Diagnostic Performance and Prognostic Value of Serum Amyloid A in Patients with Bloodstream Infection

Background: Serum amyloid A (SAA) is an acute-phase reactant that increases rapidly in response to inflammatory stimuli and infection, earlier and more markedly than conventional markers such as C-reactive protein (CRP). However, large-scale evidence of its clinical utility in bloodstream infections (BSIs) remains limited. This study aimed to evaluate the diagnostic and prognostic value of SAA in patients presenting to the emergency department (ED) with suspected infection. Methods: We retrospectively reviewed the electronic medical records of adult patients who underwent simultaneous SAA and blood culture (BC) testing at the ED of a tertiary referral hospital between January and December 2025. Initial laboratory data, including CRP, procalcitonin (PCT), white blood cell (WBC) count, and absolute neutrophil count (ANC), were collected, and BSI was defined as the isolation of pathogenic organisms in BC. Correlations and agreement between SAA and other markers were assessed, and the diagnostic performance of BSI was evaluated using receiver operating characteristic curves and the area under the curve (AUC). Survival outcomes were analyzed using the Kaplan-Meier method. Results: Among the 3321 included patients, 379 patients (11.4%) had positive BCs. Median SAA levels were significantly higher in BSI patients than in non-BSI patients (202.0 vs. 71.0 mg/L, p < 0.001), with the highest levels observed in Gram-negative infections. SAA showed a strong correlation with CRP (r_s = 0.884) and a moderate correlation with PCT (r_s = 0.576). The AUC for BSI diagnosis was highest for PCT (0.789), followed by SAA (0.650). The SAA demonstrated high sensitivity (90.5%) but low specificity (26.9%). Higher SAA levels were significantly associated with increased mortality rates. Conclusions: In adult ED patients, SAA is significantly associated with BSI and mortality and is a sensitive biomarker for the early detection of BSIs. Although SAA alone showed inferior discriminative performance compared to PCT, it may serve as an adjunctive screening tool in the ED, particularly in settings where PCT availability is limited.