Comment on: Strong site-specific association of pharyngeal cultures with the onset of psoriatic arthritis and psoriasis, regardless of pathogen.

Abstract

Dear Editor, I have read with great interest the study by Thrastardottir et al. reporting a strong site-specific association between pharyngeal culture samples and an increased risk of PsA and psoriasis, regardless of the pathogen, thereby indicating that the site of infection, rather than the pathogen, is associated with increased risk for both conditions [1]. Data from this population-based cohort study are interesting as well as surprising, as they contradict classic observations on the relationship between streptococcal infections and psoriatic disease [2]. Thus, having a positive streptococcal pharyngeal culture was protective against psoriatic disease. Sensitivity analyses showed that when pharyngeal samples were compared with all other culture sites, an HR of 9.51 for PsA and 5.80 for psoriasis was seen during the first 50 days of follow-up, while the risk was significantly reduced after that period [1]. In any case, the study raises certain methodological concerns. For example, doubts remain as to whether the unit of observation was the individual or the culture sample. If it was the individual, what culture sample was taken into account? If it was the culture sample, was it taken into account which different cultures are from the same person? If it was not taken into account, how is the lack of independence of the observations justified?