AbstractBackgroundIt is unknown whether serum procalcitonin (PCT) concentration monitoring can differentiate between bacterial infection or cytokine release syndrome (CRS) when chimeric antigen receptor T‐cell (CAR‐T) recipients present with a constellation of signs and symptoms that may represent both complications.ObjectiveThe objective of the study was to assess the utility of serum PCT concentrations as a biomarker of bacterial infection in CAR‐T recipients.Study designThis single‐center, retrospective, medical record review evaluated patients prescribed CAR‐T therapy until death or 30 days after infusion. Logistic regression modeling determined the association between elevated serum PCT concentrations within 48 h of fever onset and microbiologically confirmed infection. Secondary outcomes included clinically suspected infection, CAR‐T toxicity rates, and broad‐spectrum antibiotic usage. Predictive performance of PCT was assessed by area under the receiver operating characteristic curve (AUC).ResultsThe 98 included patients were a median age of 63 (IQR: 55–69) years old, 47 (48%) were male, and 87 (89%) were Caucasian. Baseline demographics and clinical characteristics were similar between patients with and without a bacterial infection. Serum PCT >0.4 ng/mL within 48 h of fever was significantly associated with a microbiologically confirmed bacterial infection (OR: 2.75 [95% CI: 1.02–7.39], p = 0.045). Median PCT values in patients with and without confirmed infections were 0.40 ng/mL (IQR: 0.26, 0.74) and 0.26 ng/mL (IQR: 0.13, 0.47), respectively. The AUC for PCT to predict bacterial infection was 0.62 (95% CI 0.48–0.76). All patients experienced CRS of some grade, with no difference in CRS severity based on elevated PCT. Broad‐spectrum antibiotics were used for a median of 45% and 23% of days in those with and without confirmed infection, respectively (p = 0.075).ConclusionElevated serum PCT concentrations above 0.4 ng/mL at time of first fever after CAR‐T infusion was significantly associated with confirmed bacterial infection. Furthermore, rigorous, prospective studies should validate our findings and evaluate serial PCT measurements to optimize antimicrobial use after CAR‐T therapy.