TABLE 2.

Characterization of nonspecific samples

Sample group	Sample(s)	Observations	Plausible interference mechanisms
I	1, 2, 3, 4, 5, 6, 7, 9, 10, 12, 13, 15, 16	Reactivity with HRP conjugates of different specificities and utilizing different conjugation procedures (see Table 1); microbial antigen is not needed to produce nonspecific signals; negative in immunoblot assay; negative in indirect IgM EIA; variable reactivity with coated HRP; abolishment or drastic reduction of signals when polymerized HRP is added; intensities of nonspecific responses are variable	Polyreactive (anti-immunoglobulin and anti-HRP) heterophilic antibodies of IgM class
II	8, 11	Positive in our test model in the presence of specific microbial antigen and in some commercial IgM capture methods; negative in immunoblot assay and in some other commercial IgM capture methods; negative in indirect IgM EIA	Polyreactive heterophilic IgM antibodies of very low affinity; some uncertainty remains if sample 8 is a true positive
III	14	Strong reactivity with all tested conjugates and with free HRP in the capture method; reaction with solid phase coated with HRP; drastic reduction of signals when polymerized HRP is added	Predominantly anti-HRP IgM class antibodies
IV	Ang	Positive in indirect T. gondii IgG and low positive in immunoblot assays; other characteristics as in group I	Specific anti-T. gondii IgM response combined with nonspecificity by polyreactive heterophilic antibodies