Q3i / 17A09 / 14B08: Outline tests of coagulation

17A09: Exam Report

Outline how the following tests assess coagulation:

61% of candidates passed this question.

a. Prothombin Time (PT)
b. Activated Partial Thromboplastin Time (APTT)
c. Activated Clotting Time (ACT)
d. Thromboelastography (TEG or ROTEM)

Many candidates incorrectly stated that the PT assessed the intrinsic system and that the APTT assessed the extrinsic system.

This led to subsequent errors in relating a coagulation test to the appropriate coagulation factors that it assessed.

Some candidates produced elaborate diagrams of the coagulation cascade in isolation without relating it to the question.

14B08: Exam Report

Outline how the following tests assess coagulation:

0% of candidates passed this question.

a. Prothombin Time (PT)
b. Activated Partial Thromboplastin Time (APTT)
c. Activated Clotting Time (ACT)
d. Thromboelastography (TEG or ROTEM)

It was expected candidates would cover all aspects of testing for each test listed. This would include normal, abnormal or therapeutic values, a comment on methods (either laboratory or point of care) and coagulation pathway assessment.

General statements about the overall purpose of the test, collection methods, plasma vs whole blood as sample scored additional marks. Diagnoses or errors associated with abnormalities in each test would also have scored marks but were not mentioned in most answers.

Overall there was a lack of depth of knowledge and incorrect facts. Many candidates knew about TEG, but did not know details about the other tests.

Q3i / 17A09 / 14B08: Outline tests of coagulation

  • All labs tests of coagulation are adversely affected by poor sampling technique
  • Collected blood sample is mixed with a calcium chelating agent (ie EDTA/citrate) at a ratio of 9:1 to prevent clotting

Testing Pathway

PT

Extrinsic & Common

APTT

Intrinsic Pathway

All factors except VII & XIII

ACT

Intrinsic Pathway

Point of care testing (less transport, whole blood used, does not require lab)

Used when assessment of systemic heparinisation requires instant testing ie cardiac surgery

TEG/ROTEM

Clot formation to Clot Lysis

Anti-Xa

Activity of heparins

Bc LMWH are many different lengths their concentration is difficult to measure so anti-Xa activity is used as surrogate

LMWH bind ATIII and increase its activity x1000 fold

LMWH primarily affect Xa levels

So anti-Xa activity is proportional to concentration of LMWH

Prolonged

PT

Most sensitive to ¯VII

Also prolonged with I, II, V, X deficiencies, liver disease, Vit K deficiency, DIC

APTT

Heparin

Antiphospholipid Ab

Hemophilia

Deficiency in I, II, V, VIII, IX,  X, XI, XII, vWF (causing low VIII)

ACT

Systemic heparinisation

CPB requires ACT >400sec to prevent clotting in circuit

TEG/ROTEM

See below

Anti-Xa

High dose LMWH

Renal failure

Normal Value

PT

12 – 13 seconds

INR 0.8-1.2

APTT

30 – 50 seconds

ACT

120 – 140 sec

TEG/ROTEM

5 parameters measured:

Reaction Time

From start of test until first measurable clot forms

  • Affected by coagulation factors
  • Normal 5-10mins
  • Deficit in clotting factors → FFP

K Time

Time elapsed until clot reaches certain strength (20mm)

  • Affected by Fibrinogen
  • Normal 1-3mins
  • Elevation = deficit in Fibrinogen → Cryo

Alpha Angle

Speed of Fibrinogen accumulation

Anti-Xa

Tx dose LMWH 1-2IU/ml

Prophylactic LMWH 0,2-0,4 IU/ml

Errors

PT

Different Thromboplastins (in diff labs) give different PT times

So INR standardizes the Thromboplastin reagent

INR is the ration of patient’s PT : control plasma PT, raised to the power of a correction factor KA Interational Sensitivity Index specific for each thromboplastin reagent

APTT

Contamination of the sample with heparin at the time of collection, or an inaccurate whole blood:citrate ratio will artificially prolong the APTT

Prolonged APTT doesn’t = bleeding tendency, for example, in vivo, lupus anticoagulant predisposes to thrombosis, but it is an inhibitor of the intrinsic pway therefore prolongs APTT

ACT

Underfilling shortens ACT

Overfilling, inadequate mixing, thrombocytopenia, warfarin, dilution prolong ACT

TEG/ROTEM

Machine requires regular calibration

Anti-Xa

Antithombin deficiency

Wide variation between labs

NB

Fondaparinux and danaparoid also work by Xa inhibition so their activity can be monitored by anti-Xa-assay

Great Thromboelastography Video