Table 1.

Cefazolin MIC₅₀, MIC₉₀, range and IQR (mg/L) at standard inoculum and high inoculum among 131 MSSA isolates demonstrating the InE by clinical source

Clinical source	Parameter	Cefazolin MICs (mg/L)		P valuea
		SI	HI
Bloodstream (n = 97)	mean	0.65	87.75	<0.001
	95% CI	0.59–0.70	64.1–111.3
	range	0.25–2	16–512
	IQR	0.5–1.0	16–64
	MIC50	0.5	64
	MIC90	1.0	64
Osteomyelitis (n = 34)	mean	0.80	120.94	<0.001
	95% CI	0.65–0.95	70.0–121.8
	range	0.5–2	16–512
	IQR	0.5–1	32–256
	MIC50	0.5	64
	MIC90	1.0	256
Total of isolates (n = 131)	mean	0.69	96.36	<0.001
	95% CI	0.63–0.74	74.4–118.29
	range	0.25–2	16–512
	IQR	0.5–1.0	16–64
	MIC50	0.5	64
	MIC90	1.0	256

SI, standard inoculum (∼10⁵ cfu/mL); HI, high inoculum (∼10⁷ cfu/mL); MIC₅₀, cefazolin concentration at which 50% of S. aureus isolates were inhibited; MIC₉₀, cefazolin concentration at which 90% of S. aureus isolates were inhibited.

^aStatistically significant differences were calculated by Mann–Whitney U-test comparing mean MICs at HI and SI.