Based on the conflicting evidence before us, we are unable to determine what value the person
of ordinary skill in the art would use as the background absorbance. Without knowledge of the
appropriate background level, any determinations based on a Scatchard analysis, including the
estimate of the affinity constant, are speculative.
b) The value of Ab*
Engvall’s example 1 also fails to explicitly describe the value of Ab*, the assay result for a
sample having a great excess of antigen. As we indicated above, the value of Ab* is significant
because it is used in the calculations of both the x- and y-axis values that are used in the Scatchard
Plot. Bergland used a value of 1.6 for Ab*, the maximum reported absorbance value reported in
Engvall’s example 1. E128, p. 3. We find however, that the person of ordinary skill in the art would
not necessarily recognize 1.6 as an appropriate value for Ab*.
As noted in the Walker article, Ab* is the value for a sample with a great excess of ligand.
E57, p 588. In other words, it is the immunometric assay result for a system which has nearly
reached saturation --a condition in which a large increase in concentration (L ) would have only a
a
relatively minor effect on the assay value (B*). What Walker means by “a great excess of ligand” can
be seen from Graphs 2 and 3 which we have prepared from the data in Walker’s tables. E57, p. 589.
Walker’s Tables 1 and 2 are reproduced in the Appendix to this opinion.
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