Although spirometry is a simple, portable test and recommended for the diagnosis of asthma and chronic obstructive pulmonary disease (COPD), it is not routinely used in the primary care setting. Minorities and underserved populations are less likely to have spirometry assessment, leading to both over and misdiagnosis of asthma and COPD. Because dyspnea is a common symptom across multiple diseases, use of spirometry as a diagnostic tool is important. Missed, delayed, or misdiagnosis of asthma and COPD, which are considered diagnostic errors (DE), can lead to poor quality of care, increased morbidity and mortality, and increased costs to patients and health systems. Barriers to the use of spirometry have been identified at clinician/clinic and health systems levels. The REDEFINE program is designed to overcome identified barriers to spirometry use in primary care by utilizing health promoters (HPs) who perform spirometry within primary care clinics and work collaboratively with clinicians to incorporate the results at the point of care without interrupting clinic workflow. The REDEFINE trial is a comparative effectiveness study comparing outcomes of the REDEFINE program with usual care (UC) in primary care patients determined to be at increased risk of DE for asthma and COPD. The primary outcome will be all-cause hospitalizations. The secondary outcomes will be the proportion of accurate diagnosis of COPD, asthma, or asthma-COPD overlap based on initial diagnosis and spirometry and all cause and respiratory-related acute outpatient care and emergency department visits. In this report, we describe the design and methods for the REDEFINE trial.