Introduction NMB facilitates intubating conditions in general anesthesia. However, it is associated with significant residual postoperative paralysis and morbidity. Objective To investigate the rate of underdiagnosed residual NMB based on two TOFR criteria (< 0.91 and < 1.00). Methods We performed a retrospective study adhering to STROBE guidelines. We included patients undergoing ENT surgery using single-dose neuromuscular block for balanced general anesthesia from June to December 2018. We collected demographic and anthropometric data, ASA score, NMBA dose, TOFR recordings at 5, 30 and 60 min and end of the surgery, anesthesia and surgery time, and administration of reversal agent. Statistical analysis included descriptive and dispersion measures statistics, curve and cross tables for residual NMB on different TOFR criteria with sub-analysis for AR, RR, and OR in patients over 65 years old. Results We included 57 patients, mean age 41; 43 females and 14 males. Mean anesthetic and surgical time were 139.4 and 116.1 min, respectively. All the patients received rocuronium under a mean ponderal single-dose of 0.48 mg/kg. Residual NMB rates were 29.9 and 49.1% for a TOFR < 0.91 and < 1.00, respectively. Older adults had an OR of 6.08 for residual NMB. Conclusions The rate of residual NMB was 29.9 to 49.1%, depending on the criteria used (TOFR < 0.91 and < 1.00, respectively). Patients above 65 years old had an increased risk of residual NMB (6.08 OR) and clinical symptoms related to residual NMB (11.75 OR). We recommend future research aiming to provide a specific surveillance protocol for patients above 65 years old, including shorter-action NMB, early reversal, and prolonged surveillance using the TOFR criteria of < 1.00 to identify patients at risk of residual NMB readily.