Test serum samples were diluted 1:200 in PBS-T20 containing 0.01% merthiolate (Sigma). Dilute test serum samples (0.2 ml each) were added totest wells in triplicate, and 11 control serum samples were added to each plate. Plates were incubated at 37°C for 1 h in a sealed humid container and washed four times as before.