Screening test for bleeding disorder? Patho exam
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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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