We found only 1 instance of seroconversion among the 87 participants (including the 6 with PCR-confi rmed infection), each of whom had 2 serum samples taken for antibody measurement. Miller et al. reported that 89.1% of participants with pandemic (H1N1) 2009 had an antibody titer of >32 three weeks after infection, although a baseline serum sample was not taken; therefore, seroconversion could not be demonstrated (17) . None of the participants with positive PCR results reported taking NIs, and all had serum samples taken >2 weeks after the positive nose swab specimen, allowing suffi cient time for seroconversion. Our results are likely to be true positives, as all swabs were only taken when patients were symptomatic. Previously, virus isolation has been the gold standard for infl uenza detection but RT-PCR is now considered to be more sensitive and specifi c. A previous study by some of the current authors has shown that seroconversion occurs in 80%-90% of serum samples if they are tested a suffi cient time after infection (confi rmed by RT-PCR) (19) . Nasal swabs are a relatively peripheral type of sample (20) . If viral load is low in the nose, the sample may be insuffi cient as an antigenic stimulus to induce a detectable level of seroconversion in the serum. This may be the explanation for the lack of seroconversion seen in some PCR-positive cases in this study. Because the number of pandemic (H1N1) 2009 cases in Victoria was low by the time this study commenced, we used a single antibody measurement for diagnosis in most patients. This is not ideal, because some participants may have had preexisting cross-reactive antibodies, as reported by others (15, 16) . However, this cross-reactivity has been most commonly found in older persons >65 years of age, a population which was underrepresented in our study. The explanation given for presence of cross-reactive antibodies in older persons has been past exposure to other antigenically similar viruses or a lifetime exposure to infl uenza A virus (17) . Because this exposure could not have occurred in our younger study participants (median age 38 years) and serum samples were collected toward the end of the pandemic wave when many would have already been exposed, reactivity likely was specifi c to pandemic (H1N1) 2009. These factors support the use of a single antibody measurement for diagnosis.