a reagent for an elisa determination of an antibody , the reagent comprising a peptide covalently linked to beta - lactamase . the reagent can be used in the following method to detect antibodies in a sample which involves a . contacting the sample with protein a linked to a solid support , b . incubating the sample - protein a linked to the solid support , c . washing the incubated sample - protein a linked to the solid support , d . contacting the washed sample - protein a with the reagent , e . incubating the sample - protein a and reagent , f . washing the incubated sample - protein a - reagent , and g . determining the enzymatic activity of the resultant mass .