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The Fontan procedure is the definitive surgical approach for managing congenital heart defects causing functionally univentricular physiology. 1 Performed between 4–6 years of age, it involves connecting the systemic venous return to the pulmonary circulation. However, over time, the Fontan circulation causes venous congestion and decreased cardiac output, leading to hepatic fibrosis and ischemia.2 Consequently, patients develop Fontan-Associated Liver Disease (FALD); the most common cause of end-organ dysfunction in Fontan patients.3 Although this phenomenon is well-documented within the literature, no study clarifies when FALD begins. This study aimed to determine the initiation and extent of FALD in the paediatric population, thus helping inform effective follow-up protocols for liver surveillance post-Fontan.We conducted a retrospective longitudinal analysis of 353 Fontan patients at a major paediatric centre in England. Patients born between 2003–2022 who underwent a Fontan operation before 2023 were included. Patients who subsequently underwent heart/liver transplants or were lost to follow-up were excluded. LFTs and FBCs were obtained for each patient preoperatively and postoperatively at 5,10, and 15 years, where available. The variables included Bilirubin, ALP, ALT, AST, GGT, Albumin and Platelets. At each time point, an abnormal LFT was defined as a derangement of any of the above variables, and these results were compared between time points. Further comparison of levels of each variable between time points was performed using the Wilcoxon signed-rank test. The extent of liver damage was also assessed using post-Fontan ultrasound scans (USS).Preoperative, 5-year, 10-year, and 15-year postoperative LFTs were available in 155/353, 298/353, 166/353 and 51/353 patients, respectively. Of these, the proportions of abnormal LFTs were 17%, 56%, 71% and 63%, respectively. Table 1 summarises the statistically significant changes in each variable between the preoperative and postoperative time points. No analysis was conducted for AST and GGT due to missing data. Pertinently, Bilirubin (p=0.012), ALT (p<0.001) and Platelets (p<0.001) levels showed significant changes within the first 5 years post-Fontan. Changes in Albumin and ALP were attributed to confounding variables, including bone growth and Fontan comorbidities. 82 USS were available at a median of 8 [IQR: 5–10] years post-Fontan, with 59% exhibiting abnormal echotexture. In three cases, focal lesions were visualised on USS but were confirmed to be normal on biopsy and MRI.Although it is uncertain whether the statistically significant LFT changes correlate to clinical significance, we can conclude that the liver experiences some insult within the first 5 years post-Fontan. In current practice, global follow-up protocols for FALD surveillance begin 5–10 years post-Fontan. Considering our findings, we propose preoperative and annual postoperative LFTs be performed for all Fontan patients. Furthermore, we recommend that paediatric centres introduce and regularly collect a Fontan liver panel for all patients to enhance early detection of FALD. These may be reviewed periodically in Fontan liver clinics where Cardiologists and Hepatologists work in conjunction, as has been successfully implemented in our centre.Abstract OC8 Table 1Summary of LFT changes post-Fontan Table showing LFT changes between time points. ↑ = significant increase. ↓ = significant decrease. ⟷ = no significant change. References Fredenburg TB, Johnson TR, Cohen MD. The Fontan procedure: anatomy, complications, and manifestations of failure. RadioGraphics 2011 Mar;31(2):453–63.de Lange C, Möller T, Hebelka H. Fontan-associated liver disease: diagnosis, surveillance, and management. Front Pediatr. 2023;11:1100514.Schleiger A, Kramer P, Sallmon H, et al. Morphologic alterations precede functional hepatic impairment as determined by 13C-Methacetin liver function breath test in adult Fontan patients. Front Cardiovasc Med. 2021;8:764009.