Laboratory diagnosis of vWD is most often based on results of von Willebrand factor antigen assay (abbreviated vWF:Ag). This test measures the amount or concentration of vWF in a blood sample. The Comparative Coagulation Section reports each dog's result as %vWF:Ag compared to a 100% standard. Dogs having low plasma vWF:Ag (below 50%) are at risk for transmitting or expressing the vWD trait. In general, the most severely affected dogs have marked reduction in plasma vWF:Ag, with values of less than 15%.
The methods used to draw, process, and ship samples are important for accurate results. Samples containing clots or hemolysis (red cell breakdown) are the most likely to yield inaccurate or unreproducible results. Use of a standard sampling technique ensures optimum sample quality.
Plasma vWF levels fluctuate from day to day in normal, healthy dogs. This fluctuation is exaggerated during pregnancy or heat in bitches, and in any dog having a systemic illness (especially liver disease or inflammatory disorders). Baseline values of vWF:Ag are more accurate genetic predictors for vWD status, therefore samples for genetic screening should be drawn from healthy dogs and bitches not pregnant or in heat. Puppies can be sampled as young as 6 to 8 weeks of age.
Diagnostic ranges of vWF:Ag are used to identify vWD-affected dogs and as an aid for predicting genetic status for the vWD trait in asymptomatic dogs.
|Normal||70 to 180|
|Borderline||50 to 69|
|Abnormal||0 to 49|
Dogs testing in the normal range are considered clear of the vWD trait, and at low risk for expressing or transmitting vWD.
Dogs testing in the borderline range can not be accurately classified as carrier or clear on the basis of that measurement. This is an overlap region of plasma vWF:Ag, where some individuals are clear and some carriers of vWD. On a second test, some dogs fall in the normal or abnormal range, thereby enabling a prediction of their genetic status. A test mating can be performed by breeding a borderline range dog to a high-testing clear mate. If the borderline parent is clear of vWD, then all pups in the litter are predicted to be clear. The presence of 1 or more abnormal range pup indicates that the borderline parent is a carrier of vWD. Dogs testing in the borderline range are not expected to have a bleeding tendency. They need no restriction on activity or special management.
Dogs testing in the abnormal range are considered carriers of the vWD trait. They are at risk for transmitting an abnormal vWF gene to offspring, and some will express a bleeding tendency. Dogs affected with the most severe form of vWD (type 3 vWD) have very low values of vWF:Ag (1% or less). Severely affected dogs should not undergo surgical procedures without transfusion.