Overview
Haemostasis means "arrest of bleeding." It involves an intricate interplay of the vascular phase, platelet phase, coagulation phase and fibrinolytic phase, working together to seal vessel injury while keeping blood flowing normally elsewhere.
Understanding these phases is essential for interpreting screening tests like bleeding time, PT, APTT and TT, and for diagnosing bleeding disorders such as Von Willebrand's disease, thrombocytopenia and Haemophilia.
Learning Objectives
After this lesson you will be able to…- Describe the four phases of Haemostasis.
- Describe the coagulation of blood via intrinsic, extrinsic and common pathways.
- Explain Von Willebrand's Disease and its laboratory diagnosis.
- Explain the causes of thrombocytopenia.
- Discuss the Bleeding Time test and interpret PT, APTT and TT.
- Explain INR and describe the significance of D-dimer.
Clinical Story
Why This MattersA young man scheduled for knee surgery mentions his uncle had "a bleeding problem." The surgeon orders a coagulation screen before proceeding. The lab technologist finds a prolonged APTT with a normal PT and bleeding time — a pattern that flags possible haemophilia, prompting a Factor VIII/IX assay before a potentially catastrophic intraoperative bleed.
Core Concepts
Vascular Phase: Endothelial cells maintain smooth blood flow, secrete VWF, and provide anticoagulant, antiplatelet and fibrinolytic properties. Platelet Phase: Platelets adhere to exposed sub-endothelial collagen via VWF/GPIb-IX, then aggregate via GPIIb-IIIa and fibrinogen, forming the primary platelet plug. Coagulation Phase: A cascade of enzyme reactions converts fibrinogen to fibrin. Fibrinolytic Phase: Plasmin, generated from plasminogen, digests fibrin clots to restore vessel patency.
The Intrinsic Pathway begins with FXII activation on contact with a non-endothelial surface, proceeding through FXI, FIX and the tenase complex (FIXa + FVIIIa + phospholipid + calcium). The Extrinsic Pathway begins with tissue factor complexing with FVII. Both converge on the Common Pathway: FXa forms prothrombinase with FVa, converting prothrombin to thrombin, which cleaves fibrinogen to fibrin. FXIIIa then stabilizes the clot.
PT (Prothrombin Time) tests the extrinsic and common pathways (FI, II, V, VII, X); normal is 12–14 seconds; reported as INR for patients on oral anticoagulants. APTT tests the intrinsic and common pathways (all factors except FVII and FXIII); normal is 30–35 seconds. TT (Thrombin Time) measures clottable fibrinogen; normal is 12–14 seconds. Correction studies (mixing test + control plasma) distinguish factor deficiency (corrects) from inhibitors like lupus anticoagulant (does not correct).
VWD is an autosomal dominant disorder of VWF, which mediates platelet adhesion and carries FVIII. Lab findings: increased bleeding time and APTT, decreased FVIII activity, normal PT and platelet count. Thrombocytopenia causes include decreased production (aplastic anemia, leukemia, marrow infiltration), increased destruction (ITP, DIC, TTP/HUS, sepsis), splenic sequestration, and dilution (massive transfusion).
Haemophilia A (FVIII deficiency) and B (FIX deficiency) are sex-linked recessive disorders presenting with haematoma formation and haemarthrosis; APTT is prolonged, PT and TT normal. Vitamin K deficiency affects FII, VII, IX and X; seen in newborns (hemorrhagic disease of the newborn), obstructive jaundice, liver disease and with oral anticoagulant use — PT and APTT both markedly prolonged.
Laboratory Principle
Coagulation screening tests measure the time taken for citrated plasma to clot after recalcification and the addition of specific activators — tissue thromboplastin for PT, a contact activator and phospholipid for APTT, and thrombin itself for TT. The clotting endpoint reflects the functional integrity of the corresponding coagulation pathway; prolongation indicates either factor deficiency or the presence of an inhibitor.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Tissue thromboplastin + calcium chloride | PT reagent | Tests extrinsic and common pathway | 2–8°C |
| Partial thromboplastin + activator + calcium chloride | APTT reagent | Tests intrinsic and common pathway | 2–8°C |
| Thrombin reagent | TT reagent | Tests fibrinogen to fibrin conversion | 2–8°C |
| 3.2% sodium citrate | Anticoagulant | Prevents clotting for plasma-based tests | Room temperature |
Step-by-Step Procedure
Draw 4.5 mL venous blood into 0.5 mL of 3.2% sodium citrate via clean venepuncture; process within 1–2 hours.
Centrifuge the citrated sample to separate platelet-poor plasma for clotting tests.
