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CEACCP > Conventional and near-patient tests of coagulation > Flashcards

Conventional and near-patient tests of coagulation Flashcards

1
Q

What is primary haemostasis?

A

Coagulation is initiated within seconds of a breach of the vasculature, with platelets forming a plug at the site of injury

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2
Q

What is secondary haemostasis?

A

Complex interaction between plasma coagulation factors, resulting in fibrin strands forming to strengthen the platelet plug

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3
Q

What does 100ml of blood need to clot?

A
  • 0.2 mg FVIII
  • 2mg FX
  • 15mg prothrombin (FII)
  • 250mg fibrinogen (FI)
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4
Q

What prevents propagation of haemostasis throughout the vasculature?

A

Inhibitors, of which the most important are:

  • tissue-factor pathway inhibitor (TFPI)
  • protein C
  • antithrombin
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5
Q

What are the two pathways in secondary haemostasis?

A

Extrinsic and intrinsic, uniting at a final common pathway

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6
Q

What do patients with severe deficiencies of high molecular weight kininogen (HMWK), prekallikrein and FXII have?

A

All have a prolonged APTT but no bleeding disorder

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7
Q

What are the 5 steps of coagulation?

A
  1. Initiation
  2. Amplification
  3. Propagation
  4. Stabilisation
  5. Inhibition of further coagulation
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8
Q

What happens in the initiation stage of coagulation?

A

Tissue factor (TF) binds to circulating FVIIa and in the presence of FV converts FIX to FIXa, and FX to FXa.

FXa binds to prothrombin to generate a small amount of thrombin.

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9
Q

What % of total circulating FVII is present as FVIIa?

A

Only 1% is present as FVIIa and available for the first step of initiation

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10
Q

What is amplification in coagulation?

A

The small amount of thrombin generated by initiation is not enough to convert fibrinogen into fibrin.

  1. Tissue factor, thrombin, FIXa, FXa complex activate more FVII
  2. Thrombin activates FVIII to increase action of FXIa on FX
  3. Thrombin activates FV to increase the action of FXa on prothrombin
  4. Thrombin activates FXI to FXIa increasing FIXa
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11
Q

What is propagation in coagulation?

A

The tissue factor/FVIIa complex ensures supply of FIXa

FIXa with FVIIIa activates FX to ensure an adequate supply of FXa and maintains continuous thrombin generation (thrombin burst)

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12
Q

What is stabilisation in coagulation?

A

High levels of thrombin stimulate FXIII to cross link the soluble monomers and the protection of the clot by thrombin-activatable-fibrinolysis-inhibitor

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13
Q

How is inhibition of further coagulation activated?

A

Overwhelming thrombosis is controlled by:

  1. Thrombin activated protein C (aPC) cleaves FVa and FVIIIa
  2. TFPI inhibits TF-VIIa and FXa by binding them in a quaternary complex
  3. Antithrombin inhibits thrombin, FIXa and FXa
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14
Q

How is blood clotting prevented in the sample tubes for coagulation?

A

Whole blood is mixed with calcium-chelating agent such as EDTA or citrate in a ratio of 9:1

Underfilling the tube or haematocrit > 0.55 both reduce plasma volume in the sample and prolong clotting time artefactually due to over-anticoagulation

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15
Q

What are coagulation tests performed on?

A

They’re performed on platelet poor plasma after centrifugation

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16
Q

What do PT, APTT and TCT measure?

A

The time taken to form a clot in vitro after recalcification of the sample in the presence of the appropriate test reagent.

Time to fibrin strand formation is detected using either a photo-optical or electromechanical device

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17
Q

What is the prothrombin time?

A

The time taken to fibrin strand formation when platelet-poor plasma is recalcified in the presence of thromboplastin (tissue factor and phospholipid)

18
Q

Which test is most sensitive to a decrease in FVII?

A

INR

19
Q

What deficiencies and diseases will prolong the INR?

A
  • FI (fibrinogen)
  • FII (prothrombin)
  • FV (proaccelerin)
  • FX (thrombokinase)
  • liver disease
  • vit K deficiency
  • DIC
  • high doses of heparin
20
Q

What is an INR?

A

It is a ratio of the patient’s PT to a control plasma PT, raised to the power of a correction factor known as the interntaional sensitivity index specific for each thromboplastin reagent.

21
Q

Why does INR not detect bleeding tendency associated with haemophilia?

