Testing principle, advantages, and disadvantages of commercially available assays to evaluate VWF activity
| . | Assay principle . | Advantages . | Disadvantages . |
|---|---|---|---|
| VWF:Ab | Measures binding of a monoclonal antibody to the VWF A1 domain epitope | Not affected by VWF exon 28 benign variantsa | • Not a true functional assay • Misses the type 2M VWF c.3971G>C, p.Gly1324Ala variant |
| VWF:RCo | Measures the ability of VWF in patient plasma to agglutinate donor platelets in the presence of ristocetin | Broadly available | • Highest coefficient of variation • Limited lower level of quantificationc • Affected by VWF exon 28 benign variants • Can miss type 2B patients, especially those with the VWF c.3916C>T, p.Arg1306Trp variant |
| VWF:GPIbM | Measures the spontaneous binding of VWF to a gain-of-function mutant GPIba fragment | Not affected by VWF exon 28 benign variants Low variability | • Can miss some type 2B patients • Can overestimate activity in patients with very low VWF:Ag |
| VWF:GPIbR | Measures the ristocetin-induced binding of VWF to a recombinant wild-type GPIb fragment | Least variable and lowest level of detectionb | May be affected by VWF exon 28 benign variantsd |
| . | Assay principle . | Advantages . | Disadvantages . |
|---|---|---|---|
| VWF:Ab | Measures binding of a monoclonal antibody to the VWF A1 domain epitope | Not affected by VWF exon 28 benign variantsa | • Not a true functional assay • Misses the type 2M VWF c.3971G>C, p.Gly1324Ala variant |
| VWF:RCo | Measures the ability of VWF in patient plasma to agglutinate donor platelets in the presence of ristocetin | Broadly available | • Highest coefficient of variation • Limited lower level of quantificationc • Affected by VWF exon 28 benign variants • Can miss type 2B patients, especially those with the VWF c.3916C>T, p.Arg1306Trp variant |
| VWF:GPIbM | Measures the spontaneous binding of VWF to a gain-of-function mutant GPIba fragment | Not affected by VWF exon 28 benign variants Low variability | • Can miss some type 2B patients • Can overestimate activity in patients with very low VWF:Ag |
| VWF:GPIbR | Measures the ristocetin-induced binding of VWF to a recombinant wild-type GPIb fragment | Least variable and lowest level of detectionb | May be affected by VWF exon 28 benign variantsd |
VWF exon 28 benign variants affecting ristocetin sensitivity include VWF c.4138A>G, p.Ile1380Val (I1380V); VWF c.4304A>G, p.Asn1435Ser (N1435S); and VWF c.4414G>C, p.Asp1472His (D1472H).
When performed by chemiluminescence immunoassay.
Lowest sensitivity activity assay. Does not allow for accurate subtype classification in 18% of patients.10
The effect of these variants has been hypothesized to be similar to VWF:RCo; however, a large study comparing activity assays did not confirm this finding.