BACKGROUND: Early identification of TB cases, followed by treatment to completion, are essential for controlling and preventing the disease. Previous studies have found some factors associated with both loss to follow-up (LTFU) and patient delay. We aim to build a causal model to investigate the association between TB patient delay and LTFU.METHODS: Pulmonary TB cases were identified using the national surveillance system in Portugal between 2008 and 2017. A directed acyclic graph was used to identify the minimal set of variables to adjust for when studying the association between delay (exposure) and LTFU (outcome). Crude and adjusted hazard were estimated using Cox regression.RESULTS: Nearly 4% of the patients did not follow up treatment. There was no association between patient delay and LTFU, even after adjustment with the minimal set of covariates. Factors associated with a higher risk of LTFU were being younger, being unemployed, living in urban areas, having HIV and the abuse of alcohol and drugs.CONCLUSION: Patient delay was not associated with LTFU, while social conditions were. Future research should investigate the underlying reasons why patients discontinue TB treatment and use these findings to develop targeted interventions that can support patients in completing their treatment regimen.