Evaluation of nasal methicillin-resistant Staphylococcus aureus screening for intraabdominal infections

Author(s):
Gabrielle Gibson; Emily Owen; John Mazuski; Jessica Kramer

Background:

Intra-abdominal infections (IAIs) are associated with significant morbidity and mortality in the ICU. Early diagnosis, adequate source control, and appropriate antimicrobial therapy are the cornerstones of treatment. Most IAIs are caused by gram-negative organisms, while methicillin-resistant Staphylococcus aureus (MRSA) is an uncommon organism in IAIs. Despite MRSA being a rare organism in IAIs, if isolated it does confer an increased risk of mortality. Guideline recommendations suggest only adding anti-MRSA antibiotics when certain risk factors are present, however, anti-MRSA agents are frequently utilized for empiric treatment of IAIs. Therefore, it would be useful if nasal MRSA screening could be used to help limit exposure of anti-MRSA agents. This study aimed to evaluate the performance of nasal MRSA screening to predict MRSA IAIs in critically ill adult patients.

Hypothesis:

Nasal MRSA screening identifies patients with intra-abdominal infection who will have positive cultures for MRSA

Methods:

This was a retrospective, cohort study at single center that took place between June 2018 and December 2020. Adult patients admitted to an ICU with a diagnosis code for an IAI and had an intra-abdominal culture were eligible for inclusion. Patients were excluded if they died within 96 hours of admission. The primary outcome was to determine the performance of the MRSA nasal swab by assessing sensitivity, specificity, positive and negative predictive values.

Results:

There were 79 patients that had a MRSA nasal swab that was positive (NS+) and 57 patients that had a negative MRSA nasal swab (NS-). Patients in the NS+ group were older (59 years vs 53 years; p=0.019) and had higher APACHE II scores (13 vs 10; p=0.01) compared to the NS- group. Nine patients (11.4%) in the NS+ group had IAI cultures positive for MRSA compared to 38 patients (66.7%) in the NS- group (p<0.001). MRSA nasal swab screening demonstrated a sensitivity of 19% (95% CI: 9%-33%) and specificity of 21.35% (95% CI: 13.4%-31.3%).The positive predictive value of the nasal swab predicting an IAI with positive MRSA culture was 11.4% (95% CI: 6.6%-19%), while the negative predictive value was 33.3% (95% CI: 24.7%-43.3%). The ICU length of stay was similar between groups (6.8 days in NS+ group vs 5.7 days in the NS- group; p=0.584), however, patients in the NS+ group spent more time in the hospital (15 days vs 11 days; p=0.026).  There was no difference between groups in in-hospital mortality.

Conclusions:

A negative MRSA nasal swab was not useful in excluding MRSA as a causative organism in IAIs.