Assessment

View Details

Coagulation disorders and lab findings

Condition	PT	APTT	Bleeding time	Platelet count
Warfarin or Vit K deficiency	Prolonged	Normal or mildly Prolonged	Normal	Normal
DIC	Prolonged	Prolonged	Prolonged	Reduced
Uremia	Normal	Normal	Prolonged	Normal
Aspirin/Clopidogrel	Normal	Normal	Prolonged	Normal
Thrombocytopenia	Normal	Normal	Prolonged	Reduced
Liver failure	Prolonged	Normal or Prolonged	Normal or Prolonged	Normal or Reduced
Von Willebrand’s disease	Normal	Normal or Prolonged	Prolonged	Normal
Factor V deficiency	Prolonged	Prolonged	Normal	Normal
Hemophilia	Normal	Prolonged	Normal	Normal
Factor XII deficiency	Normal	Prolonged	Normal	Normal
Glanzmann’s thrombasthenia	Normal	Normal	Prolonged	Normal

PT monitors extrinsic and common pathway, reflecting activity of factors I, II, V, VII and X.

PTT monitors intrinsic and common pathway, reflecting activity of all factors except VII and XIII.

Bleeding time assesses platelet function.

Test

Basic

PT and INR = extrinsic

Thromboplastin is added to blood to activate the extrinsic pathway. Clotting time is measured in seconds (PT). This is compared to the normal value (12–13s) to get the INR for ease of comparison (normal 0.8–1.2).

Aid to memoire: WEPT – Warfarin Extrinsic Prothrombin Time

Prior to the common pathway, only factor 7 is involved (isolated deficiencies are rare), so PT/INR is only really affected by global reduced clotting factor synthesis or increased consumption

Warfarin/vitamin K deficiency
Liver disease
DIC

APTT = intrinsic

Phospholipid, a contact activator, and calcium are added to blood to activate the intrinsic pathway. Clotting time is measured in seconds (normal = 30–50s).

Involves similar clotting factors to the extrinsic pathway plus some others (8, 9, 11) so affected by

Like the extrinsic path

Warfarin/vitamin K deficiency
Liver disease
DIC

PLUS anything which affects factors 8 (haemophilia A/Von Willebrand’s), factor 9 (haemophilia B), factor 11 (haemophilia C)
Notes

Factor 12 is also involved in the intrinsic pathway but deficiencies are rare
Anti-phospholipid syndrome is a common cause of a misleadingly prolonged APTT, due to antibody-mediated inactivation of the phospholipid added to activate the intrinsic pathway in the laboratory

Bleeding time = platelet function

Involves making a patient bleed and timing how long it takes to stop

Measures platelet plug formation so only affected by conditions involving platelet quantity/function

Thrombin time = fibrinogen test

Thrombin is added to blood to test the conversion of fibrinogen to fibrin to form a clot

This tests the level and function of fibrinogen

Coagulation assays and interpretation

Assay	Description	Interpretation
Thrombin clotting time (TT)	Thrombin added to anticoagulated (citrate/oxalate tube) blood and time to clot formation measured	Prolonged with heparin, fibrin degradation products and fibrinogen deficiency
Bleeding time	Not a laboratory test— involves making a cut in skin and measuring time for bleeding to stop. Has been replaced by platelet function assays	Prolonged in platelet dysfunction, but also other circumstances where bleeding tendency not reflected in laboratory tests (e.g. uremia)
Platelet function assay	Blood clotting assessed on collagen/epinephrine (Col/Epi) and collagen/ADP membranes. Normal Col/Epi closure time (< 180 s) excludes significant platelet function defect. If Col/Epi time prolonged, Col/ADP (Normal < 120 s) is automatically performed	Abnormal Col/Epi but normal Col/ADP suggests aspirin induced platelet dysfunction. Abnormal Col/Epi and Col/ADP suggests anemia, thrombocytopenia, or von Willebrand disease
INR/PT	Tests extrinsic + common pathway— factors I, II, V, VII and X. Performed by adding tissue factor (III) to anticoagulated sample and measuring time to clot formation. Each batch of tissue factor has differences; therefore this is standardized in INR	Prolonged PT due to vitamin K deficiency, warfarin, liver disease or DIC
Activated partial thromboplastin time (aPTT)	Tests intrinsic + common pathway I, II, V, VIII, IX, XI, XII— will be normal despite factor VII or XIII deficiency. Anticoagulated blood mixed with phospholipid, activator and calcium and time to clot formation measured. “Partial” = absence of tissue factor (III) from mixture	Prolonged APTT suggests heparin, antiphospholipid antibody, factor deficiency or anti-factor antibodies
Mixing (50:50) study	Plasma of patient with abnormal PT/APTT mixed with normal plasma in 50:50 ratio	Correction of PT/APTT suggests factor deficiency. Persistence of abnormal PT/APTT suggests presence of inhibitor (warfarin, heparin, lupus anticoagulant, coagulation factor antibodies)
Factor V Leiden	Requires genetic test. Mutated factor V cannot be inactivated by Protein C	—
Dilute Russell’s viper venom time (aRVVT)	Test for lupus anticoagulant (antiphospholipid antibodies) as venom requires phospholipid for activation	Prolonged aRVVT as antiphospholipid antibodies impair venom activation

Made with Bullet