A

The amount of TF present in the initiating thromboplastins is so large that it negates the effect of TFPI and renders the test independent of FVIII, FIX and FXI.

22
Q

What does APTT measure?

A

All procoagulant factors except FVII and FXIII.

23
Q

How is APTT distinguished from PT?

A

The term “partial thromboplastin” indicates that the reagent contains phospholipids (as a substitute for platelet membrane) but no tissue factor, distinguishing it from the PT.

24
Q

What happens in the APTT test?

A

Platelet-poor plasma is “activated” by a 3 min pre-test incubation with the APTT reagent. In addition to phospholipids, this reagent contains a contact activator that is a fine suspension of negatively charged particles (kaolin, celite or ellagic acid)

The sample is then recalcified and the time taken to fibrin strand formation is the APTT

25
Q

What is APTT used to monitor?

A

Unfractionated heparin therapy and as a screening tool for haemophilia A, B and coagulation inhibitors

26
Q

What is APTT prolonged by?

A

A reduction in procoagulant activity to <30-40% of normal (factor and reagent dependent) by the presence of a specific clotting factor inhibitor, or by the presence of heparin or the lupus anticoaglant, which acts as a non specific intrinsic pathway inhibitor.

27
Q

What will artificially prolong the APTT?

A
  • The contamination of the sample with heparin at the time of collection
  • an inaccurate whole blood: citrate ratio
28
Q

What does thrombin time measure?

A

Thrombin time (TCT or TT) tests the fibrin polymerisation process

29
Q

How is the thrombin time test performed?

A

A standard concentration of human thrombin is added to citrated, platelet-poor plasma and time to clot formation is measured

Clot formation only requires presence of fibrinogen and the absence of thrombin inhibitors

TCT is independent of all other factor deficiences

30
Q

What is the commonest thrombin inhibitor? Which test is most sensitive for this?

A

Heparin (TCT is more sensitive than APTT for detection of heparin)

31
Q

What is TCT prolonged by?

A
  • hypofibrinogenaemia
  • dysfibrinogenaemia
  • the presence of fibrin degradation products
  • myeloma proteins
32
Q

What are the most frequently used tests in routine clinical practice for measuring fibrinogen?

A
  • Clauss assay
    • addition of high concentration thrombin to diluted test plasma, clotting time compared with a calibration curve - result in g/litre
    • results adversely affected by heparin eg ECMO
    • heparin effect can be negated by use of ion exchange resin or heparinase enzymes
  • PT-derived fibrinogen level (PT-Fg)
    • compare the PT of the test sample with a reference curve, indirectly measuring fibrinogen
    • rapid and at no extra cost if PT is measured
33
Q

What is the haemochron activated clotting time?

A
  • 2mls of whole blood into a black topped test tube containing celite and ferro-magnetic bar, warmed to 37 degrees and rotated
  • time to coagulation measured (detected by magnetic sensor)
34
Q

When is haemochron activated clotting time used?

A
  • where assessment of systemic heparinization is required almost instantaneously
    • cardiac surgery
    • cardiac catheterisation
    • haemofiltration
35
Q

What are normal values for HACT? What values are required for cardiopulmonary bypass and haemofiltration?

A

Normal is 120-140s

For bypass: ACT >480s

In the presence of aprotinin: >700s

For haemofiltration: ACT > 200s

36
Q

What will prolong the ACT when using the haemochron activated clotting time?

A
  • Overfilling
  • Inadequate mixing
  • thrombocytopaenia
  • warfarin
  • pro-coagulant (dilutional) deficiencies
37
Q

What can the hemochron Jr signature give values for?

A
  • deriving the ACT
  • APTT
  • and PT
38
Q

What does the test consist of in the Hemochron Jr signature?

A
  • test-specific cuvettes are pre-warmed to 37 degrees
  • 50 microlitres of fresh/citrated whole blood is placed onto this
  • 15 microlitres is aspirated into the test channel
  • blood mixed with test-specific reagent and moved back and forth within the test channel
  • as coagulation occurs, optical sensors detect the impeded movement, timer stops and audible tone sounds
39
Q

What reagent is used in the hemochron Jr signature?

A

A mix of silica, kaolin and phospholipids

40
Q

Which is faster, the Haemochron, or Haemochron Jr?

A

The haemochron Jr.

41
Q

Does high dose aprotinin therapy affect the Haemochron Jr?

A

No

42
Q
A

CEACCP (16 decks)

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