Table 4.

Adjusted risk of infection in patients with rheumatoid arthritis (RA)

	Adjusted IRR (95% CI)	p Value
Current RA treatments
TNF antagonists	1.52 (1.30 to 1.78)	<0.001
MTX	1.30 (1.12 to 1.50)	<0.001
Prednisone	1.05 (0.97 to 1.15)	0.251
Other DMARDs	Reference	–
Clinical variables
Smoking (ever)	1.52 (1.38 to 1.67)	<0.001
Chronic lung disease	1.31 (1.15 to 1.50)	<0.001
Diabetes mellitus	1.33 (1.15 to 1.52)	0.001
Tender joint count	1.01 (1.00 to 1.02)	0.009
ACR functional class	1.23 (1.12 to 1.34)	<0.001
Body mass index	1.01 (1.00 to 1.02)	0.002

The interaction term for MTX and TNF antagonist combination therapy was significant (p = 0.001), indicating that patients on combination therapy had a reduced risk (IRR 0.75, 95% CI 0.62 to 0.89) compared with the expected multiplicative risk based on individual risk contributions.

The multivariable model included demographic variables (age, gender, race and education level) as well as the treatment and clinical variables listed in the table. Female gender (IRR 1.43, 95% CI 1.28 to 1.60) and age (IRR 0.995, 95% CI 0.99 to 1.00) were both significantly associated with the risk of infection in this model; race and education were not.

DMARD, disease-modifying antirheumatic drug; IRR, incident rate ratio; MTX, methotrexate; TNF, tumour necrosis factor.