Background Evaluating severity of illness of patients with prolonged mechanical ventilation (PMV) is important to adopt the best appropriate care management for each individual. Yet, no severity-of-illness scoring system has been specifically designed for this type of patients. The aim of this study was to develop and validate a new instrument, the Multi-INdependence Dimensions (MIND) questionnaire designed to comprehensively measure the severity of illness of patients under PMV. Methods The validation of the MIND questionnaire was performed during a longitudinal observational study conducted with PMV subjects in weaning facilities in three countries (Argentina, Colombia and Germany). The questionnaire validity was tested in 3 stages: 1) Specification of components, with description of item responses, inter-item and Cronbach alpha correlations; 2) Creation of the composite scores; 3) Measurement properties determination including test-retest reliability after 30 days, clinical validity (Medical Research Council (MRC) muscle strength score, Sepsis-related Organ Failure Assessment (SOFA), Glasgow Coma Scale (GCS), Dependence Nursing Scale and EuroQol-5 Dimension evaluated at inclusion), and ability to detect change. Results A total of 128 subjects participated in the validation study. Eleven component scores and four composite scores were created. MIND scores significantly correlated with MRC muscle strength, SOFA, DNS, GCS and EQ-5D, supporting the validity of the new scores. Intraclass Correlation Coefficient greater than 0.82 were observed for all composite scores, indicating good test-retest reliability. MIND scores were able to detect improvement in subject severity of illness. Conclusion The MIND questionnaire is a valid and reliable instrument for measuring comprehensively the multiple dimensions characterizing the severity of illness of PMV patients. Trial registration NCT02255058 . Electronic supplementary material The online version of this article (10.1186/s12890-019-0870-2) contains supplementary material, which is available to authorized users.

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