återfallsperiod

Återfallsperiod is a term used primarily in medical contexts to describe the interval between successive episodes of a disease after a period of remission or inactivity. It refers to how long elapses before a relapse or recurrence occurs and can be used to compare the relapse risk across treatments, patient groups, or disease subtypes. The exact definition of what constitutes a relapse can vary by condition, so the term is often used with field-specific nuances.

The concept appears in several medical disciplines. In oncology, the period until cancer returns after achieving remission is a key measure, sometimes termed time to recurrence or recurrence-free interval. In neurology and psychiatry, it may describe the time between attacks or episodes, such as multiple sclerosis relapses or depressive/manic episodes. In infectious diseases, relapses can occur after apparent clearance of infection, for example malaria relapses due to latent liver forms or reactivation of tuberculosis.

Measurement and analysis typically treat återfallsperiod as time-to-event data. It is commonly summarized by the median recurrence time or by recurrence-free survival in cohort studies. Population-level analyses may report incidence rates of relapses per person-year. Statistical methods include Kaplan-Meier estimates and Cox proportional hazards models; in studies with multiple relapses per patient, recurrent-events models (such as Andersen-Gill or Prentice-Williams-Peterson) may be used.

Notes on usage emphasize that definitions and endpoints can differ by disease, affecting comparability. Factors such as treatment changes, adherence, comorbidities, and censoring influence estimates of the återfallsperiod. The term is more commonly used in Swedish medical literature; English-language equivalents include “recurrence interval,” “time to recurrence,” or “relapse interval.”