Discussion

Repeat nasal MRSA PCRs detected rare conversions from negative to positive results within 14 days. Although the NPV for ruling out MRSA pneumonia has repeatedly been demonstrated, ^{Reference Turner, Seligson, Parag, Shea and Hobbs3–Reference Dangerfield, Chung, Webb and Seville6} positive nasal MRSA PCRs have shown low positive predictive value (35–45%) suggesting limited value for diagnosing MRSA pneumonia. ^{Reference Dangerfield, Chung, Webb and Seville6,Reference Parente, Cunha, Mylonakis and Timbrook7} The rare conversion from negative to positive within 14 days, low rate of MRSA isolation from culture among converted results, and the poor positive predictive value indicate repeating nasal MRSA PCRs within 14 days is of limited utility. Additionally, these results suggest that escalation to anti-MRSA therapy for pneumonia within 14 days of a negative nasal MRSA PCR is likely not indicated, as also noted by Turner et al. ^{Reference Turner, Seligson, Parag, Shea and Hobbs3}

To our knowledge, no published studies have evaluated nasal MRSA PCR concordance to previously collected molecular results. Turner et al. evaluated the impact of timing on the predictive value of nasal MRSA PCR in relation to positive respiratory cultures. A high NPV of nasal MRSA PCR overall (94.9%) was reported with consistent results across all cohorts (within 24 hours, 25 to 48 hours, 49 hours to 7 days, 8 to 14 days, and more than 14 days). No difference in predictive value based on level of care or culture specimen was identified. ^{Reference Turner, Seligson, Parag, Shea and Hobbs3} Palisano et al. compared patients with concordant nasal MRSA screening to respiratory cultures with those found to have discordant (negative nasal MRSA PCR and a positive MRSA respiratory culture) results to identify risk factors for discordance. A history of MRSA infection and greater than 7 days between nasal screen and respiratory culture collection were associated with discordant results; however, high-quality respiratory samples should be pursued in clinically suspected MRSA pneumonia rather than repeated nasal MRSA PCR due to continued high NPV of nasal screens beyond 7 days. ^{Reference Palisano, Yen and Mercuro5} These results support our findings by describing an overall high NPV (94.9%) within 14 days and low rate of discordance (0.28%) indicating that repeating nasal screening without respiratory culture should not alter treatment. ^{Reference Turner, Seligson, Parag, Shea and Hobbs3,Reference Palisano, Yen and Mercuro5}

This evaluation has several limitations. The dates included periods with high incidence of patients infected with Delta and Omicron severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. The surge of infections with these variants may have resulted in fewer nasal MRSA PCRs being collected due to admission for known viral illness, thus underestimating rates of repeat testing and/or MRSA. Additionally, infection precautions implemented during the pandemic may have influenced PCR conversion. Chart review was not performed which limited our ability to include respiratory microbiological data for all results; however, literature has already explored predictive values associated with MRSA PCRs and respiratory cultures as noted throughout. Medication administration data for all encounters was not collected, so we were unable to identify whether MRSA PCR was repeated to discourage empiric MRSA-active antibiotic use. Nevertheless, our data suggest that re-initiation of anti-MRSA therapy is likely unwarranted within 14 days of a negative nasal MRSA PCR given low rates of conversion to positive and four of five converted results without isolation of MRSA from culture. Finally, the small incidence of negative to positive conversions precluded the assessment of factors associated with conversion. However, it is unclear if additional data would further affect findings, given the persistently high NPV demonstrated until and potentially beyond 14 days from initial screen.

In summary, these data suggest that nasal MRSA PCRs should not be repeated within 14 days of a prior test. Local antimicrobial stewardship interventions should be implemented to limit repeated nasal MRSA PCRs and empiric MRSA-active antibiotics within 14 days of a negative screen, potentially through the use of clinical decision support tools.