Covariates

We measured the following covariates as of the surgery date: age (in categories; 65–74, 75–79, 80–84, 85–90 and 90+ years), sex, BMI (in categories; underweight: BMI <18.5 kg/m², normal weight: BMI ≤18.5–24.9 kg/m², overweight: BMI 25.0–29.9 kg/m² and obese: BMI ≥30.0 kg/m²), comorbidity level, marital status (married, not married), type of fracture (femoral neck and sub-/per-trochanteric fractures), type of surgery (osteosynthesis and hemi/total arthroplasty) and surgery year (biannual). Furthermore, we included the use of corticosteroids, anti-osteoporotic medicine, non-steroidal anti-inflammatory drugs, oral anticoagulants, statins, selective serotonin reuptake inhibitors and antibiotics due to the potential association between these drugs and infection risk. Patients were categorized into nonusers (no redemption of a prescribed specific drug in the year prior to surgery), former users (redemption of at least one prescription drug 91–365 days prior to hip fracture surgery) and current users (at least one prescription drug ≤90 days prior to hip fracture surgery).

The comorbidity level was measured with the Charlson Comorbidity Index (CCI) score. We defined three comorbidity levels; none, given to patients with no previous record of diseases included in the CCI; a medium level of comorbidity and a high level of comorbidity, given to patients with a record of diseases equaling CCI-index scores at 1 to 2 and 3 or more, respectively. All primary and secondary diagnoses included in the CCI (see appendix 1 for ICD, revision 10 codes) and registered in relation to hospitalizations and outpatient visits over a ten-year period before the hip fracture formed the basis of the CCI calculation. In addition to the CCI, we included the presence of an alcoholism-related disease as an individual comorbid condition.

All relevant ATC codes and ICD codes used to define the study population, infections and comorbidities are available in appendix 1. The distribution of diseases from the Charlson index according to delay in surgery is also available in appendix 1.