Effects of socioeconomic status on physical and mental health of hemodialysis patients in Japan: differences by age, period, and cohort

Figures & data

Table 1 Differences in characteristics between studied analytic sample and total dialysis patients in Japan during each survey year

Characteristics		1996		2001		2006		2011
		Samplea	Totalb	Samplea	Totalb	Samplea	Totalb	Samplea	Totalb
		Age categories (years)	≤39	9.5	7.7	6.1	5.7	3.6	4.1	2.0	2.8
40–49	26.4		17.9	16.7	10.8	10.4	7.8	8.9	7.2
50–59	30.5		25.1	35.5	25.5	33.1	22.7	22.3	15.5
60–69	24.9		27.6	28.9	28.9	34.8	28.4	41.6	30.8
≥70	8.7		21.7	12.8	29.1	18.1	37.0	25.3	43.6
Sex	Male	56.4	59.4	55.8	60.0	54.9	61.2	55.2	62.6
	Female	43.6	40.6	44.2	40.0	45.1	38.8	44.8	37.4
Cause of renal disease	Chronic glomerulonephritis	64.6	59.9	59.6	52.6	55.1	45.4	48.6	40.0
	Diabetic nephropathy	10.7	23.4	13.3	28.8	16.5	34.7	21.4	37.9
	Others	24.7	16.7	27.1	18.6	28.4	19.9	30.0	22.1
Duration of dialysis (years)	<5	32.0	54.3	30.3	51.6	24.2	49.8	23.7	47.8
	5–9	27.9	22.9	27.1	24.3	27.1	24.9	24.4	25.3
	10–14	18.8	12.1	16.7	11.5	18.7	12.1	18.8	12.7
	>15	21.2	10.7	26.0	12.6	30.0	13.2	33.1	14.2
Totalc		100.0	100.0	100.0	100.0	100.0	100.0	100.0	100.0

Notes:

a Sample means analytic sample in each survey conducted in 1996, 2001, 2006, and 2011.

b Total means total dialysis patients in Japan in 1996, 2001, 2006, and 2011. Data were obtained from an annual national statistical survey of dialysis facilities conducted by the Japanese Society for Dialysis Therapy.

c Total percentages for each characteristic were calculated by exclusion of cases including “unknown” variables.

Table 2 Summary statistics for dialysis complications, depressive symptoms, level-1 variables, and level-2 variables among aggregated data from 1996 to 2011 surveys after raking adjustment

Variables	Description	n	Mean (range), SD
Outcome variables
Dialysis complications	Count of the number of organs/systems with dialysis complicationsa	28,561	2.735 (0–9), 1.670
Depressive symptoms	Short version of Center for Epidemiologic Studies-Depression	28,561	8.231 (0–33), 4.895
		n	%
Level-1 variables
Household equivalence income scale	Reference = fourth quartile	7,001	21.1
	1 = first quartile	6,021	24.5
	1 = second/third quartile	12,705	44.5
	1 = no answer	2,833	9.9
Sex	Reference = female	11,201	39.2
	1 = man	17,360	60.8
Age category at survey year (years)	Reference =≥70	9,458	33.1
	1=≤39	1,459	5.1
	1=40–49	3,012	10.6
	1=50–59	6,396	22.4
	1=60–69	8,216	28.8
Cause of renal disease	Reference = chronic glomerulonephritis	13,858	48.5
	1= diabetic nephropathy	9,128	32.0
	1= others	5,575	19.5
Duration of dialysis (years)	Reference =<5	14,502	50.8
	1= from 5 to 9	6,925	24.2
	1= from 10 to 14	3,486	12.2
	1= >15	3,648	12.8
Period (survey year)	Reference =1996	6,140	21.5
	1=2001	7,495	26.2
	1=2006	8,137	28.5
	1=2011	6,789	23.8
Level-2 variables
Birth cohort	1915–1919	472	1.7
	1920–1924	1,233	4.3
	1925–1929	2,783	9.7
	1930–1934	4,089	14.3
	1935–1939	4,619	16.2
	1940–1944	4,494	15.7
	1945–1949	3,876	13.6
	1950–1954	3,008	10.5
	1955–1959	1,634	5.7
	1960–1964	1,029	3.6
	1965–1969	699	2.4
	1970–1974	456	1.6
	1975–1979	167	0.6

Note:

a Existence of intradialytic complications added one point to total.

Abbreviation: SD, standard deviation.

