4.5 Laboratory Tests of COagulation

Question 1

The routine tests of the coagulation cascade include:
• Prothrombin time (PT) and activated partial thromboplastin time (APTT): routine coagulation screens
• Thrombin time (TT): part of routine screen in some hospitals
• Fibrinogen: routine in some hospitals
• D-dimers/fibrin degradation products (FDPs)

FBC test VS Coagulation studies

• Blood must be taken with the ____________ VS Blood must be taken into a citrated tube (blue topped) → citrate reversibly chelates calcium in the sample
• Proper coagulation studies cannot be done VS _____________ ratio must be met, or the samples will be thrown away

Answer 1

irreversible anticoagulant EDTA;

Correct blood : citrate

Question 2

APTT measures the intrinsic pathway of the classical coagulation system, which begins from factor XII up to the production of fibrin:

Spin citrated sample and remove cells → Add source of ___________ as conversion of factor IX to IXa and X to Xa require it → Activate factor XII by adding _______________ to trigger contact activation → Add ________ to start the reaction (because all of it has been chelated by citrate)

• The time taken to form a fibrin clot is then measured (usually about 25 – 30 seconds)
• If APTT is normal, it suggests normal amounts of factors I (fibrinogen), II (prothrombin), V, VIII, IX, X, XI, XII → intact common pathway

Answer 2

phospholipids;

inert substances (e.g. silica);

calcium

Question 3

[Low APTT}

Reasons

• Low coagulation factor in intrinsic pathway: Low factor XII, low factor XI, low fibrinogen
• Multiple deficiencies in coagulation factors: ________
• Inhibitor of coagulation factor: Antibodies to factor VIII
• Inhibitor of phospholipid: Anti-phospholipid antibody (________________)
• Drugs: _____________

There are several steps to determine the cause of the long APTT:
1. Consider if the prolonged APTT is due to a factor deficiency or antibody interference:
• Sample is mixed with normal plasma (50 : 50 mix) and APTT is repeated
• If normal, it suggests that the problem has been corrected by adding normal plasma containing normal factors (prolonged APTT due to factor deficiency and not an inhibitor)
• If not normal, the prolonged APTT is due to an inhibitor (non-specific inhibitors can be present in sick patients or as part of an anti-phospholipid syndrome like lupus anticoagulant, anti-phospholipid antibody or ____________________
2. Measure relevant factors and find out which one is low
• Factors XII, XI, IX, X, VIII, V, fibrinogen
• If TT (measuring common pathway) is normal, it is not fibrinogen; if PT (measuring extrinsic pathway) is normal, it is not fibrinogen, factors VIII and V

Answer 3

Liver diseases ;

lupus anticoagulant;

Heparin;

anti-β2 glycoprotein 2 antibodies)

Question 4

PT measures the extrinsic pathway of the classical coagulation system, which begins from tissue factor and factor VII up to the production of fibrin:

Spin citrated sample and remove cells → Add recombinant tissue factor to activate factor VII to VIIa → Add source of ___________ as conversion of factor IX to IXa and X to Xa require it → Add ________ to start the reaction (because all of it has been chelated by citrate)

• The time taken to form a fibrin clot is then measured (usually about _______________ → shorter due to fewer number of steps involved)
• If APTT is normal, it suggests normal amounts of _________________

Answer 4

phospholipids;

inert substances (e.g. silica);
calcium

9 – 11 seconds;

factors I (fibrinogen), II (prothrombin), V, VII, X

Question 5

[Long PT}

Reasons

• Low coagulation factor in extrinsic pathway: Low factor VII, low factor X, low factor V
• Multiple deficiencies in coagulation factors: Liver diseases, vitamin K deficiency
• Drugs: Warfarin (leads to deficiency of factors ________)

There are several steps to determine the cause of an unexplained long PT:

1. Consider if the patient has a factor deficiency by mixing the sample 50 : 50 with normal plasma (which corrects the deficiency) and redoing the PT
2. Consider if the APTT, fibrinogen levels and TT are normal
3. Measure ____________ (all other factors are in the common pathway and would affect APTT as well)

Answer 5

II, VII, IX, X;

factor VII

Question 6

TT and fibrinogen levels measure different aspects of the final common pathway of the classical coagulation system, which begins from factor X up to the production of fibrin:
• Fibrinogen levels are easily obtained from laboratory results
• TT is measured by adding thrombin to the patient’s plasma sample and the time taken to form a fibrin clot is then measured (usually about 11 – 17 seconds)
o If TT is normal, it suggests normal amounts of functionally normal fibrinogen (not a ___________) and nothing interfering with conversion of fibrinogen to fibrin (e.g. ________________)

• In normal coagulation, fibrinogen is converted to fibrin, which is then cross-linked by factor XIIIa, then plasmin degrades fibrin and fibrinogen to form FDPs:
o Large amounts of fibrin formed means increase in FDPs
o Fibrinogen may increase in the ______________, so FDPs rise non-specifically
• D-dimers are another type of FDP, but they refer to ________________ → increased D-dimers indicates increased clot formation (more specific for fibrin formation than FDPs)

The fibrinogen and FDP levels should be measured in patients with prolonged TT to determine the cause, and patient must be checked for heparin use:
• Prolonged TT may be caused by low fibrinogen, abnormal fibrinogen (dysfibrinogenaemia), something interfering with conversion to fibrin (e.g. D-dimers or FDPs), or _____________ (most common cause in hospitalised patients)

Answer 6

dysfibrinogenaemia;

fibrinogen degradation products/FDPs;

acute phase of inflammation (acute phase protein) or infection;

cross-linked fibrin degradation ;

heparin use

Question 7

DISORDERS OF PRIMARY HAEMOSTASIS
- Primary haemostasis is the formation of the primary platelet plug which involves platelets, blood vessel wall and von Willebrand factor (vWF):

Component affected

• Platelets: Thrombocytopenia (low platelet count). Abnormal platelet function (inherited/acquired (e.g. _________, renal failure)
• Vessel wall: Hereditary vascular disorders, Scurvy (vitamin C deficiency), steroids, age (e.g. ___________)
• von Willebrand factor: von Willebrand disease (a group of different conditions including reduction in vWF and dysfunctional vWF)

Answer 7

aspirin;

senile purpura

Question 8

PLATELET DISORDERS
To test for platelet disorders, the platelet count, bleeding time and PFA 100 (in specialised laboratories) are measured:
• Platelet count: commonly measured
• Bleeding time: subjective → incision in patient’s forearm must be made to measure time taken to stop bleeding (varies individually; affects bleeding)
• Platelet function analyser (PFA)-100: place citrated blood into the machine, where it passes through an aperture coated with _________________ (causes platelet adhesion, activation and aggregation that close the aperture)
o Measures the time taken for platelets to form across the aperture

Answer 8

collagen and epinephrine or ADP

Question 9

VON WILLEBRAND DISEASE
If von Willebrand disease is suspected, how much vWF and whether the vWF works properly must be considered:

• Quantity: ____________ (if normal, indicates dysfunctional vWF instead)
• Function: _____________, collagen binding assay (specialised tests only performed in specific hospitals)
• Bleeding time, PFA-100 may be used (vWF sticks platelets together and would prolong these timings if dysfunctional)
  • Normal bleeding time suggests normal platelet and vWF count and function

Answer 9

vWF antigen level;

Ristocetin cofactor (Ricof)

Question 10

VASCULAR DISORDERS
There are no easy tests for vascular problems, and a clinical assessment is important:
• May present with ______________ (do genetic studies)

Answer 10

loose skin or positive family history