Preexisting frailty at ICU admission has been well investigated and linked to adverse outcomes, the continuum and determinants of new onset of frailty remain poorly characterized. This study aimed to estimate transition probabilities and intensities across non-frailty, frailty, and death after critical illness, and explore a set of potential variables associated with these transitions.

We conducted a multicenter prospective cohort study of ICU survivors, followed from ICU discharge to 6 months. Frailty was assessed with the Clinical Frailty Scale (CFS), with scores ≥5 defined as frail and <4 as non-frail. Multistate Markov models were used to estimate transition probabilities and intensities among three states: frailty, non-frailty, and death. Univariate and multivariable Markov models were used to identify predictors of state transitions.

At follow-up, 66.1% of frail patients reversed to non-frail, 25.7% remained frail, and 8.2% died. Among those non-frails at baseline, 72.9% remained stable, 21.2% developed frailty, and 5.9% died. Recovery from frailty occurred more than three times as often as progression from non-frailty to frailty (transition intensity ratio, 3.12; 95% CI, 2.22–4.38). The transition from frailty to death was over twice as frequent as from non-frailty to death (ratio, 2.52; 95% CI, 0.39–16.37). Sepsis-related complications significantly increased the risk of frailty onset and mortality, whereas higher body mass index, muscle mass, handgrip strength, and phase angle consistently promoted recovery and survival. Older age, female sex, comorbidities, malnutrition, and prolonged ICU or hospital stay impaired recovery.

Frailty after critical illness is a dynamic and potentially reversible condition, with transitions shaped by sepsis-related complications, nutritional and muscular status, and patient vulnerability factors. Early post-ICU frailty assessment and targeted interventions addressing infection, nutrition, and physical function may facilitate recovery and improve survival in ICU survivors.