Poisson regression for the effect of month on dSSI rates over time (laminectomy and fusion/revision fusion).
| Period | Patients (n = 22,568) | No. of dSSI (n = 225) | dSSI Rate (%) | Rate Ratio (95% CI)a | P Value |
| Overall | 22,568 | 225 | 0.99 | 0.90 (0.86, 0.94) | <0.01 |
| Month | |||||
| July (reference variable) | 1882 | 21 | 1.12 | --- | --- |
| January | 2039 | 22 | 1.08 | 1.09 (0.60, 1.98) | >0.05 |
| February | 1895 | 17 | 0.89 | 0.91 (0.48, 1.73) | >0.05 |
| March | 2107 | 17 | 0.81 | 0.79 (0.42, 1.51) | >0.05 |
| April | 1944 | 22 | 1.13 | 1.35 (0.74, 2.45) | >0.05 |
| May | 2016 | 25 | 1.24 | 1.11 (0.62, 1.99) | >0.05 |
| June | 2052 | 23 | 1.12 | 1.05 (0.58, 1.90) | >0.05 |
| August | 1897 | 22 | 1.16 | 1.13 (0.62, 2.06) | >0.05 |
| September | 1642 | 16 | 0.97 | 1.04 (0.54, 1.99) | >0.05 |
| October | 1764 | 13 | 0.74 | 0.62 (0.31, 1.24) | >0.05 |
| November | 1675 | 11 | 0.66 | 0.82 (0.40, 1.70) | >0.05 |
| December | 1618 | 16 | 0.99 | 0.92 (0.48, 1.76) | >0.05 |
Abbreviation: dSSI, deep surgical site infection.
Note: Boldface indicates statistically significant findings.
↵a Rate ratio obtained from Poisson regression.