Chlorhexidine (CHG) susceptibilities of blood culture isolates from patients receiving CHG bathing

Background: Bathing patients with chlorhexidine has been associated with reductions in health care-associated bacteremia in intensive care units (ICU). The concern for the emergence of decreased CHG susceptibility with repeated exposure exists.

Objective: To compare CHG susceptibility of blood culture isolates recovered from patients undergoing CHG bathing with blood culture isolates from patients not receiving CHG baths.  

Methods: The medical ICU and respiratory care unit of a tertiary care hospital has used 2% CHG gluconate cloths [Sage Products Inc, Cary Ill.] for bathing since October 2008. Blood culture isolates recovered from October 2009 to September 2010 from patients on these units, and from patients housed on units not using CHG baths, were saved. These isolates were tested for CHG minimum inhibitory concentrations (MICs) using Clinical Laboratory Standards Institute broth micro-dilution methods and guidelines. Serial two fold dilutions of CHG were prepared from a 20% CHG solution (USP grade, Sigma, St. Louis MO) in cation-adjusted Mueller-Hinton broth. For each isolate, a 0.5 McFarland suspension was prepared from a fresh culture grown on appropriate medium for 16-24 hours. Approximately 5 x 10⁴ organisms were inoculated into each microplate well with 0.1 ml of media containing CHG concentrations ranging from 0.25 µg/ml to 256 µg/ml. One well without CHG was used as growth control per isolate. Plates were incubated overnight at 35⁰ C for 16-24 hours for bacteria, and 48 hours for yeast. Median [range] MICs were compared by the Wilcoxon Rank-sum Test. Days bathed with CHG was calculated from date blood culture isolate collected minus date of admission, and was analyzed in relation to MIC using a non-parametric test of trend.

Results: 118 blood culture isolates from 90 patients were tested; 49 isolates were from patients receiving CHG baths; 69 isolates were from patients not receiving CHG baths. For CHG susceptibilities, see Table 1. The median [range] of days bathed with CHG was 16 [1,243] days. By test of trend, there was no significant association between MIC and days bathed with CHG for gram positive bacteria (p=.48) and gram negative bacteria (p=.69). CHG median [range] MICs of isolates from patients receiving CHG baths were similar to median [range] MICs of isolates from patients not receiving CHG baths for gram positive bacteria [0.5 µg/ml  [0.25, 2] vs. 1.0 µg/ml [0.25, 8], p=0.11] and for gram negative bacteria [8 µg/ml [0.5,16] vs. 8 µg/ml [1,32], p=.47].              

Conclusions: In a medical ICU and respiratory care unit using CHG bathing for more than one year, there was no statistical difference in CHG MICs for blood culture isolates recovered from patients receiving CHG bathing compared with blood culture isolates recovered from patients not receiving CHG bathing. There was no significant trend between MIC and days bathed with CHG for either gram positive or gram negative blood culture isolates.