Serological testing is widely used to monitor swine herds for Actinobacillus pleuropneumoniae (APP). Several serological tests are presently used, most often the complement fixation test, the long-chain lipopolysaccharide enzyme-linked immunosorbent assay (ELISA), and the ApxIV ELISA. Serological testing occasionally generates ambiguous results. In such situations, bacterial isolation and polym...