A New Automated Screening Assay for the Diagnosis of von Willebrand Disease
A New Automated Screening Assay for the Diagnosis of von Willebrand Disease
A new, automated assay for von Willebrand factor (vWF) activity has recently become commercially available (HemosIL vWF activity assay, Instrumentation Laboratories, Lexington, MA). We prospectively studied 61 specimens from 58 patients undergoing laboratory testing for suspicion of von Willebrand disease with this new method, in comparison with the established ristocetin cofactor method. Assays for factor VIII and vWF antigen were also performed using an established method on an MDA-180 coagulation analyzer (bioMérieux, Durham, NC) and a new method on an ACL TOP coagulation analyzer (Instrumentation Laboratories). Blood types were determined. The results showed no significant difference between the assays for factor VIII (mean, 97% for MDA-180 and ACL TOP; P = .494) or vWF antigen (mean, MDA-180, 109%; ACL TOP, 111%; P = .766). The mean result for the ristocetin cofactor assay was 106% vs 93% with the automated vWF activity (P = .007). The automated activity assay was 100% sensitive and 86% specific for detecting vWF abnormalities and seems to be a suitable screening test. Abnormal results should be followed up with a ristocetin cofactor activity assay for confirmation. Further study is recommended to confirm these conclusions.
von Willebrand disease (vWD) is the most common inherited bleeding disorder in humans, occurring in approximately 1% of the population. von Willebrand factor (vWF) is essential for normal blood coagulation because it mediates platelet adhesion to injured endothelium, and it binds and protects factor VIII in the circulation. vWD is a caused by a quantitative or qualitative defect of vWF, leading to easy bruising, epistaxis, excessive postoperative or posttraumatic bleeding, gum bleeding, and menorrhagia. Diagnosis can be difficult, requiring correlation of clinical findings with multiple laboratory test results and family studies. Laboratory testing is complicated by the fact that vWF is an acute phase reactant, such that vWF increases with stress, injury, surgery, pregnancy, and oral contraceptive use and in the newborn period. Type 1 vWD is a quantitative deficiency, and type 2 vWD variants are qualitative deficiencies. Type 3 vWD is a rare, severe quantitative deficiency, with a prevalence of 1 in 1 million. Laboratory tests for vWD are necessary not only for diagnosis but also for determining the subtype of disease. Initial tests include factor VIII activity, vWF antigen, and ristocetin cofactor (a vWF activity assay).
In addition to these complexities with vWD testing, the ristocetin cofactor assay is labor-intensive and requires technologist expertise. An easier, automated vWF activity assay could simplify laboratory testing for vWD. The HemosIL vWF activity assay (Instrumentation Laboratories, Lexington, MA) is a latex turbidimetric immunoassay designed for use on Instrumentation Laboratory analyzers as an aid in the diagnosis and classification of vWD. The reagent contains latex particles coated with a monoclonal anti-vWF antibody directed against the platelet binding site on vWF (the platelet glycoprotein Ib receptor). If vWF in the patient plasma is normal, it binds to and agglutinates the latex particles. The degree of agglutination is directly proportional to vWF activity in the patient plasma and is measured by light transmittance, which decreases when aggregates form.
The main objective of this study was to evaluate the HemosIL vWF activity assay as a screening assay for vWD and to compare it with the ristocetin cofactor assay as the reference method, in conjunction with vWF antigen, factor VIII, and blood type results. In addition, 2 vWF antigen methods and 2 factor VIII methods were compared. We studied the hypothesis that the automated vWF activity assay (along with vWF antigen and blood type) would function as a screening assay for vWD such that normal results with the automated activity assay would require no further testing for vWD (eliminating the need for a ristocetin cofactor assay), whereas abnormal results with the automated activity assay would result in reflex testing with a ristocetin cofactor assay for definitive vWD assessment.
Abstract and Introduction
Abstract
A new, automated assay for von Willebrand factor (vWF) activity has recently become commercially available (HemosIL vWF activity assay, Instrumentation Laboratories, Lexington, MA). We prospectively studied 61 specimens from 58 patients undergoing laboratory testing for suspicion of von Willebrand disease with this new method, in comparison with the established ristocetin cofactor method. Assays for factor VIII and vWF antigen were also performed using an established method on an MDA-180 coagulation analyzer (bioMérieux, Durham, NC) and a new method on an ACL TOP coagulation analyzer (Instrumentation Laboratories). Blood types were determined. The results showed no significant difference between the assays for factor VIII (mean, 97% for MDA-180 and ACL TOP; P = .494) or vWF antigen (mean, MDA-180, 109%; ACL TOP, 111%; P = .766). The mean result for the ristocetin cofactor assay was 106% vs 93% with the automated vWF activity (P = .007). The automated activity assay was 100% sensitive and 86% specific for detecting vWF abnormalities and seems to be a suitable screening test. Abnormal results should be followed up with a ristocetin cofactor activity assay for confirmation. Further study is recommended to confirm these conclusions.
Introduction
von Willebrand disease (vWD) is the most common inherited bleeding disorder in humans, occurring in approximately 1% of the population. von Willebrand factor (vWF) is essential for normal blood coagulation because it mediates platelet adhesion to injured endothelium, and it binds and protects factor VIII in the circulation. vWD is a caused by a quantitative or qualitative defect of vWF, leading to easy bruising, epistaxis, excessive postoperative or posttraumatic bleeding, gum bleeding, and menorrhagia. Diagnosis can be difficult, requiring correlation of clinical findings with multiple laboratory test results and family studies. Laboratory testing is complicated by the fact that vWF is an acute phase reactant, such that vWF increases with stress, injury, surgery, pregnancy, and oral contraceptive use and in the newborn period. Type 1 vWD is a quantitative deficiency, and type 2 vWD variants are qualitative deficiencies. Type 3 vWD is a rare, severe quantitative deficiency, with a prevalence of 1 in 1 million. Laboratory tests for vWD are necessary not only for diagnosis but also for determining the subtype of disease. Initial tests include factor VIII activity, vWF antigen, and ristocetin cofactor (a vWF activity assay).
In addition to these complexities with vWD testing, the ristocetin cofactor assay is labor-intensive and requires technologist expertise. An easier, automated vWF activity assay could simplify laboratory testing for vWD. The HemosIL vWF activity assay (Instrumentation Laboratories, Lexington, MA) is a latex turbidimetric immunoassay designed for use on Instrumentation Laboratory analyzers as an aid in the diagnosis and classification of vWD. The reagent contains latex particles coated with a monoclonal anti-vWF antibody directed against the platelet binding site on vWF (the platelet glycoprotein Ib receptor). If vWF in the patient plasma is normal, it binds to and agglutinates the latex particles. The degree of agglutination is directly proportional to vWF activity in the patient plasma and is measured by light transmittance, which decreases when aggregates form.
The main objective of this study was to evaluate the HemosIL vWF activity assay as a screening assay for vWD and to compare it with the ristocetin cofactor assay as the reference method, in conjunction with vWF antigen, factor VIII, and blood type results. In addition, 2 vWF antigen methods and 2 factor VIII methods were compared. We studied the hypothesis that the automated vWF activity assay (along with vWF antigen and blood type) would function as a screening assay for vWD such that normal results with the automated activity assay would require no further testing for vWD (eliminating the need for a ristocetin cofactor assay), whereas abnormal results with the automated activity assay would result in reflex testing with a ristocetin cofactor assay for definitive vWD assessment.
