfusion threshold for stable critical care patients 7g/dL
FFP
15 ml/kg if APTT or PT > 1.5 X normal
Cryoprecipitate
5ml/kg if Fibrinogen < 1.0
FFP contains some fibrinogen, so if received FFP, recheck fibrinogen level before ordering cryoprecipitate
Platelets
10-15ml/kg
Transfusion thresholds (for impaired platelet production)
< 10
< 20 and DIC, or platelets likely to fall by 10 by time of next sample
< 50 and acute bleeding
< 100 and CNS bleeding or multiple trauma
D/W consultant if ITP or previous intrauterine transfusions
Check platelet increment 30mins after transfusion, high consumption eg. sepsis may need repeat transfusions