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Lab Diagnosis of Bleeding Disorders + Why aPTT is Prolonged in von Willebrand Disease
(Robbins & Kumar Basic Pathology)
Part 1: Lab Diagnosis of Bleeding Disorders
The key screening tests used are:
| Test | Normal Range | What it measures |
|---|---|---|
| aPTT (Activated Partial Thromboplastin Time) | 25-37 seconds | Intrinsic pathway (Factors XII, XI, IX, VIII) + common pathway (X, V, II, fibrinogen) |
| PT/INR (Prothrombin Time) | 9.4-12.5 sec / INR 0.9-1.1 | Extrinsic pathway (Factor VII) + common pathway (X, II, fibrinogen) |
| Platelet count | 150,000-400,000/µL | Quantitative platelet defect |
| Bleeding time | Reflects platelet function & primary hemostasis | |
| vWF antigen (55%-200%) | Quantity of von Willebrand factor | |
| vWF:Ristocetin cofactor assay | Functional assay for vWF activity | |
| Factor VIII activity (55%-200%) | Factor VIII level (bound to and stabilized by vWF) |
Interpretation Pattern:
| Disorder | Bleeding Time | aPTT | PT | Platelet count |
|---|---|---|---|---|
| von Willebrand disease | Prolonged | Prolonged | Normal | Normal |
| Hemophilia A (FVIII deficiency) | Normal | Prolonged | Normal | Normal |
| Hemophilia B (FIX deficiency) | Normal | Prolonged | Normal | Normal |
| Thrombocytopenia | Prolonged | Normal | Normal | Low |
| Liver disease / Warfarin | Variable | Prolonged | Prolonged | Variable |
Part 2: Why aPTT is Prolonged in von Willebrand Disease
This is the key concept, and it comes down to the dual role of vWF:
FIG. 10.31 from Robbins - Structure and function of the Factor VIII-vWF complex
The Mechanism (Step by Step):
1. vWF normally acts as a carrier/stabilizer for Factor VIII:
"vWF also stabilizes factor VIII; thus, vWF deficiency leads to a secondary deficiency of factor VIII."
- Robbins & Kumar Basic Pathology, p. 430
2. In von Willebrand disease, vWF is reduced or dysfunctional:
- Without vWF to stabilize it, Factor VIII is rapidly degraded in plasma
- This leads to a secondary reduction in Factor VIII levels
3. Factor VIII is a key component of the intrinsic (contact activation) pathway:
- Factor VIII acts as a cofactor for Factor IXa, forming the "tenase complex" that activates Factor X
- aPTT measures the intrinsic pathway (Factors XII → XI → IX → VIII → X → thrombin)
- When Factor VIII is reduced secondary to vWF deficiency, the intrinsic pathway is slowed
- Result: aPTT is prolonged
Summary in one sentence:
vWF deficiency → secondary Factor VIII deficiency → defective intrinsic coagulation pathway → prolonged aPTT
"In addition to its role in platelet adhesion, vWF also stabilizes factor VIII; thus, vWF deficiency leads to a secondary deficiency of factor VIII."
- Robbins & Kumar Basic Pathology, p. 430
Important Note on vWD Subtypes:
| Type | Defect | aPTT |
|---|---|---|
| Type I (most common) | Reduced quantity of vWF | Mildly prolonged |
| Type II | Dysfunctional vWF (qualitative defect) | Usually normal or mildly prolonged |
| Type III (homozygous, severe) | Near-complete absence of vWF | Markedly prolonged (resembles hemophilia A) |
In Type III, Factor VIII falls so severely that features resemble hemophilia - Robbins & Kumar Basic Pathology, p. 431.
The Special Test for vWD:
The Ristocetin platelet agglutination test is used specifically for vWD:
- Ristocetin enhances binding of vWF to platelet glycoprotein Ib, causing platelet agglutination
- In vWD, this agglutination is reduced or absent (abnormal ristocetin cofactor assay)
- This test serves as a functional bioassay for vWF activity
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