Most serological analyses for PRRSV are made with ELISA. There are several ELISA kits on the market but most of them detect antibodies against nucleocapsid protein. This protein is being produced in large quantities in infected cells and stimulates strong seroconversion. However, these antibodies have no impact on protection against reinfection.
So, we can only interpret the seroconversion detected in ELISA as a proof of contact of a pig with PRRSV. Not as a proof of immunity against infection or clinical symptoms. The problem with serological assessment of sows using ELISA is that most of kits give some percent of false positive results. The phenomenon has been known for many years but the problem was not fully solved with new generations of ELISA kits.
Another aspect of interpreting ELISA results is that the method can be used for assessment of populations and not for individuals. After infection or vaccination antibodies start to appear at 10-14 days, the level grows for several weeks and then decreases. Some animals can become seronegative at 4-6 months post exposure.
It is dependent of the immunizing strain and also of the dose. In a stable sow herd (no virus circulation, no vertical transmission) gilts can be highly positive if immunized at quarantine, while old sows could be often low positive or negative. In unstable farm (virus circulates and is being vertically transmitted) naïve replacement gilts will seroconvert and may exhibit clinical symptoms. Small farms can spontaneously clear off PRRSV if internal replacement is applied.
If naïve replacement gilts are introduced, they will remain seronegative, while old sows can show some levels of antibodies. Answering the question, yes, low positive sows can show acute clinical symptoms of PRRS.
To be sure that the suspected symptoms are caused by PRRSV the best is to test fetuses or weak piglets by PCR. Detecting PRRSV will be an ultimate proof of PRRS caused reproductive failure. Serological analysis (unless it takes into account detailed analysis of multiple age sows) will be of little diagnostic help.