Antibodies negative results

Measurement of plasma tryptase levels immediately (1-4 h) following an episode may be useful to confirm that a histaminoid reaction has occurred. Some four weeks later, intradermal skin tests and specific antibody tests (radioallergosorbent PAST, enzyme-linked immunosorbent ELISA) may help to identify the causative drug, although these tests do produce false-positive and false-negative results. 

The innovative use of bioassays based on biological molecules such as antibodies is changing the way analysts approach many traditional problems. The current tendency within residue analysis is toward the development of either fast screening procedures that are optimized for maximum sample throughput and a minimum chance of obtaining false-negative results, or confirmatory methods. 

In a separate line of research, the majority of 129 strain mice were shown to be resistant to a primary infection with 5. japonicum (Mitchell et al., 1984). Moreover, these mice had a dominant antibody response to a 26 kDa antigen from adult worms that was also strongly reactive with sera from rabbits multiply immunized with adult worm extract (Beall and Mitchell, 1986). This allowed the cDNA to be cloned from an expression library and identified as a GST (Smith et al., 1 986). The native protein was easily purified on a glutathione affinity column and used for protection experiments, with mostly negative results except for >50% on one occasion in C57BL/6 mice (Mitchell et al., 1988). The natural resistance of 129 strain mice for S. mansoni was also demonstrated (Tiu et al., 1 986) but unfortunately proved to be the result of a defective portal vasculature rather than acquired immunity (Cox, 1990). This finding undermined the whole concept of immunity induced by GST in 129 strain mice and as a consequence vaccine work with the Philippines strain of S. japonicum largely petered out. 

As antibodies may cross-react with other proteins both methods could lead to false positive results. Further, the alteration of the structure of the protein could render the protein unrecognizable for the antibodies causing false negative results. Changes in the structure of proteins can be induced through any kind of processing, no-... 

One of the problems with the diagnosis and treatment of EPM is that the sensitivity and specificity of the tests available currently to diagnose this disease continue to be debated and antibodies to S. neurona may persist in the cerebrospinal fluid (CSF), making it difficult to evaluate treatment efficacy. A negative result on Western blot of a CSF sample is evidence of absence of infection (Daft et al 2002). It also indicates resolution of infection and is the most reliable predictor that a horse will not relapse if the treatment is discontinued. 

In this study, from 957 specimens analyzed, GC-MS results indicate 151 positive and 806 negative results. These are compared with results of the immunoassay methods in Table 16.7. This study concluded that lowering the cannabinoid cutoff concentration from 100 to 50 pg/L THCCOOH increased the percent correct identification of true-positive specimens by all of the commercial immunoassays tested. Therefore, the sensitivities of all of the immunoassays were enhanced, resulting in improved efficiency for these assays. As expected, the specificity decreased slightly when the lower (50 pg/L) cutoff value was used. It is also evident from the results shown in Table 16.7 that there are discrepancies between the results of the eight commercial immunoassays. Preliminary tests suggest that these discrepancies can be attributed to differences in antibody selectivities, since different antibodies are used in the different assays these antibodies may be expected to show different cross-reactivities with other cannabinoid metabolites, as well as other interfering species. 

Currently, CgA is measured by immunoassay. Depending on the assay, polyclonal or monoclonal antibodies are used. Care must be taken in choosing an assay since CgA and the other chromogranins are heavily processed after release, which may render them nondetectable by the assay and produce false-negative results. Therefore an assay that recognizes both the intact and processed molecule may be desirable. 

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