Utilization of Methicillin Resistant Staphylococcus aureus Nares Screens for Empiric Antibiotic Therapy

Background: Methicillin resistant Staphylococcus aureus (MRSA) continues to be a burdensome hospital and community acquired pathogen, leading to excess morbidity, mortality and high heath care cost. As the empiric drug of choice for MRSA infections, vancomycin therapy has limitations such as side effects, emerging resistance, and selective pressures on nosocomial bacteria such as Enterococcus. Efficacy of MRSA surveillance screening to decrease in-hospital transmission is controversial, but colonization with MRSA is a known risk factor for developing infection with potential utility in designing empiric antibiotic therapy.

Methods: A retrospective chart review was completed on 3341 patients with any positive culture and nares screen performed from May 2013 to December 2014. Culture source, organism, and duration between screen and culture were collected. Patient in all clinical settings, and all specimen types were included.

Results: Of the 3341 cultures, 1962 (58.7%) were urine cultures, 316 (9.5%) were blood cultures, 732 (21.9%) were skin/soft tissue (SST) cultures, and 165 (4.9%) were respiratory cultures. The performance characteristics of nares screening is shown in the table below with no statistical difference between blood, SST or respiratory tract infections. Nares screening was statistically least predictive of urine infection. Average time between screen and culture was 124 days with the median time of 42 days. 23.6% of patients were colonized with MRSA and 58.4% of S. aureus cultured was resistant to methicillin.

Source	Sensitivity	Specificity	PPV	NPV	OR (95% CI, p)
Blood	51.0	74.5	26.9	89.2	3.05 (1.63-5.71, p=0.0005)
Urine	42.6	78.6	5.4	97.9	2.73 (1.57-4.74, p=0.0002)
Skin/Soft Tissue	49.6	80.7	35.9	88.0	4.12 (2.77-6.13, p<0.0001)
Respiratory Tract	57.6	81.8	44.2	88.5	6.1 (2.69-13.86, p<0.0001)
All	50.0	78.9	18.3	94.4	3.73 (2.92-4.78, p<0.0001)

Conclusion: Given the high NPV of nares screens, regardless of time between screen and culture proven infection, excluding empiric vancomycin may be considered in certain clinical scenarios, including de-escalation of antibiotics. This could potentially limit unnecessary vancomycin exposure and associated risks. These results show that MRSA colonization is a significant risk factor for developing subsequent infections, and further research aimed at reducing such risk with decontamination or other strategies is needed.