Estimation of seasonal effect on the psoriasis recurrence using time dependent coefficient rates model for recurrent events

Psoriasis is an inflammatory skin disease, which is related to the immune system and is characterized by the rapid growth of skin cells. The aim of this study was to investigate some risk factors in psoriasis and also to check that whether the rate of recurrence is fixed or variable in different seasons by using a time-dependent coefficients rate model. After reviewing the files of psoriasis patients that had been hospitalized in March 2011 to March 2014 in the dermatology ward of Imam Khomeini Hospital in Tehran, a total of 160 patients who had relapses leading to hospitalization, entered to the study. A rate model with time dependent coefficients by using R software version 3.1 was fitted and risk factors for recurrence were identified. Effect of the season on the recurrence of psoriasis was evaluated. Recurrence rate of psoriasis was 7.2 times higher in winters than in springs, which had the lowest recurrence rate. Spring and summer were associoated with lower recurrence rates. Effect of season on the relapse of the disease was significantly different for different seasons (P<0.01). Also, infection, stress, positive family history, hypocalemia, the normal number of white blood cells and temperature changes were associated with psoriasis relapse (P<0.05). Recurrence rate of psoriasis is different in different seasons. Psoriasis recurrence rate was 7 times higher in winters in comparison with springs.