Some observations on the indirect Elisa for antibodies toMycoplasma iowaeserovar i in sera from turkeys considered to be free from Mycoplasma infections

Abstract

A number of sera from turkeys considered to be uninfected with Mycoplasma iowae were examined by indirect ELISA using whole cell Ai. iowae serovar I antigen. In an attempt to reduce the number of ‘false positive’ sera, assays were made (a) using plates blocked with foetal calf serum or skim milk before, or, bovine serum albumin after, coating with antigen, (b) using the absorbance difference method in which each serum was tested against mycoplasma antigen and medium antigen and (c) with unblocked plates but with serum heated to 56 C for 30 min. The upper limit of absorbance for the negative sera was determined using formulae already used by others including, the mean plus twice the standard deviation, the mean plus three times the standard deviation, (M + 3SD), twice the mean, 115% of the mean and four times the standard deviation. Some reduction in the number of ‘false positive’ sera occurred after heating sera or using the formula of M + 3SD. This formula might, however, reduce the sensitivity of the test.