"Conclusions about etiology are more difficult to determine if viral sequences are detected by PCR in the absence of clinical symptoms". In other words its important to consider the much greater sensitivity of PCR compared to old culture-based diagnostics, in the context of false positives (environmental or laboratory cross-contamination) and what a weak H7N9-positive might practically mean.
A good seroconversion still can't be beat in these cases!
IMHO - experienced diagnostic labs should have long ago dealt with this issue. PCR has been in use as the "gold standard" for years.
In a situation like the one ongoing, I assume that all H7N9 positives are being suitably confirmed using follow-up confirmatory testing using previously established protocols.