Objective Biochemical and cytological pericardial fluid (PF) analysis is essentially based on the knowledge of pleural fluid composition. The aim of the present study is to identify reference intervals (RIs) for PF according to state-of-art methodological standards.

Methods We prospectively collected and analysed the PF and venous blood of consecutive subjects undergoing elective open-heart surgery from July 2017 to October 2018. Exclusion criteria for study enrolment were evidence of pericardial diseases at preoperatory workup or at intraoperatory assessment, or any other condition that could affect PF analysis.

Results The final study sample included 120 patients (median age 69 years, 83 men, 69.1%). The main findings were (1) High levels of proteins, albumin and lactate dehydrogenase (LDH), but not of glucose and cholesterol (2) High cellularity, mainly represented by mesothelial cells. RIs for pericardial biochemistry were: protein content 1.7–4.6 g/dL PF/serum protein ratio 0.29–0.83, albumin 1.19–3.06 g/dL, pericardium-to-serum albumin gradient 0.18–2.37 g/dL, LDH 141–2613 U/L, PF/serum LDH ratio 0.40–2.99, glucose 80–134 mg/dL, total cholesterol 12–69 mg/dL, PF/serum cholesterol ratio 0.07–0.51. RIs for pericardial cells by optic microscopy were: 278–5608 × 10⁶ nucleated cells/L, 40–3790 × 10⁶ mesothelial cells/L, 35–2210 × 10⁶ leucocytes/L, 19–1634 × 10⁶ lymphocytes/L.

Conclusions PF is rich in nucleated cells, protein, albumin, LDH, at levels consistent with inflammatory exudates in other biological fluids. Physicians should stop to interpret PF as exudate or transudate according to tools not validated for this setting.