OBJECTIVELoss to follow-up has been shown to bias outcomes assessment among studies utilizing clinical registries. Here, the authors analyzed patients enrolled in a national surgical registry and compared the baseline characteristics of patients captured with those lost to follow-up at 2 years.METHODSThe authors queried the Quality Outcomes Database for patients with grade I lumbar degenerative spondylolisthesis undergoing a surgical intervention between July 2014 and June 2016. Only those patients enrolled in a multisite study investigating the impact of fusion on clinical and patient-reported outcomes (PROs) among patients with grade I spondylolisthesis were evaluated.RESULTSOf the 608 patients enrolled in the study undergoing 1- or 2-level decompression (23.0%, n = 140) or 1-level fusion (77.0%, n = 468), 14.5% (n = 88) were lost to follow-up at 2 years. Patients who were lost to follow-up were more likely to be younger (59.6 ± 13.5 vs 62.6 ± 11.7 years, p = 0.031), be employed (unemployment rate: 53.3% [n = 277] for successful follow-up vs 40.9% [n = 36] for those lost to follow-up, p = 0.017), have anxiety (26.1% [n = 23] vs 16.3% [n = 85], p = 0.026), have higher back pain scores (7.4 ± 2.9 vs 6.6 ± 2.8, p = 0.010), have higher leg pain scores (7.4 ± 2.5 vs 6.4 ± 2.9, p = 0.003), have higher Oswestry Disability Index scores (50.8 ± 18.7 vs 46 ± 16.8, p = 0.018), and have lower EQ-5D scores (0.481 ± 0.2 vs 0.547 ± 0.2, p = 0.012) at baseline.CONCLUSIONSTo execute future, high-quality studies, it is important to identify patients undergoing surgery for spondylolisthesis who might be lost to follow-up. In a large, prospective registry, the authors found that those lost to follow-up were more likely to be younger, be employed, have anxiety disorder, and have worse PRO scores.