Transition from ICD-9-CM to ICD-10-CM/PCS in Portugal: An heterogeneous implementation with potential data implications.

Dear Editor, In 2013, the planning process to implement the International Classification of Diseases – 10th revision – Clinical Modification/Procedure Coding System (ICD-10-CM/ PCS) for clinical coding in all public hospitals in Portugal began, replacing its predecessor, the International Classification of Diseases – 9th revision – Clinical Modification (ICD-9-CM), after almost three decades in use (Diário da República, 2013). The main rationale behind this transition is to improve specificity as the ICD-9-CM might fail to adequately represent all the current diversity of diagnoses and procedures (ACSS, 2016a; Lopes, 2012; Watzlaf et al., 2015). Moreover, the use of ICD-10-CM/PCS would allow an adaptation to newer definitions of the Diagnosis-Related Groups (DRG) software, which determines a case mix system for hospital reimbursement in Portugal and several other countries. The transition process to ICD-10-CM/PCS started in Portugal in August 2016, with three public hospitals being selected as pilots to implement the new classification system in October 2016, while the remaining hospitals would shift to ICD-10-CM/PCS by January 2017. In Portugal, clinical coding is done by trained medical coders based on information concerning patient disease progress, procedures and treatments that are routinely reported in health records, discharge notes, as well as pathology and surgical reports (ACSS, 2016b, 2021). Following the clinical coding of episodes, an audit phase is internally performed in hospitals usually by auditing clinicians (Barros and Braun, 2017). These audit processes are quite heterogeneous and not systematically performed in a hospital setting. In fact, despite some internal reviews takingplace, some medical coders have stated the need for relevant information for health records audits, namely, clearly defined audit processes, widely accepted official guidelines and health records standards (Alonso et al., 2020a). The resulting coded data concerning all inpatient, surgical or day hospital outpatient episodes from all hospitals within the Portuguese National Health Service (NHS) are gathered and held at the Hospital National Morbidity Database, managed by the Central Authority for Health Services, I.P. (ACSS). Despite its main purpose of hospital reimbursement, this comprehensive database is reused in several subdomains, ranging from clinical or health services research, to management and epidemiological studies, by either national or international reusers. During the transition process to ICD-10-CM/PCS, clinical coders’ training played a critical role. However, there are still sources of heterogeneity among hospitals regarding clinical coding practices that also impacted this transition, as resources, documents and tools used and abstracted during the clinical coding process (either electronically or on paper) differ considerably according to the hospital (Alonso et al., 2020a, 2020b). This process is also highly dependent on the information systems (also heterogeneous) of each hospital. These existing differences were inevitably reflected during the ICD-10-CM/PCS transition period (October 2016 to January 2017). Only one pilot hospital managed to start the newly issued clinical coding system in October 2016, whereas the other two pilot institutions