Table 3. Unadjusted and adjusted odds ratios of association between variables and risk of ankylosing spondylitis.

	Univariate	Multivariate
	OR (95% CI)	aOR (95% CI)
Tonsillitis (ICD9:474.0, 463)	1.52 (1.49–1.56)	1.46 (1.43–1.50)
Acute tonsillitis (ICD9:463)	1.52 (1.49–1.56)	1.46 (1.43–1.50)
Chronic tonsillitis (ICD9:474.0)	1.63 (1.45–1.82)	1.52 (1.36–1.70)
Number of visits for tonsillitis
No tonsillitis	1.00 (reference)	1.00 (reference)
1 visit	1.31 (1.27–1.35)	1.28 (1.24–1.32)
2 visits	1.48 (1.42–1.54)	1.43 (1.37–1.49)
3–4 visits	1.59 (1.52–1.65)	1.52 (1.46–1.59)
>4 visits	1.96 (1.89–2.03)	1.82 (1.75–1.89)
Cumulative cost of tonsillitis-related visits (New Taiwan dollars)
No tonsillitis	1.00 (reference)	1.00 (reference)
Q1 (0< dollars ≤396)	1.30 (1.25–1.35)	1.27 (1.22–1.32)
Q2 (396< dollars ≤770)	1.38 (1.33–1.44)	1.34 (1.29–1.39)
Q3 (770< dollars ≤1,611)	1.58 (1.52–1.64)	1.52 (1.47–1.58)
Q4 (> 1,611 dollars)	1.92 (1.85–1.99)	1.79 (1.72–1.85)
Interval between the first tonsillitis visit and the index date
No tonsillitis	1.00 (reference)	1.00 (reference)
Q1 (0≤ years <1.8)	1.31 (1.26–1.36)	1.27 (1.22–1.32)
Q2 (1.8≤ years <3.3)	1.44 (1.38–1.49)	1.38 (1.33–1.44)
Q3 (3.3≤ years <5.4)	1.60 (1.54–1.67)	1.53 (1.48–1.59)
Q4 (≥5.4 years)	1.93 (1.85–2.01)	1.84 (1.76–1.91)

Adjusted variables include periodontal disease, CCI, and appendicitis. Abbreviation: OR, odds ratio; CI, confidence interval; CCI, Charlson comorbidity index