Procalcitonin and interleukin-6 to diagnose infection in cardiac surgery patients with hyperinflammation: a two-centre, prospective cross-sectional study

Background Inflammation after cardiac surgery complicates infection diagnosis. The diagnostic value of procalcitonin (PCT) in patients with suspected infection remains unclear. We studied the diagnostic performance of PCT in elective cardiac surgery patients suspected of early-onset infection. Secondarily, we analysed C-reactive protein (CRP), leucocytes, interleukin-6 (IL-6), and neutrophil/lymphocyte ratio (NLR). Methods A two-centre, prospective study was conducted. Patients were suspected of infection if they developed abnormal body temperature (<36.0°C or >38.0°C), had blood cultures drawn, or received antibiotic treatment within the first 3 postoperative days. A positive and negative diagnostic threshold was assessed at the time infection was suspected, with a negative likelihood ratio (–LR) <0.2 and a positive likelihood ratio (+LR) >5 deemed clinically relevant for ruling out and ruling in infection, respectively. Results Infection was confirmed in 21/284 (7%) patients. The area under the curve (AUC) for PCT was 0.59 (95% confidence interval [CI] 0.47–0.71). No PCT threshold met predefined clinical relevance criteria, with a minimum –LR of 0.25 at 0.15 μg L^–1 and maximum +LR of 2.07 at 7.16 μg L^–1. The AUC for IL-6 was 0.70 (95% CI 0.59–0.80), achieving a –LR of 0.15 and a +LR of 5.37 at thresholds of 73 and 531 pg ml^–1, respectively. This enabled infection to be ruled out for 82 (29%) patients and ruled in for three (1%) patients. CRP, leukocytes, and NLR did not differentiate between infected and non-infected patients. Conclusions PCT did not meet clinically important thresholds, offering no valuable diagnostic information in patients with suspected infection early after cardiac surgery, whereas IL-6 did. IL-6 appears promising for ruling out infection, warranting further investigation. Trial registration number clinicaltrials.gov, registration number NCT05199025, Open Science Framework (OSF) (https://doi.org/10.17605/OSF.IO/6DGJS.