Screening test for bleeding disorder? Patho exam
The key screening tests for bleeding disorders are PT (prothrombin time), aPTT (activated partial thromboplastin time), platelet count, and bleeding time. PT screens the extrinsic/common pathway, aPTT screens the intrinsic/common pathway, and bleeding time assesses platelet function and vascular integrity.
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Screening Tests for Bleeding Disorders
Overview
The standard screening battery for a suspected bleeding disorder consists of 4 core tests, covering both primary and secondary hemostasis:
| Test | Normal Value | What It Measures |
|---|---|---|
| Platelet count | 150-400 × 10⁹/L | Quantitative platelet assessment |
| Bleeding time (BT) | 2.5-10 min | Primary hemostasis: platelet-subendothelium interaction + vWF |
| PT / INR | PT: 11-13 s; INR: 1.0 | Extrinsic + common pathway (Factors VII, X, V, II, fibrinogen) |
| aPTT | 22-34 s | Intrinsic + common pathway (Factors XII, XI, IX, VIII, X, V, II, fibrinogen) |
Additional tests (fibrinogen level, thrombin clotting time, mixing studies, factor assays) are ordered as indicated.
1. Platelet Count
- Normal: 150-400 × 10⁹/L
- Bleeding is generally not a problem until count falls below 50,000/mm³
- Spontaneous bleeding risk (including CNS hemorrhage) occurs with count < 10,000/mm³
- Low count = thrombocytopenia (decreased production or increased destruction)
2. Bleeding Time (BT)
- Normal: ~2.5-10 min (template method)
- Measures platelet adhesion/activation/aggregation AND von Willebrand factor function in vivo
- Prolonged BT caused by:
- Thrombocytopenia (platelet count < 50,000/mm³)
- Abnormal platelet function: von Willebrand disease, antiplatelet drugs (aspirin), uremia, liver disease
- Limitation: Technically challenging, operator-dependent, insensitive to many hereditary disorders (including some types of vWD)
- Modern replacement: Platelet Function Analyzer-100 (PFA-100) - measures primary hemostasis in vitro
3. Prothrombin Time (PT) / INR
- Tests: Extrinsic pathway (Factor VII) + common pathway (Factors X, V, II, fibrinogen)
- Normal PT: 11-13 s; INR: 1.0 (INR of 1.7 ≈ ~30% factor activity)
- Prolonged PT caused by:
- Warfarin (inhibits Vit K-dependent factors: II, VII, IX, X)
- Liver disease (decreased factor synthesis)
- Certain antibiotics (interfere with Vit K-dependent factors)
- Factor VII deficiency (isolated prolonged PT only)
- DIC
4. aPTT (Activated Partial Thromboplastin Time)
- Tests: Intrinsic pathway (Factors XII, XI, IX, VIII) + common pathway (X, V, II, fibrinogen)
- Normal: 22-34 s
- Prolonged aPTT caused by:
- Heparin therapy (most common)
- Factor deficiencies < 30% of normal (especially Factors VIII → Hemophilia A; IX → Hemophilia B; XI → Hemophilia C)
- Lupus anticoagulant / antiphospholipid antibodies
- Contact factor deficiencies (XII, XI, HMWK, prekallikrein)
Interpretation Pattern (HIGH-YIELD for Exams)
| PT | aPTT | Likely Diagnosis |
|---|---|---|
| Normal | Prolonged | Hemophilia A (↓FVIII), Hemophilia B (↓FIX), Hemophilia C (↓FXI), Heparin |
| Prolonged | Normal | Factor VII deficiency, early Warfarin, early liver disease |
| Both prolonged | Both prolonged | ↓FX, ↓FV, ↓FII, ↓Fibrinogen, DIC, Liver disease, Warfarin (advanced), Supratherapeutic heparin |
| Normal | Normal | Platelet/vascular disorder (check BT, PFA-100), mild vWD, Factor XIII deficiency |
5. Mixing Study (Follow-up when PT or aPTT is prolonged)
- Patient's plasma mixed with normal pooled plasma (1:1)
