Is a single set of negative blood cultures sufficient to ensure clearance of bloodstream infection in patients with Staphylococcus aureus bacteremia? The skip phenomenon.

Background: The most recent version of the Infectious Diseases Society of America guidelines for the treatment of methicillin-resistant Staphylococcus aureus infections states that a single set of negative blood cultures is sufficient to demonstrate clearance of bacteremia. However, S. aureus might exhibit fluctuating blood culture positivity, labeled as "the skip phenomenon". Our objectives were to determine the prevalence of the skip phenomenon in a cohort of hospitalized patients with S. aureus bacteremia and to determine the associated clinical variables.

Methods: We conducted a nested case-control study, using a previous cohort of 757 adult inpatients between July 2006 and June 2011 with ≥ 3 days of S. aureus bacteremia. Each case of S. aureus bacteremia with the skip phenomenon was matched to 2 to 4 controls based on age, gender, and duration of bacteremia. The association of clinical characteristics with the skip phenomenon was analyzed via conditional logistic regression.

Results: Of the 757 patients in the cohort, 29 (4%) had the skip phenomenon. 26 (90%) patients in the cases group were male. The median age was 69.4 years (interquartile range [IQR] 58.7 to 80.3). Although an attempt to match for the duration of bacteremia was done, there was a statistically longer duration in patients with cases as compared to that in controls (median [IQR], 10 [7-12] days, vs 8 [6-10] days; P = 0.015). Accordingly, duration of bacteremia was adjusted for in regression models. Notably, 26 (90%) patients in the case group were receiving chronic immunosuppressive therapy, as compared to 69 (79%) patients in the control group (P = 0.427).

Conclusion: Our findings prompt consideration of a practice chance to obtain serial negative blood cultures to ensure clearance of bacteremia among patients with S. aureus bacteremia.