Aernoud Fiolet

318 Chapter 13 represent the major EHR software vendors in the Netherlands (Epic [Hospital A], ChipSoft [Hospital B], CSC Care solutions [Hospital C]; cumulatively used in 80% of the Dutch hospitals and almost 10% of the hospitals worldwide. 15,16 Participants of the LoDoCo2 trial were retrieved on their trial identification number and unique on-site identifier as recorded in their EHR files. Participants for which no trial identifiers were reported in the EHR were ignored since they could not be linked to CRF data functioning as the reference standard. Participant identification methods Automatic, using text-mining from EHRs. A Boolean retrieval query to obtain the required data was developed in adherence with the eligibility criteria of the LoDoCo2 trial by two authors (WBvD and ATLF) (Supplement 1a). For developing the query a graphic user interface data mining tool with text-mining features was used (CTcue, version 2.0.12; Amsterdam, The Netherlands). This data mining tool integrally searched structured and unstructured EHR data (including clinical letters, in-hospital consultations, procedures, diagnostic tests, and drug prescriptions). Both authors who developed the query were considered to have content expertise from their medical backgrounds and had extensive experience in query development.Additionally, one of these authors (ATLF) was also a lead investigator of the LoDoCo2 trial. The query consisted of regular expressions of the eligibility criteria as given by the LoDoCo2 trial, their synonyms, and negations (e.g., “no hypertension” instead of “hypertension”). Synonyms were added using the automatic synonym expander built into the datamining tool and supplementedwith synonyms and abbreviations commonly used by the query developing authors (Supplement 1a). For precluding automatic retrieval of information entered in the EHR after trial participation, only data registered in EHRs prior to the screening of the trial were used. No site-specific optimizations were added to the query, except for the retrieval of trial participants and periprocedural drug recognition adjustments. To approximate data collection as would have been performed in the trial, the most recent status on any data point before entering the trial was taken. Additionally, drug use data were limited to data registered within a year of enrollment. When no measurement of a variable was found, it was assumed to be absent for the participant.