More surgery in December among US patients with commercial insurance is offset by unrelated but lesser surgery among patients with Medicare insurance.

Study Objective: Evaluate whether there is more surgery (in the US State of Florida) at the end of the year, specifically among patients with commercial insurance.

Design: Observational cohort study.

Setting: The 712 facilities in Florida that performed inpatient or outpatient elective surgery from January 2010 through December 2019.

Results: Among patients with commercial insurance, December had more cases than November (1.108 [1.092-1.125]) or January (1.257 [1.229-1.286]). In contrast, among patients with Medicare insurance (traditional or managed care), December had fewer cases than November (ratio 0.917 [99% confidence interval 0.904-0.930]) or January (0.823 [0.807-0.839]) of the same year. Summing among all cases, December did not have more cases than November (ratio 1.003 [0.992-1.014]) or January (0.998 [0.984-1.013]). Comparing December versus November (January) ratios for cases among patients with commercial insurance to the corresponding ratios for cases among patients with Medicare, years with more commercial insurance cases had more Medicare cases (Spearman rank correlation +0.36 [+0.25], both p < 0.0001).

Conclusions: In the US State of Florida, although some surgeons' procedural workloads may have seasonal variation if they care mostly for patients with one category of insurance, surgical facilities with patients undergoing many procedures will have less variability. Importantly, more commercial insurance cases were not causing Medicare cases to be postponed or vice-versa, providing mechanistic explanation for why forecasts of surgical demand can reasonably be treated as the sum of the independent workloads among many surgeons.