hair samples with no drug and with drug at the cut-off concentration in a cocaine RIA used in the authors' laboratory. The distribution of 100 negatives is shown in the histogram nearest the y-axis, and the distribution of these same negatives spiked with cocaine at the cut-off concentration is shown to the right in the figure. If there is a great variability in the responses of the negative samples (termed the B0, which is the amount of binding in the absence of nonradioactive drug), this variability will likely also occur at the cut-off, creating greater uncertainty in the correct identification of samples containing the cut-off concentration of cocaine. In this assay, the mean of the negatives shown was 99.1% B/B0, with a standard deviation (SD) of 2.4. (% B/B0 is the response of the unknown divided by the negative or B0 reference, expressed as a percent). The spread of such a population of samples is a result not just of matrix effects, but also of the many factors that affect precision. One can estimate the contribution of matrix differences among different samples to this spread by comparing the precision of replicates of the same sample at zero and at the cut-off concentration of drug. In this case, the mean of 20 replicates of the same negative sample had a mean of 98.4% B/B0 and a SD of 1.74 (Table 1), indicating that the matrix effects were quite small, because the precision among the 20 replicates of the same samples had nearly the same amount of error.

Figure 1 also illustrates another desirable feature of a screening assay (i.e., a clear separation between the negative population and the population at or beyond the cut-off). In this example, the lower edge of the negative 3 SD distribution of the zero-drug samples is a full 30% B/B0 units above the upper edge of the 3 SD distribution of the samples at the cut-off. Note that at the cut-off there will always be one-half of the samples falling above and one-half falling below the cut-off. A sizable separation between the negatives (zero drug) and the cut-off must not be achieved, however, at the expense of operating in the optimal region of the assay. An assay usually has a working range for quan-titation purposes of one to two orders of magnitude at best, with the optimum precision in the steeper part of the curve (in the case of a competitive RIA or EIA). Although assays using only a cut-off calibrator do not require a full dose-response curve, knowing the nature of such a curve is helpful in determining the optimum point for the cut-off. Placement of the cut-off in the most linear region of the curve facilitates achieving maximal precision at the cut-off and at points 25% and 50% above and below the cut-off.

Achieving acceptable statistical precision for samples containing drug at ±25% and ±50% of the cut-off has been a challenge for hair-screening assays. Controlling matrix effects, especially at the levels of sensitivity required, is likely the largest single factor in doing so.

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