Assessment of differential item functioning of the PHQ-9, HADS-D and PROMIS-depression scales in persons with and without multiple sclerosis

•We observed differential item functioning (DIF) in two of the three scales tested.•PROMIS-Depression scale was unaffected by DIF for age, gender, BMI or diagnosis.•DIF by diagnosis (multiple sclerosis (MS) vs. non-MS) for the PHQ-9 was confounded.•DIF by diagnosis (MS vs. non-MS) for the HADS-D was confounded.•Key confounders of DIF by diagnosis were age, gender and BMI.