A Negative Nasal Screen for Methicillin-Resistant Staphylococcus aureus (MRSA) At The Time Of Hospital Admission Is A Predictor Of Culture-Negativity for MRSA

Background: Nasal MRSA screening is often used to determine whether patients should be put in isolation to limit nosocomial spread of MRSA.  Little is known about the clinical utility of these results in predicting clinical culture results. Predicting future culture results may decrease the use of unnecessary broad-spectrum antibiotics.

Objective: We sought to determine whether admission MRSA screening results could be used to predict clinical culture results. 

Methods: At our community hospital, nasal swabs for MRSA DNA by PCR are routinely performed on patients being admitted from the Emergency Department to Med/Surg/ICU floors. Patients known to have a history of MRSA by prior screen or culture are excluded from further screening. We reviewed the records of 310 consecutive patients who underwent MRSA screening. Culture results for the first three hospital days were assessed. We subsequently reviewed all MRSA screens and MRSA cultures for admitted patients for the period 1.1.10 - 6.30.10, and calculated the positive and negative predictive values. Finally, we reviewed the charts of all admitted patients with positive MRSA cultures within the same time frame and calculated the number and proportion with evidence of MRSA by recent screen or history.

Results: Of 310 consecutive patients screened, 15 (4.8%) had a positive MRSA screen. Of those 15, seven  (46.7%) had clinical cultures performed and only one had a clinical culture positive for MRSA (positive predictive value = 6.7%). Of 295 patients with a negative screen, 62 had clinical cultures performed (21%), and none had a clinical culture positive for MRSA (negative predictive value =100%). During the six-month period there were 238 positive screens and 3472 negative screens performed.  Twenty of the 238 patients with positive screens had MRSA positive cultures (positive predictive value = 8.4 %). Of 3472 negative screens, only two patients had a positive MRSA culture within three days (negative predictive value = 99.9%). Forty-two patients had a total of 45 sites positive for MRSA by culture. Of those 42, 40 had either a positive screen at the time of admission or a history of MRSA in the past. All three patients with MRSA bacteremia, 16 patients with respiratory MRSA, and eight patients with MRSA bacteruria had either a positive screen or history.  Eleven out of twelve (91.7%) patients with a positive wound culture and one out of two patients with MRSA found in stool had a positive screen. 

Conclusions: For patients admitted to our Med/Surg/ICU units, a recent negative MRSA screen correlated with an extremely low risk of having a positive MRSA clinical culture within three days of admission. These findings may have significant implications for the selection of antibiotic therapy for hospitalized patients.