“…Thus, as test conditions and reagents vary, results vary. Although the choice of a constant multiple of the mean absorbance yielded by negative sera to be the threshold for positivity in an ELISA is common (3, 6, 12), some authors have chosen as threshold the mean absorbance of negative controls plus a constant multiple (usually 2, 3, or 5) of the standard deviation (5,14). In theory, this method avoids some of the problems mentioned above; however, unless an impractically large number of wells of negative controls are tested on each plate, the chance variability in the standard deviation is too large to make this method useful (12).…”