Risk Factors for Colistin Resistance among Gram-Negative Rods and Klebsiella pneumoniae Isolates.

Infections due to colistin-resistant (Col) Gram-negative rods (GNRs) and colistin-resistant isolates in particular result in high associated mortality and poor treatment options. To determine the risk factors for recovery on culture of Col GNRs and Col, analyses were chosen to aid decisions at two separate time points: the first when only Gram stain results are available without any bacterial species information (corresponding to the Col GNR model) and the second when organism identification is performed but prior to reporting of antimicrobial susceptibility testing results (corresponding to the Col model). Cases were retrospectively analyzed at a major academic hospital system from 2011 to 2016. After excluding bacteria that were intrinsically resistant to colistin, a total of 28,512 GNR isolates (4,557 isolates) were analyzed, 128 of which were Col (i.e., MIC > 2 μg/ml), including 68 of which that were Col In multivariate analysis, risk factors for Col GNRs were neurologic disease, residence in a skilled nursing facility prior to admission, receipt of carbapenems in the last 90 days, prior infection with a carbapenem-resistant organism, and receipt of ventilatory support (-statistic = 0.81). Risk factors for Col specifically were neurologic disease, residence in a skilled nursing facility prior to admission, receipt of carbapenems in the last 90 days, receipt of an anti-methicillin-resistant antimicrobial in the last 90 days, and prior infection with a carbapenem-resistant organism (-statistic = 0.89). A scoring system derived from these models can be applied by providers to guide empirical antimicrobial therapy in patients with infections with suspected Col GNR and Col isolates.