Table 4.

Models of Additive Interactions Between Significant Covariates for Prediction of HCV Treatment Failure Among Patients Living With HIV in the HCV-TREN Cohort

Any HCV Treatment Failure			HCV Clinical Treatment Failure		HCV Virologic Treatment Failure
	OR Estimate (95% CI)	P Value	OR Estimate (95% CI)	P Value	OR Estimate (95% CI)	P Value
Any drugs, NOT Mental illness	3.35 (1.45–7.78)	.005	4.70 (1.48–14.87)	.009
Mental illness, NOT Any drugs	3.57 (1.61–7.95)	.002	4.17 (1.32–13.18)	.015
Any drugs AND Mental illness	7.47 (3.60–15.50)	<.0001	9.18 (3.25–25.95)	<.0001
Any drugs, NOT F3/F4 fibrosis					3.79 (0.99–14.39)	.050
F3/F4 fibrosis, NOT Any drugs					2.90 (0.90–9.36)	.075
Any drugs AND F3/F4 fibrosis					6.66 (1.90–23.34)	.003
Relative excess risk due to interaction (RERI)a	1.55 (−3.39 to 6.38)	.53	1.31 (−6.63 to 9.26)	.75	0.97 (−5.45 to 7.42)	.77
Attributable proportionb	0.20 (−0.37 to 0.78)	.48	0.14 (−0.66 to 0.95)	.73	0.15 (−0.75 to 1.04)	.75
Synergy indexc	1.31 (0.56–3.10)	.53	1.19 (0.41–3.45)	.75	1.21 (0.35–4.23)	.77

Abbreviations: CI, confidence interval; HCV, hepatitis C virus; HIV, human immunodeficiency virus; OR, odds ratio; TREN, Transatlantic Research Network.

^aRERI is also sometimes referred to as the “interaction contrast ratio” and gives the direction (positive, negative, or zero) of the additive interaction. Only the direction, rather than the magnitude of RERI, is needed to draw conclusions about the public health relevance of interaction.

^bAttributable proportion is another measure of additive interaction and essentially measures the proportion of the risk in the doubly exposed group that is due to the interaction itself.

^cSynergy index measures the extent to which the risk ratio for both exposures together exceeds 1, and whether this is greater than the sum of the extent to which each of the risk ratios considered separately each exceeds 1 (reference [15]).