In the discovery phase of the study, a total of 50 pig sera, 25 from PRRSV-positive and 25 from PRRSVnegative piglets, as determined by PCR (see below), were analyzed [Additional file 1: Table S1 ]. The validation phase was performed with the same experimental conditions as the discovery phase. A total of 35 new PRRSVpositive and 35 new PRRSV-negative piglets were examined [Additional file 2: Table S2 ]. The actual duration of infection for each individual PRRSV-positive piglet was unknown, as sera were collected and analyzed once for each piglet (at weaning: 45-50 days of age). None of the piglets was treated, as they did not show any symptom of the disease.