Case definition

We obtained HCUP-NIS data from 2009 to 2016. The study cohort was restricted to episodes of care that involved those aged 65 years and over, the age commonly used to define older persons in the USA [21]. The International Classification of Diseases 9th Revision and 10th Revision (ICD-9 and ICD-10) primary procedure and diagnostic codes were used to identify admissions with a primary diagnosis of UTIs. Following coding practice in the USA, ICD-9 codes were used for data before the fourth quarter of 2015, and ICD-10 codes were used for data from the fourth quarter of 2015 until the end of 2016. Cases were identified as admissions with a primary diagnosis of UTIs based on the definition of AHRQ Prevention Quality Indicator 12 for Urinary Tract Infections [22]. This definition comprises urinary tract infection, acute cystitis, cystitis, acute pyelonephritis, renal/perirenal abscess, pyeloureteritis cystica, pyelonephritis or pyonephrosis not specified as acute or chronic, and infection of kidney; it excludes cases with any codes for kidney/urinary tract disorder or transferred from another health care facility.

Infections due to Methicillin-resistant Staphylococcus aureus (MRSA) have been associated with high rates of mortality and prolonged hospital stay [23], and MRSA is the only resistant organism for which specific codes have been created by the ICD system since October 2008 [24]. Therefore, we decided to report resistance due to MRSA separately from other types of AR. Episodes with all types of AR, including beta-lactam resistance (BR), resistance due to MRSA, multidrug resistance (MR), quinolone resistance (QR), were identified using ICD codes (See S1 Appendix). We were also particularly interested in BR, resistance due to MRSA, MR, and QR as these types of resistance can develop in patients with UTIs [18] and have a major impact on patient outcomes [3,5]. AR was defined as having any ICD-9 V09.XX codes or diagnosis codes for MRSA (038.12, 041.12, 482.42), and equivalent ICD-10 codes. The ICD-9 codes were also used to define BR (V09.0, V09.1), QR (V09.5X), and MR (V09.81, V09.91), along with equivalent ICD-10 codes. An age-adjusted Deyo-Charlson Comorbidity Index (ACCI) was established using ICD codes to identify any of the following comorbidities for each admission: congestive heart failure, chronic pulmonary disease, cerebrovascular disease, diabetes mellitus with or without chronic complications, dementia, myocardial infarction, rheumatic disease, peripheral vascular disease, mild, moderate or severe liver disease, peptic ulcer disease, renal disease, hemiplegia or paraplegia, and HIV/AIDS. We used the Deyo-Charlson Comorbidity Index as this provided details on ICD-9 codes to define comorbidities, and applied age-adjusted version of Charlson Comorbidity Index as older patient data were used in this study [25,26]. Episodes with comorbidities were then weighted using the Deyo-Charlson algorithm and ACCI score calculated [25,26].