ObjectivePatient‐related outcome measures (PROMs) can guide clinicians in providing evidence‐based treatment and have the potential to empower patients, support clinical decision making, and improve quality of care. In order to make the information coming from PROMs useful, it is important to know to what extent the use of PROMs is biased in any way. Therefore, we assessed (a) the current level of use of PROMs among primary care physiotherapists and (b) which factors on the patient level, therapist level, and primary care practice level are associated with the use of PROMs in patients with nonspecific low back pain (LBP).DesignAn observational study based on electronic health record data recorded routinely in Nivel Primary Care Database.ParticipantsA total of 2916 patients aged 18 years or older with nonspecific LBP consulting a primary care physiotherapist.MethodsMultilevel logistic regression analyses were used to identify factors at the level of the patient, physiotherapist, and primary care practice, which may affect the use of PROMs.ResultsPROMs were used in 46% of the patients, by 72% of the physiotherapists, and in 71% of the physiotherapy practices. None of the included independent variables were associated with the use of PROMs. Only 1% of the variance was explained by the final model.ConclusionThis study shows that the use of PROMs is mostly dependent on characteristics of patients. However, we did not succeed in identifying characteristics of patients that are responsible for that. This could mean that therapists randomly choose patients for PROMs or that there is some other unmeasured patient characteristic determining the use of PROMs. The former explanation implies no systematic bias in the information resulting from PROMs. More research is needed to examine possible related factors to improve implementation and a more frequent use of PROMs in the future.