Table 2. Timing and Magnitude of CDI Risk Associated with Antibiotic Exposures.

				Unadjusted		Adjustedc
	CDI Cases	Follow-up	IRa	IRR	Δ AICb	IRR	Δ AICb
	(N)	(days)		(95% CI)		(95% CI)
Antibiotic use on current day
No	63	141019	4.47	Reference		Reference
Yes	64	75959	8.43	1.89 (1.33, 2.67)	0	1.79 (1.24, 2.59)	0
Time since end antibiotic therapy (d)
0 (current receipt)	64	75959	8.43	2.86 (1.73, 4.72)		2.41 (1.41, 4.13)
1–5	30	36950	8.12	2.75 (1.56, 4.85)		2.16 (1.17, 4.00)
6–30	13	36288	3.58	1.21 (0.60, 2.44)		0.98 (0.48, 2.00)
>30, or no antibiotic use	20	67781	2.95	Reference	−9.5	Reference	−4.6
Time since start of first antibiotic (d)
0–2	7	21060	3.32	1.08 (0.47, 2.49)		1.39 (0.56, 3.46)
3–6	40	38007	10.52	3.43 (2.11, 5.59)		3.10 (1.73, 5.54)
7–14	33	37530	8.79	2.87 (1.72, 4.77)		1.76 (0.98, 3.13)
15–30	20	32313	6.19	2.02 (1.13, 3.60)		1.56 (0.84, 2.90)
>30, or no antibiotic use	27	88068	3.07	Reference	−14.6	Reference	−0.7
Cumulative duration of antibiotic use (d)
0 (no prior receipt)	20	67781	2.95	Reference		Reference
1–3	25	45201	5.53	1.87 (1.04, 3.37)		1.60 (0.85, 3.03)
4–6	28	34596	8.09	2.74 (1.55, 4.87)		2.27 (1.24, 4.16)
7–11	28	35968	7.78	2.64 (1.49, 4.68)		2.10 (1.12, 3.94)
>11	26	33432	7.78	2.64 (1.47, 4.72)	−0.07	2.84 (1.39, 5.81)	5.1

Abbreviations: AIC, Akaike's Information Criterion; CDI, Clostridium difficile infection; CI, confidence interval; d, days; IR, incidence rate; IRR, incidence rate ratio.

^aIncidence rate, per 10,000 patient-days.

^bThe difference in AIC relative to the reference model (current antibiotic use): negative numbers denote an improvement in fit. Δ AIC <−2 was considered a statistically significant improvement in fit at p<0.05.

^cAdjusted for time since hospital admission, age, sex, number of previous hospital admissions, infection pressure, and current or prior ICU admission.