Add the appropriate reagent to test and control plasma; incubate and measure time to clot formation for each test.
Compare test results against control plasma; PT >3 sec difference, APTT >5 sec difference, TT >2 sec difference are considered significant.
Mix equal parts test and control plasma and repeat; correction indicates factor deficiency, lack of correction indicates an inhibitor.
Flow Diagram
Quality Control
Always run normal and abnormal control plasma with every batch of PT/APTT/TT testing. Verify the blood-to-citrate ratio is correct (under-filled tubes falsely prolong clotting times).
Participate in coagulation EQAS programs to validate ISI calibration for INR reporting and ensure inter-laboratory comparability.
Reference Values
Normal Ranges⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Prolonged APTT, normal PT and BT | Intrinsic pathway factor deficiency (e.g. Haemophilia A/B) | Perform FVIII/FIX assay; do correction studies |
| Prolonged PT, normal APTT | FVII deficiency or early liver dysfunction / warfarin effect | Assess liver function; review anticoagulant therapy |
| Prolonged BT and APTT, decreased FVIII activity | Von Willebrand's Disease | Confirm VWF antigen/activity assay; treat with FFP or cryoprecipitate before surgery |
Common Errors & How to Avoid Them
Cause: Under-filled citrate tubes cause relative excess of anticoagulant, falsely prolonging clotting times.
Prevention: Always fill citrate tubes to the exact marked volume (9:1 ratio).
Cause: Factor V and VIII degrade rapidly at room temperature, falsely prolonging PT/APTT if testing is delayed.
Prevention: Process citrated samples within 1–2 hours of collection.
Cause: Tissue factor contamination from a difficult draw can shorten clotting times or cause clots in the sample.
Prevention: Use clean, atraumatic venepuncture technique; discard first few mL if drawing from a central line.
Laboratory Tips from the Bench
Bleeding time is no longer recommended as a routine pre-surgical screening test — a thorough bleeding history is equally or more predictive.
A persistently short APTT (<25 seconds) may indicate a hypercoagulable state, not just lab error — always double-check before dismissing.
"PT = Play Tennis outside (Extrinsic); APTT = All Pathways Trapped Together inside a vessel (Intrinsic)" — a quick way to recall which pathway each test screens.
Important Notes
Since FXIIIa acts only after fibrin formation, standard clotting tests miss its deficiency — a separate clot solubility test (5M urea or 1% acetic acid) is needed.
INR accounts for differences in thromboplastin sensitivity (ISI) between laboratories, allowing consistent monitoring of patients on oral anticoagulants across different labs.
Interactive Quiz
Test Your KnowledgeFlashcards
Tap to flipClick or tap any card to reveal the answer.
Clinical Case Study
Apply Your KnowledgePresents with a swollen, painful knee joint after minor trauma while playing. Maternal uncle has a known "bleeding disorder."
Isolated prolonged APTT with normal PT, bleeding time and platelet count, combined with haemarthrosis and a positive family history, is classic for Haemophilia A or B.
- →Isolated APTT prolongation with normal PT points to an intrinsic pathway factor deficiency.
- →Haemarthrosis is a hallmark clinical sign of haemophilia.
- →Family history is important since haemophilia is sex-linked recessive.
Frequently Asked Questions
Bleeding time is highly operator-dependent and a poor predictor of abnormal surgical bleeding. A well-taken bleeding history is considered just as reliable for screening purposes.
Most coagulation factors are synthesized in the liver, so hepatocellular dysfunction causes multifactor deficiency, presenting as prolonged PT and APTT, low fibrinogen, and increased D-dimers.
DIC involves widespread activation of both coagulation and fibrinolysis, with low platelets and low fibrinogen alongside raised D-dimer, while primary fibrinolysis lacks the coagulation activation component.
Quick Revision
10-Minute ReviewKey Takeaways
- Haemostasis is a tightly regulated four-phase process to arrest bleeding while preventing pathologic clotting.
- PT, APTT and TT screen distinct segments of the coagulation cascade.
- INR standardizes anticoagulant monitoring across laboratories with different thromboplastin sensitivities.
- Von Willebrand's Disease and Haemophilia have distinct, recognizable screening test patterns.
- Thrombocytopenia has diverse causes — production failure, destruction, sequestration or dilution.
- Mixing studies help distinguish true factor deficiency from an inhibitor.
Competency Checklist
Track Your MasteryReferences
- NIOS Hematology and Blood Bank Technique — Lesson 24: Haemostasis.
- Dacie JV, Lewis SM. Practical Haematology. 11th ed.