an assay method and kit for detecting the presence of a predesignated , target igg antibody in a sample selected from one or more patient bodily fluids . the method comprises the following steps : contacting the sample of one or more patient bodily fluids with a membrane - bound recombinant protective antigen to bind to the target igg antibody in the sample ; previously , simultaneously or subsequently to step , binding the protective antigen with a conjugated label producing a detectable signal ; and detecting the signal whereby the presence of the target igg antibody is determined in the sample by the intensity of the signal . the method can further comprise the step of evaluating immunization status of the patient from whom the sample came by comparing the signal or lack thereof with immunizations previously received by the patient . in a preferred embodiment , the recombinant protective antigen specifically binds to anthrax protective antigen - specific igg antibodies . preferably , the immunoassay of the present invention comprises a lateral - flow assay comprising a membrane , a conjugated label pad , and a recombinant protective antigen bound to the membrane .