Circulating Surfactant Protein-D for Risk Stratification in Paediatric Acute Lung Infections: A Systematic Review

Background and Objectives: Surfactant protein-D (SP-D) enters the circulation when the alveolo-capillary barrier is injured. We synthesised evidence on the diagnostic and prognostic performance of circulating SP-D in children with acute infectious lung disease. Methods: We searched MEDLINE, Embase and Scopus (inception-1 June 2025) for human studies reporting serum/plasma SP-D in patients <18 years with community-acquired pneumonia (CAP), viral pneumonitis or paediatric ARDS (PARDS). Two reviewers independently screened, extracted data and assessed risk of bias (ROBINS-I). Primary outcomes were discrimination of severe versus non-severe disease and prediction of hard outcomes (mechanical ventilation, PARDS and mortality). Heterogeneity in assays and outcome definitions precluded meta-analysis; a narrative synthesis was undertaken. Results: Five studies (n = 723) from emergency and PICU settings met inclusion criteria. Admission SP-D was consistently higher in severe versus mild CAP; reported AUCs ranged 0.699-0.802. Thresholds of 110-180 ng/mL yielded sensitivities of 67-85% and specificities of 45-70%. In influenza-associated respiratory failure, SP-D correlated with ventilator days (r ≈ 0.45) and ICU length of stay (r ≈ 0.44). In multicentre PARDS cohorts, each 10 ng/mL increase in SP-D was associated with higher odds of severe PARDS and death (adjusted OR 1.02 per 10 ng/mL). Overall risk of bias across studies was low-to-moderate, with one study rated serious due to sampling and adjustment limitations. Conclusions: Across pathogens and care settings, elevated circulating SP-D correlates with radiographic consolidation, evolving PARDS and worse short-term outcomes. Although assay standardisation and external validation are needed, current evidence supports incorporating SP-D into multiparametric, age-aware risk-stratification algorithms for childhood pneumonia and viral lung injury.