Objective
We aim to create a web-based calculator for assessing depressive symptoms with the Patient Health Questionnaire 9 (PHQ-9), utilizing IRT-based standardized scores, to improve measurement precision, standardization, and practical application in clinical practice.

Methods
This study developed a web-based calculator using a graded response IRT model for assessing depressive symptoms with the PHQ-9, using data from the Brazilian National Health Survey 2019 (n = 90,846, aged 15 to 107 years old, 52.8% female). The tool calculates latent depressive symptoms and converts them into T-scores, with stratification by sex and age groups. The application respects patient confidentiality by deleting sensitive information postcalculation.

Results
Estimated models resulted in a mean sample size of 3244.5 participants in each group (SD: 1066). The calculator can be accessed at https://mheg.shinyapps.io/phq9-score.

Discussion
The development of an IRT-based web calculator for the PHQ-9 represents an advancement in depressive symptoms’ assessment, offering precision and potential clinical utility. By standardizing scores into a common metric, this tool facilitates the interpretation of depressive symptoms and comparison across different instruments. The study's scope is limited to the Brazilian population and external validity for other contexts is warranted. Future studies should evaluate the clinical validity and the threshold of the tool for predicting real life problems.