Results of immunoassays for which no positive standards are available are often expressed
as endpoint titers. The endpoint titer is defined as the reciprocal of the highest analyte
dilution that gives a reading above the cutoff. Unfortunately, there is no generally accepted
rule for the determination of these cutoff values. In enzyme-linked immunosorbent assays
(ELISA) a value two or three times the mean background or negative control reading is
sometimes used. Other investigators set the cutoff arbitrarily at a certain absorbance value …