Timing of Acquisition of Methicillin-Resistant Staphylococcus aureus Nasal Carriage: Can we Limit Repeat Screening?

Broad-spectrum empiric antibiotics are routinely administered to hospitalized patients with potential infections. These antibiotics provide protection; however, they come with their own negative effects. The utility of Methicillin-resistant Staphylococcus aureus (MRSA) polymerase chain reaction (PCR) nasal screening to steward anti-MRSA empiric antibiotics in hospitalized patients is established. With this current study, we look to determine the optimal frequency of MRSA nasal testing to help limit unnecessary testing consistent with the efforts of Choosing Wisely. We hypothesize that MRSA PCR nasal swab conversion will be low within the first 2 wk after index swab collection.

We performed a single-center retrospective chart review of all adult patient encounters from October 2019-July 2021 with MRSA PCR nasal testing. We excluded duplicate patient encounters. Further exclusion criteria included patients with a single MRSA PCR swab and those who tested positive for MRSA colonization on their index swab. We evaluated how many conversions from negative to positive there were, and the timing of those relative to those that did not develop colonization while in the hospital.

263 patients had multiple MRSA nares screening. 215 patients had 2 swab collections, 35 patients had 3 swab collections, 9 patients had 4 swab collections, and 4 patients had 5 swab collections. 14 converted from negative to positive. The time of conversions ranged from within 0-36 d, with an overall cumulative conversion of 5%. The rate of cumulative conversion from one week was 1.9%, for 2 wk it was 3.4%.

Findings suggest that MRSA PCR nasal swab conversion is unlikely to occur within 2 wk. Therefore, to optimize resources, further investigation should be conducted to target guidelines as well as systems to limit repeat swab testing. We will investigate the utility of this after implementation.