Abstract Body (Do not enter title and authors here): Introduction: Leukocyte activation is reported to be an important inflammatory pathway underlying atrial fibrillation (AF). The lymphocyte-to-monocyte ratio (LMR) is a novel, cost-effective, and widely available marker of inflammation.

Research Questions: The usefulness of LMR in predicting late recurrence of AF (LRAF) after catheter ablation (CA), and the impact of AF type on its usefulness, have not yet been elucidated.

Goals: We aimed to elucidate the association between LMR measured just before CA and the risk of LRAF in patients who underwent CA for AF, and to investigate whether the type of AF impacts the predictability of LMR for LRAF.

Methods: We enrolled 597 consecutive patients with AF (median age, 65 years; 32% female; 334 patients with paroxysmal AF [PAF] and 263 patients with persistent AF [PeAF]) who underwent initial radiofrequency CA. LMR was measured within 3 days prior to the date of CA, and calculated by dividing the absolute lymphocyte count by the absolute monocyte count using the same blood samples. Any detectable atrial tachyarrhythmia lasting for longer than 30 s beyond a 3-month blanking period after the procedure was considered as LRAF. Patients were followed-up for a maximum of 3 years.

Results: During a median follow-up of 24 months, LRAF occurred in 215 patients (96 with PAF and 119 with PeAF). Although there was no significant difference in LMR between patients with and without LRAF in PAF (5.3 [4.1–6.7] vs 5.2 [4.0–6.6], p=0.403), LMR in patients with LRAF was significantly lower than those without LRAF in PeAF (4.7 [3.9–6.4] vs 5.6 [4.5–6.8], p=0.015). Multivariate Cox analysis showed that LMR was an independent predictor of LRAF in PeAF (p=0.008), but not in PAF (p=0.662). Receiver operating characteristic curve analysis revealed that an LMR of 4.8 was a fair discriminator for LRAF in PeAF. Although there was no significant difference in the risk of LRAF between the patients with lower (LMR<4.8) and higher LMR (LMR≥4.8) in PAF, patients with lower LMR had a significantly higher risk of LRAF than those with higher LMR in PeAF (Figure).

Conclusions: Pre-ablation LMR was shown to be useful for predicting LRAF in patients with PeAF who underwent CA for AF, while not in patients with PAF.