Studies will be skewed if they examine the effects of biologics without accounting for the demographics of which patients are prescribed which medications, according to new research from Swedish scholars.

“Unless differences in age, medical history, and disease activity are accounted for, they will substantially confound non-randomised comparative studies of available bDMARDs’ (biological disease-modifying anti-rheumatic drugs) safety and effectiveness,” write Thomas Frisell, of Karolinska Institutet in Stockholm, and colleagues in Annals of the Rheumatic Diseases.

The researchers studied 6,481 Swedish rheumatoid arthritis patients, or all of the RA patients in Sweden between 2011 and 2015 who initiated tumor necrosis factor inhibitor (TNFi), rituximab, abatacept, or tocilizumab. Of those patients, 2,829 switched from TNFi during that span.

The patients starting TNFi tended to be younger than those who started non-TNFi, and the older patients also tended to be poorer and less educated and to have higher disease activity and burden, including diabetes, serious infections, and malignancy, they found.

See a report in MedPage, which includes five other findings of the research.