Risk factors, biomarkers, and mechanisms for persistent inflammation, immunosuppression, and catabolism syndrome (PICS): a systematic review and meta-analysis

Introduction Persistent inflammation, immunosuppression, and catabolism syndrome (PICS) has been proposed as an endotype of chronic critical illness (CCI). The aim of this systematic review is to synthesise the available evidence of risk factors, biomarkers, and biological mechanisms underlying PICS. Methods MEDLINE, CENTRAL, and EMBASE were searched on June 2, 2023. Our population of interest was adult intensive care unit survivors. The exposure group was patients with PICS and the comparator group was patients with no PICS, CCI, or rapid recovery. Mean differences were pooled for each biomarker using a random effects DerSimonian–Laird method. Risk of bias assessment was done using the Newcastle–Ottawa Scale. Results Six papers were included. Five were single-centre retrospective cohort studies, and one was a prospective cohort study, with sample sizes ranging from 22 to 391 patients. Two studies showed an increased incidence of PICS with age, and two studies showed an association between PICS and Charlson Comorbidity Index scores. PICS was associated with requiring mechanical ventilation in four studies. Meta-analysis showed a 34.4 mg L−1 higher C-reactive protein (95% confidence interval [CI] 12.7–56.2 mg L−1; P<0.01), a 4.4 g L−1 lower albumin (95% CI 0.5–8.3 g L−1; P<0.01), and a 0.36×109 L−1 lower lymphocyte count (95% CI 0.25–0.47×109 L−1; P=0.01) in the PICS compared with the non-PICS group. There are a large variety of other potential biomarkers but limited validation studies. The overall quality of evidence is limited, and these results should be interpreted accordingly. Conclusions While older patients and those with co-morbidities could be at greater risk for PICS, acquired risk factors, such as injury severity, are potentially more predictive of PICS than intrinsic patient characteristics. There are many potential biomarkers for PICS, but limited validation studies have been conducted. Persistent myeloid-derived suppressor cell expansion, the continual release of danger-associated molecular patterns and pathogen-associated molecular patterns propagating inflammation, and bioenergetic failure are all mechanisms underlying PICS that could offer potential for novel biomarkers and therapeutic interventions. Clinical trial registration International Prospective Register of Systematic Reviews (PROSPERO; CRD42023427749).