Study group and outcome measurement
The population was composed of patients with newly diagnosed constipation (ICD-9-CM codes=564.0) from 2000 to 2012. To ensure accuracy of diagnoses, we excluded any diagnosis of constipation before 2000 and only patients with at least three outpatient visits or one hospitalization were selected for inclusion in the final analysis. The index date of this cohort was set as the first date of diagnosis of constipation. Furthermore, to ensure that all individuals had new-onset asthma, we ruled out any diagnosis of asthma (ICD-9-CM=493) happening before the index date. The non-constipation group constituted subjects who had never been diagnosed with constipation for the period 1999 to 2013.
The outcome variable was defined as a diagnosis of asthma with at least three outpatient visits or once hospitalization. The patients were followed up until the occurrence of atopic dermatitis, 31 December 2013, or withdrawal from the National Health Insurance system, whichever occurred first.