OBJECTIVE: Medication nonadherence is common in patients with systemic lupus erythematosus (SLE) and negatively affects outcomes. To better recognize and address nonadherence in this population, there is a need for an easily implementable tool with interpretable scores. Domains of Subjective Extent of Nonadherence (DOSE-Nonadherence) is a measure that captures both extent of and reasons for nonadherence. We refined and evaluated DOSE-Nonadherence for patients with SLE. METHODS: We refined the reasons for the nonadherence domain of DOSE-Nonadherence through rheumatologist feedback and patient cognitive interviewing. We then administered the refined instrument to patients prescribed oral SLE medications and compared the results to the Beliefs About Medicines Questionnaire (BMQ), the Medication Adherence Self-Report Inventory (MASRI), medication possession ratios (MPRs), and hydroxychloroquine (HCQ) blood levels using Pearson correlations. RESULTS: Five rheumatologists provided feedback; 16 patients (median age 43 yrs, 100% female, 50% Black) participated in cognitive interviews and 128 (median age 49 yrs, 95% female, 49% Black, 88% on antimalarials, and 59% on immunosuppressants) completed the refined instrument. Items assessing extent of nonadherence produced reliable scores (α 0.89) and identified 47% as nonadherent. They showed convergent validity with MASRI (r = -0.57), HCQ blood levels (r = -0.55), to a lesser extent MPRs (r = -0.34 to -0.40), and discriminant validity with BMQ domains (r = -0.27 to 0.32). Nonadherent patients reported on average 3.5 adherence barriers, the most common being busyness/forgetting (62%), physical fatigue (38%), and pill fatigue (33%). CONCLUSION: Our results support the reliability and validity of DOSE-Nonadherence for SLE medications. This refined instrument, DOSE-Nonadherence-SLE, can be used to identify, rigorously study, and guide adherence intervention development in SLE.