NOTE: To obtain an optimal baseline specimen patients should be stress free, physically well and for women, the specimen should be collected 5 - 8 days after menstruation commences.
- Citrated platelet poor plasma. Minimum volume 0.5 mL.
- If a PFA is also required, send 2 x 2.7 mL citrate tubes or 3 x 1.8 mL paediatric tubes.
or
- Tube must be adequately filled.
- Filtered plasma is unsuitable and may lead to erroneously low results.
| Test | Reference interval | Uncertainty of measurement |
|---|---|---|
| FVIII | 55 - 150% | 18% |
| vWF:Ag | 55 - 150% | 9% |
|
vWF:AC (Siemens Innovance) |
55 - 150% | 14% |
| vWF:CB | 55 - 150% | 16% |
This screen includes
- Factor VIIIC - Factor VIII assay
- vWF:Ag - von Willebrand factor antigen
- vWF:AC - von Willebrand activity
-
CBA - collagen binding assay
NOTE: If further testing is required to determine von Willebrand subtype, a sample specimen may be sent to another accredited laboratory for further investigation.
The assay principle makes use of the binding of VWF to its receptor Glycoprotein Ib (GPIb). GPIb is the main VWF receptor on platelets. Polystrene particles are coated with an antibody against GPIb. Recombinant GPIb (two gain-of-function mutations included) is added and binds to the antibody as well as to the VWF of the sample .
Background
von Willebrand Disease (vWD) is the most common inherited bleeding disorder affecting up to 1% of the population. Patients have defects in, or reduced levels of von Willebrand Factor (vWF) - a plasma protein made in platelets and endothelial cells. It has one main function; to facilitate binding of platelets to sites of vascular damage. This initiates platelet plug formation as one of the first steps in the coagulation cascade. Von Willebrand Factor also acts as a carrier protein for factor VIII and thus prevent factor VIII degradation in the circulation.
vWF exists in the plasma in different multimer sizes - the larger or high molecular weight subtypes are more important in platelet adhesion and aggregation.
vWF is a heterogeneous disorder which is divided into various subtypes. Accurate classification is important as different approaches to therapy are necessary for some subtypes.
Clinical expression is variable in individuals and within families but most patients have mild to moderate bleeding of mucocutaneous type e.g. easy bruising, epistaxis, menorrhagia. Some patients present with excess bleeding after surgery, dental extractions or aspirin ingestion.
The importance of a full personal and family bleeding history and drug history cannot be over-emphasised. This is of more clinical relevance than laboratory test results. See:
Classification
vWD is a heterogeneous disorder:
| Type I vWD | partial quantitative deficiency of vWF - accounts for 70% vWD |
| Type 2 vWD | qualitative abnormality of vWF. Four main subtypes the commonest being 2a (vWF lower than vWF:Ag) and 2b (increased binding to platelets often leads to thrombocytopenia) |
| Type 3 vWD | severe quantitative defect. |
Diagnosis
The platelet function screen (PFA) is a useful screening test for von Willebrand disease. Prolongation of both closure times supports the diagnosis of vWD, whereas a normal result virtually excludes the diagnosis. A normal PFA result makes the diagnosis of von Willebrand disease very unlikely. The interpretation of the vWF assays can be very difficult. The vWF:Ag is an acute phase protein that fluctuates that fluctuates through menstrual cycle, and rises with oestrogen therapy, pregnancy, in response to stress and chronic disease. The presence of these conditions may mask mild vWD. Levels are about 25% lower in those with blood group O.
Interpretation of results
All abnormal results should be confirmed and diagnosis only made after consistent and significant abnormalities (usually less than 35%) are shown in one or more parameters. Repeat testing is advisable in:
- patients with equivocal results
- patients with minor abnormalities in only one test
- where there is a strong personal or family history to suggest an inherited bleeding disorder and initial laboratory results are normal.
See
For further information contact the Haematology laboratory (Ext 22067) or:
| Dr Nicola Eaddy | Ext 22005 | |
| Dr Peter Bradbeer | Ext 22062 | |
| Dr Anna Ruskova | Ext 22137 | |
| Dr Nikhil Rabade | Ext 22071 |
Plasma should be sent frozen.