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Unit 3 of 3 · B.Sc MLS Sem 4

Unit 3: Coagulation screening tests

Basic Haematological Techniques-II notes · PTU syllabus (BMLS402-18)

3 min read5 topics9 exam questions
On this page
  1. Unit summary
  2. Bleeding time
  3. Clotting time
  4. Hess (tourniquet) test
  5. Prothrombin time (PT)
  6. Activated partial thromboplastin time (APTT)
  7. Key terms
  8. Quick revision
  9. Important questions

Unit summary

Screening tests show whether bleeding comes from platelets, vessels or clotting factors. This unit covers bleeding time, clotting time, the Hess (tourniquet) test, prothrombin time and activated partial thromboplastin time.

After this unit you can

  • Perform bleeding time and clotting time
  • Perform the Hess capillary fragility test
  • Perform and interpret PT and INR
  • Perform and interpret APTT

PTU syllabus topics

  • Bleeding time and clotting time
  • Hess test
  • prothrombin time (PT)
  • activated partial thromboplastin time (APTT)
ComparisonCoagulation screening tests
Normal range (approx.)
Prolonged in

Bleeding time (Duke / Ivy)

2–7 min

Platelet disorders, von Willebrand disease

Clotting time

4–9 min (Lee-White)

Severe factor deficiencies

PT

11–16 sec (lab-specific)

Warfarin therapy, liver disease, vitamin K deficiency

APTT

25–40 sec (lab-specific)

Haemophilia A and B, heparin therapy

1

Topic 1

Bleeding time

ComparisonBleeding time methods
Method
Normal

Duke

Ear lobe puncture; blot every 30 seconds

1–5 minutes

Ivy (preferred)

BP cuff at 40 mmHg; standard forearm incisions

2–7 minutes

  • Tests: platelet function and vessel integrity. Prolonged: thrombocytopenia, platelet function defects, von Willebrand disease, aspirin. Normal in haemophilia.
2

Topic 2

Clotting time

  • Lee and White method: 1 mL venous blood in each of three glass tubes at 37 °C; tilt every 30 seconds until clotted. Normal: 5–10 minutes (capillary method 2–5 minutes). Insensitive; prolonged in severe factor deficiencies and heparin therapy.
3

Topic 3

Hess (tourniquet) test

  • Capillary fragility test: inflate a BP cuff to midway between systolic and diastolic pressure for 5 minutes; count petechiae in a 2.5 cm circle on the forearm after 5 minutes. Positive: more than 20 petechiae — thrombocytopenia, vascular purpura, scurvy, dengue (used in the WHO dengue screening).
4

Topic 4

Prothrombin time (PT)

ProcessPT (one-stage, Quick's method)
  1. 1Collect blood in 3.2% citrate (9 : 1); centrifuge for platelet-poor plasma
  2. 2Warm plasma and thromboplastin–calcium reagent to 37 °C
  3. 3Add 0.2 mL reagent to 0.1 mL plasma and start the timer
  4. 4Record time to clot formation (manual tilt or coagulometer)
  5. 5Test in duplicate with a normal control
Key formulasINR

  • INR

    (Patient PT ÷ mean normal PT) raised to the power ISI

  • ISI

    International sensitivity index of the thromboplastin

  • Targets

    Normal about 1.0; warfarin therapy usually 2–3

  • Normal PT: about 11–16 seconds. Prolonged: warfarin, vitamin K deficiency, liver disease, DIC, deficiency of VII, X, V, II or fibrinogen.
5

Topic 5

Activated partial thromboplastin time (APTT)

  • Principle: plasma is incubated with a contact activator (kaolin, silica or ellagic acid) and phospholipid (cephalin, the partial thromboplastin), then calcium is added; tests the intrinsic and common pathways.
  • Normal: about 26–40 seconds (laboratory-specific). Prolonged: haemophilia A and B, heparin (monitoring unfractionated heparin), lupus anticoagulant, liver disease, DIC.
ComparisonInterpreting screening tests
Likely cause
Pattern

Thrombocytopenia or platelet defect

Platelet problem

BT prolonged; PT and APTT normal

Haemophilia A or B

Factor VIII or IX deficiency

APTT prolonged; PT and BT normal

Warfarin or factor VII deficiency

Extrinsic pathway

PT prolonged; APTT normal or mildly prolonged

Liver disease, DIC, vitamin K deficiency

Many factors

PT and APTT prolonged; DIC also low platelets, high D-dimer

von Willebrand disease

vWF deficiency

BT and APTT prolonged

  • Mixing studies: correction with normal plasma suggests factor deficiency; no correction suggests an inhibitor.

Exam tip

Pre-analytical errors — wrong blood-to-citrate ratio, clotted or haemolysed samples, high haematocrit, delay — are the commonest causes of wrong coagulation results.

Key terms

Bleeding time
Time for a standard skin wound to stop bleeding
Prothrombin time
Clotting time of plasma with thromboplastin and calcium
INR
Standardised ratio for monitoring warfarin
APTT
Test of intrinsic and common pathways
Mixing study
Test distinguishing factor deficiency from inhibitors

Quick revision

  • Duke and Ivy bleeding time; Lee–White clotting time.
  • Hess test: more than 20 petechiae.
  • PT procedure; INR formula; causes of prolonged PT.
  • APTT principle; causes of prolonged APTT; heparin monitoring.
  • Pattern table; mixing studies; pre-analytical errors.

Important exam questions

Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).

Short-answer questions

  1. Q1.What does bleeding time test?
  2. Q2.Give the normal Ivy bleeding time.
  3. Q3.What is a positive Hess test?
  4. Q4.Define INR.
  5. Q5.Which test monitors unfractionated heparin?
  6. Q6.Give the screening test pattern in haemophilia A.

Long-answer questions

  1. Q1.Describe bleeding time, clotting time and the Hess test.
  2. Q2.Describe the prothrombin time and INR.
  3. Q3.Describe the APTT and interpretation of coagulation screening tests.

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