Assessment of the effectiveness of weight-adjusted antibiotic administration, for reduced duration, in surgical prophylaxis of primary hip and knee arthroplasty

Table 1 Demographic information for patients treated with the old regime and new regime of perioperative antibiotic prophylaxis, n (%)

	Old regime	New regime
	n = 669 patients	n = 595 patients
Age (mean ± SD)	69.3 ± 11.9 yr	68.8 ± 10.5 yr
Gender (male)	279 (41.7)	214 (40.0%)
BMI (mean ± SD)	32.2 ± 11.0	31.8 ± 7.4
Operation performed
THA	THA, n = 291 (43.1)	THA, n = 243 (40.6)
TKA	TKA, n = 384 (56.9)	TKA, n = 355 (59.4)

BMI: Body mass index; SD: Standard deviation; THA: Total hip arthroplasty; TKA: Total knee arthroplasty.

Table 2 Demographic information for patients treated with the old regime and new regime of perioperative antibiotic prophylaxis (complete data)

	Old regime, n (%)	New regime, n (%)
	n = 310	n = 458
Age (mean ± SD)	67.5 ± 10.9 yr	67.0 ± 10.5 yr
Gender (male)	132 (42.6)	165 (36.0)
Weight (kg; mean ± SD)	84.82 ± 20.9	87.73 ± 22.3
Weight > 120 kg	19 (6.1)	44 (9.6)
Operation performed
Total hip arthroplasty	124 (40.0)	198 (43.2)
Total knee arthroplasty	186 (60.0)	260 (56.8)
Comorbidities
ASA grade
1	5 (1.6)	12 (2.6)
2	154 (49.7)	212 (46.3)
3	148 47.7)	227 (50.0)
4	3 (1.0)	7 (1.5)
Diabetes mellitus
Yes	49 (15.8)	48 (10.5)
No	261 (84.2)	410 (89.5)
Anticoagulation on admission
Yes	27 (8.7)	29 (6.3)
No	283 (91.3)	429 (93.7)
Antibiotic prophylaxis
Cefazolin	342 (50.7)	449 (98.0)
Clindamycin	11 (1.6)	8 (1.7)
Vancomycin	1 (0.1)	1 (0.3)
Dose appropriate for weight?
Yes	340 (50.3)	458 (100)
No	13 (2.0)	0 (0.0)

ASA: American society of anaesthesiology.

Table 3 Incidence of superficial and deep surgical site infections after 2 years follow up in patients undergoing elective primary total hip and knee arthroplasty

	Old regime, n = 669 patients	New regime, n = 595 patients
	(n = 675 THA/TKA)	(n = 598 THA/TKA)
Superficial infection, n (%)	14 (2.07)	9 (1.50)
	THA, n = 6	THA, n = 2
	TKA, n = 8	TKA, n = 7
Deep/organ space infection, n (%)	5 (0.74)	3 (0.50)
	THA, n = 1	THA, n = 2
	TKA, n = 4	TKA, n = 1

TKA: Total knee arthroplasty; THA: Total hip arthroplasty.

Table 4 Incidence of superficial and deep surgical site infections after 2 years follow up in patients undergoing elective primary total hip and knee arthroplasty (THA/TKA; Complete demographic data subgroup)

	Old regime	New regime
	n = 310 joints	n = 458 joints
Total number of infections, n (%)	9 (2.9)	9 (2.0)
Superficial infections, n (%)	8 (2.6)	7 (1.5)
	THA, n = 2	THA, n = 1
	TKA, n = 6	TKA, n = 6
Deep/organ space infections	1 (0.3)	2 (0.5)
	THA, n = 1	THA, n = 2

THA: Total hip arthroplasty; TKA: Total knee arthroplasty.

Table 5 Relative risks for all and deep surgical site infections in original dataset and after propensity score weighting

Infection	Original dataset		Weighted dataset
	RR (95%CI)	P value	RR (95%CI)	P value
Overall infection	0.69 (0.34 to 1.4)	0.311	0.66 (0.32 to 1.4)	0.273
Deep infection	0.66 (0.16 to 2.7)	0.732	0.55 (0.13 to 2.4)	0.433

¹Chi-squared test.

²Fisher’s exact test.

³Weighted log-binomial regression using robust variance estimator.

RR: Relative risk (old regime/new regime).

Table 6 Coefficient values (converted to relative risks) for interrupted time series analysis of overall infection rate over time

Parameter	Original dataset		Weighted dataset
	RR (95%CI)	P value	RR (95%CI)	P value
Slope of infection rate per 100 patients, old regime	0.99 (0.79 to 1.3)	0.98	0.95 (0.76 to 1.2)	0.62
Jump in infection rate, new vs old	1.16 (0.30 to 4.4)	0.82	1.3 (0.39 to 4.5)	0.66
Change in slope of infection rate per 100 patients, new vs old	0.83 (0.55 to 1.2)	0.37	0.88 (0.61 to 1.3)	0.46

RR: Relative risk.