Table 3.

Additive and multiplicative Interaction effects of multimorbidity and frailty risk on adverse patient outcomes, full cohort.

Mortality within 30-days post admission	Low frailty risk			Elevated frailty risk
	N with outcome	% outcome	aRR (95% CI)	N with outcome	%outcome	aRR (95% CI)
No multimorbidity	3854	4.7	1	4731	9.2	1.90 (1.82–1.99)
Multimorbidity	4046	10.7	2.40 (2.29–2.51)	16,255	18.8	4.23 (4.07–4.39)
aMeasure of effect modification on additive scale: RERI (95% CI) = 0.93 (0.81–1.04)*
aMeasure of effect modification on multiplicative scale: ratio of RR = 0.93 (0.88–0.98), p-value = 0.006*

Prolonged LOS	Low frailty risk			Elevated frailty risk
	N with outcome	% outcome	aRR (95% CI)	N with	%outcome	aRR (95% CI)
No multimorbidity	11,855	14.5	1	19,181	37.4	2.62 (2.56–2.68)
Multimorbidity	8001	21.1	1.57 (1.52–1.61)	37,548	43.4	3.34 (3.26–3.41)
aMeasure of effect modification on additive scale: RERI (95% CI) = 0.15 (0.09–0.21)*
aMeasure of effect modification on multiplicative scale: ratio of RR = 0.81 (0.79–0.84), p-value < 0.001*

Readmission within 30-days post discharge	Low frailty risk			Elevated frailty risk
	N with outcome	% outcome	aRR (95% CI)	N with outcome	%outcome	aRR (95% CI)
No multimorbidity	5457	6.9	1	4727	9.9	1.25 (1.21–1.31)
Multimorbidity	3960	11.4	1.41 (1.35–1.47)	12,120	16.5	1.79 (1.73–1.86)
aMeasure of effect modification on additive scale: RERI (95% CI) = 0.13 (0.06–0.20)*
aMeasure of effect modification on multiplicative scale: ratio of RR = 1.02 (0.96–1.07), p-value = 0.57

*Denotes significance at 5% level.

^aSignificance of an interaction on an additive scale is denoted where RERI is different from 0, and on the multiplicative scale if ratio of RR is different from 1.