- Corrects → factor deficiency present (specific factor assays next)
- Does NOT correct → inhibitor present (e.g., factor-specific antibody, lupus anticoagulant)
6. Additional Tests
| Test | Measures | Key Use |
|---|---|---|
| Fibrinogen level | Fibrinogen (acute phase reactant, made in liver) | Low in DIC; elevated in inflammation |
| Thrombin clotting time (TCT) | Fibrinogen → fibrin conversion | Prolonged by low/abnormal fibrinogen, heparin, FDPs, paraproteins |
| D-dimers / FDPs | Fibrin degradation | Elevated in DIC, PE, DVT |
| PFA-100 | Primary hemostasis (platelet + vWF) | Replaced bleeding time in most labs |
| Factor assays | Specific factor activity levels | After abnormal mixing study |
Hemostatic Component Coverage (Summary Table)
| Component | Screening Test |
|---|---|
| Blood vessel | Bleeding time |
| Platelets | Bleeding time, platelet count, PFA-100 |
| Coagulation system | PT, aPTT, thrombin time |
| Fibrinolytic system | Not measured by standard screening tests |
Note: PT and aPTT are functional global assays. They do NOT test specific proteins and are often insensitive to mild deficiencies (factor levels must typically fall below 30% of normal to cause prolongation). Direct oral anticoagulants (DOACs) do not consistently affect these screening tests.
Sources:
- Tintinalli's Emergency Medicine, Table 232-1 (Tests of Hemostasis)
- Harrison's Principles of Internal Medicine 22E, Chapter 121 (Coagulation Disorders)
- Lee's Essential Otolaryngology, Chapter 5 (Screening Tests of Coagulation)
- Harriet Lane Handbook 23E, Figure 14.3 (Coagulation Cascade & Differential Diagnosis)
Tell me in short for written exam
Screening Tests for Bleeding Disorders
The 4 Core Tests
- Platelet count – quantitative platelet assessment
- Bleeding time (BT) – platelet function + vascular integrity
- PT/INR – extrinsic + common pathway (VII, X, V, II, fibrinogen)
- aPTT – intrinsic + common pathway (XII, XI, IX, VIII, X, V, II, fibrinogen)
High-Yield Interpretation
| PT | aPTT | Diagnosis |
|---|---|---|
| Normal | ↑ | Hemophilia A/B/C, Heparin |
| ↑ | Normal | Factor VII deficiency, early Warfarin |
| ↑ | ↑ | DIC, Liver disease, FV/FX/FII/Fibrinogen deficiency |
| Normal | Normal | Platelet/vascular disorder, mild vWD, FXIII deficiency |
Key Points to Write
- PT tests extrinsic pathway; aPTT tests intrinsic pathway
- Factor levels must fall < 30% of normal to prolong PT/aPTT
- Mixing study: corrects = factor deficiency; doesn't correct = inhibitor
- Bleeding time prolonged in: thrombocytopenia, vWD, aspirin, uremia
- Fibrinolytic system is NOT measured by any standard screening test
- PFA-100 has largely replaced bleeding time in modern labs
Tell me normal values of bt ct pt aptt platel count
Normal Values - Quick Reference
| Test | Normal Value | Method |
|---|---|---|
| BT (Bleeding Time) | 2-7 min (Ivy) / 1-3 min (Duke) | Template: 2.5-10 min |
| CT (Clotting Time) | 5-11 min | Lee-White method (glass tube) |
| PT (Prothrombin Time) | 11-13 seconds | INR = 1.0 |
| aPTT | 22-34 seconds | Activated |
| Platelet Count | 1.5-4 lakh/mm³ (150,000-400,000/mm³) |
Memory Trick
"2, 5, 11, 22, 1.5L" BT=2min | CT=5min | PT=11s | aPTT=22s | Platelets=1.5 lakh (lower limit)
Exam Extras
- Therapeutic INR (on warfarin) = 2-3
- PT prolonged = problem with extrinsic pathway (Factor VII first to fall)
- aPTT prolonged = problem with intrinsic pathway (Heparin monitored by aPTT)
- CT is a crude test - rarely used now, replaced by aPTT
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