False positive Subject

Assessment and definition of sensitivity are often described for quantitative analysis but are of equal importance for qualitative devices of the dip-stick type that are very popular for farm- or field-based screening assays. Because of the somewhat subjective nature of visually assessed assays, the assay s sensitivity must be validated using a number of observers to determine at what level a test is deemed positive. The number of false positives and false negatives must be carefully determined in order to balance consumer safety and potential economic loss to animal producers. 

Patients and blood donors are routinely screened for exposure to HIV by means of ElISA and Western blot assays of blood samples (F uie 1-7-15). The assays are designed to detect antibodies to HIV in the blood of the test subject The ELISA is used as the primary screening assay because it is very sensitive. Because the reference interval for the test is set to include everyone with antibodies to HIV, it also gives false positives and thus has a rather low positive predictive value, especially in low-risk populations. The Western blot (or immunoblot) is used as the confirmatory test for HIV exposure. In the Western blot technique, specific HIV proteins are separated by gel electrophoresis and blotted to a filter. The filter is incubated with the test sample. If the sample contains antibodies to HIV, they will bind to the proteins on the filter. The filter is next washed and incubated with a labeled goat anti-human IgG to visualize any bound human antibodies. The Western blot is highly specific. The combination of an ELISA and Western blot has a positive predictive value of greater than 99%,... 

The 76 variables derived from the DEC evaluation were first analyzed using stepwise discriminant analysis to determine the variables that best predicted the presence or absence of each drug. This subset of best-predictor variables was then subjected to a discriminant function analysis that predicted and classified whether subjects were dosed or not dosed with drug. The resulting data were classified as true positive, true negative, false positive, or false negative. These parameters were then used to calculate several measures of predictive accuracy of the DEC evaluation, including sensitivity, specificity, and efficiency. 

Over 20,000 samples of microbial culture broths were subjected to our screening program for CETP inhibitors by method A. At first no BSA was added to the assay mixture, but many false-positive compounds such as fatty acids were isolated. To prevent this, the optimal concentration of BSA was tested and set up as 200 pM, resulting in a low hit rate in the primary screen. The serum albumin concentration in the assay is similar to that in human plasma. Finally, we discovered erabulenols from a fungal strain, and ferroverdins from an actinomycete strain, as novel CETP inhibitors (Fig. 5). 

An ideal tumor marker would be a molecule specific to one type of tumor (100% specificity, i.e. no false positives) and detectable right from the initial stage of the disease (100% sensitivity, i.e. no false negatives). It would be undetectable in healthy subjects, and enable the screening and diagnosis of cancer. The tumor marker level should correlate closely with tumor size, contribute to the initial extension of the profile and evaluation of therapeutic efficacy, as well as the early detection of recurrent diseases. 

Although reliable, this technique may lead to false positive results in some cases. To overcome this problem many proteomic companies are now adopting the technique of tandem mass spectrometry to unambiguously identify protein sequences. This technique subjects proteins to successive routines of fragmentation and mass analysis in order to provide the actual amino acid sequence. 

In Figure 11.1, 200 compounds that have already shown activity in other species should include 90 per cent (180) that are truly active in humans and 10 per cent (20) that lack activity. When the 180 genuinely active molecules are subjected to experiments with 90 per cent power, 162 trials will successfully detect that activity but 18 will fail to do so. Among the 20 inactive compounds, there will be the usual 5 per cent rate of false positives (one case). There are thus a total of 163 positive findings of which only one is false. 

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