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Introduction Physiological myocardial FDG uptake limits diagnostic accuracy in cardiac FDG-PET. Adequate suppression requires fasting, compliance with which is difficult to objectively verify. Beta-hydroxybutyrate (BHB) rises with fasting, reflecting metabolic shift from glucose to fatty acids. This study evaluated BHB as a marker of preparation adequacy and myocardial suppression.Methods Cardiac FDG-PET reports from St Bartholomew’s Hospital (2018–2025) were reviewed for inadequate suppression. Pre-scan BHB was measured before all FDG-PET scans using the Optium Neo® point-of-care meter. BHB in cardiac and non-cardiac scans was compared using Welch’s t-test. Logistic regression, likelihood-ratio tests, and ROC analysis assessed BHB’s relationship with suppression outcome.Results Of 716 cardiac scans, 631 (88.1%) showed no physiological uptake and 85 (11.9%) did. Following BHB implementation, 56 cardiac and 115 non-cardiac scans were performed. Mean BHB was higher in cardiac than non-cardiac scans (0.45 ± 0.31 vs. 0.27 ± 0.24 mmol/L, p < 0.001), confirming BHB as a fasting surrogate. Of cardiac scans, 47 (83.9%) achieved complete suppression, 3 (5.4%) partial, and 6 (10.7%) failed. Mean BHB was higher in complete versus inadequate suppression (0.48 vs. 0.29 mmol/L). BHB ≥0.5 mmol/L yielded PPV 95.7%, specificity 88.9%, and sensitivity 46.8% for complete suppression. ROC analysis gave AUC 0.69 (95% CI 0.52–0.86). BHB remained independently predictive after adjustment for age, indication, and preparation adherence (p = 0.041) (figures 1–5).Conclusion Pre-scan BHB testing provides an objective marker of preparation adequacy in cardiac FDG-PET. A threshold of ≥0.5 mmol/L reliably predicts complete myocardial suppression, supporting scan interpretation and diagnostic confidence. Which of the following is the currently being explored as an objective predictor of inadequate myocardial suppression in patients undergoing cardiac FDG-PET?A: Serum insulin level measured on the day of the scanB: Continuous glucose monitoring in the 24 hours prior to scanningC: Patient self-reported dietary adherence questionnairesD: Fasting plasma glucose measured at time of FDG injectionAbstract 4 Figure 5Abstract 4 Figure 4Abstract 4 Figure 3Abstract 4 Figure 2Abstract 4 Figure 1E: Point-of-care beta-hydroxybutyrate measurement prior to scanning (correct)