Methods of quantifying anti-HPA-1a antibodies in a sample, comprising (a) contacting the sample with a capture reagent that binds to the anti-HPA-1a antibodies to form a detection complex, wherein the capture reagent comprises a modified integrin psi and (b) measuring an amount of the detection complex, wherein the amount of the detection complex indicates the quantity of the anti-HPA-1a antibodies in the sample. The methods may also comprise contacting the sample with a detecting reagent that binds to the anti-HPA-1a antibodies and becomes part of the detection complex. The capture reagent and/or the detecting reagent may comprise a detection label, in which measuring the amount of the detection complex comprises measuring the amount of the detectable label.