If 2-fold dilutions are used, the quantities Xp, x, , and may be obtained from Table 3. In a one-stage factor VIII assay employing serial 2-fold dilutions the following clotting times were obtained. The estimate of potency is obtained from the ratio of the difference between patient and control readings to the slope of the lines (see below). This distance is the log of the dilution factor when serial dilutions are employed, e.g., log 2 for a series of 2-fold dilutions, log 4 for a 4-fold series. The estimation of potency from the vertical distance between the line requires also an index of slope for the portion of the curve being employed, which is given by the difference between the high and low responses of the two lines, divided by the log distance between the dilutions tested. The calculations are done in whichever transformation is necessary to allow parallel straight lines to be fitted. In fact, as events would soon prove, the newly found product had an even broader activity than had been anticipated. Success had been achieved in the quest for a fermentation product with anthelmintic activity. After an Inauspicious start, the activity of culture OS-3153 had been firmly established. A few cultures had been confirmed active previously, but none had exhibited a separation of toxicity from activity, let alone activity over at least an 8-fold range. All four levels were fully active, and this time there was little sign of toxicity. In the third test, the activity was titrated using serial 2-fold dilutions.
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