Table 3 Differences between respondents below the first quartile income and ones over the fourth quartile in the number of dialysis complications and depressive symptoms among aggregated data from 1996 to 2011 after raking adjustment

Variables		Mean of the number of dialysis complications		Relative riska	Mean of depressive symptoms		Mean differencesb
		First quartile	Fourth quartile		First quartile	Fourth quartile
Age category at survey year (years)	≤39	2.385	2.132	1.119	8.473	7.121	1.352
	40–49	2.731	2.463	1.109	8.416	7.188	1.228
	50–59	2.937	2.462	1.189	9.395	7.078	2.317
	60–69	2.944	2.717	1.084	8.820	7.207	1.612
	≥70	2.874	2.853	1.007	9.498	7.897	1.601
Period (survey year)	1996	3.415	3.134	1.090	8.270	7.106	1.164
	2001	2.742	2.413	1.136	9.233	7.584	1.649
	2006	2.586	2.414	1.071	9.602	7.363	2.240
	2011	2.813	2.517	1.118	9.145	7.291	1.854
Birth cohort	1915–1919	3.072	3.384	0.908	9.009	7.693	1.316
	1920–1924	3.014	2.925	1.031	9.809	7.983	1.826
	1925–1929	3.070	2.893	1.061	8.964	7.940	1.024
	1930–1934	2.953	2.924	1.010	9.366	7.760	1.607
	1935–1939	2.897	2.871	1.009	9.026	7.197	1.829
	1940–1944	2.947	2.741	1.075	8.967	7.417	1.540
	1945–1949	2.881	2.443	1.179	9.207	7.088	2.119
	1950–1954	2.782	2.319	1.200	9.118	6.996	2.122
	1955–1959	2.712	2.194	1.236	8.454	6.839	1.615
	1960–1964	2.450	2.351	1.042	9.494	7.605	1.888
	1965–1969	2.409	2.076	1.161	9.311	7.006	2.304
	1970–1974	2.123	1.991	1.066	7.498	7.668	–0.17
	1975–1979	2.120	2.102	1.009	8.757	5.980	2.777
Total	2.868	2.607	1.100	9.090	7.341	1.748

Notes:

a The numbers of dialysis complications were count data. What we referred to as a count was called a risk because by definition, a risk is the number of events per time. Relative risks were calculated by dividing means of the number of dialysis complications in respondents below the first quartile by means of the number of complications in ones over the fourth quartile.

b Mean differences were calculated by subtracting the means of depressive symptoms in respondents below the first quartile from means of depressive symptoms in ones over the fourth quartile.

Figure 1 Differences by age categories in relative risk of dialysis complications in first-income quartile respondents compared to fourth-quartile respondents.

Notes: Values were estimated by substitutions of mean values for independent variables without income and age variables, which were entered as fixed effects of a cross-classified random-effects model. Vertical lines indicate 95% confidence interval.

Table 4 Effect of age and period on the income differences in the number of dialysis complications and depressive symptoms: results from cross-classified random-effects two-level modela

Variables	The number of dialysis complications		Depression symptoms
	Coefficient	P-value	Coefficient	P-value
Fourth quartile of income (reference)b
First quartile of income	–0.024	0.583	1.018	0.012
Second/third quartile of income	–0.055	0.140	0.027	0.918
Interaction between age <39 years and first quartile of income	0.075	0.137	–0.611	0.429
Interaction between age 40–49 years and first quartile of income	0.070	0.123	–0.477	0.033
Interaction between age 50–59 years and first quartile of income	0.117	0.028	0.616	0.057
Interaction between age 60–69 years and first quartile of income	0.052	0.264	–0.023	0.918
Interaction between age >70 years and first quartile of income (reference)b
Interaction between age <39 years and second/third quartile of income	0.087	0.052	0.474	0.385
Interaction between age 40–49 years and second/third quartile of income	0.076	0.078	0.381	0.204
Interaction between age 50–59 years and second/third quartile of income	0.068	0.055	0.598	0.094
Interaction between age 60–69 years and second/third quartile of income	0.038	0.331	0.266	0.147
Interaction between age >70 years and second/third quartile of income (reference)b
Interaction between period 2001 and first quartile of income	0.044	0.098	0.400	0.194
Interaction between period 2006 and first quartile of income	0.037	0.609	1.180	0.002
Interaction between period 2011 and first quartile of income	0.033	0.534	1.071	0.048
Interaction between period 1996 and first quartile of income (reference)b
Interaction between period 2001 and second/third quartile of income	0.036	0.206	0.169	0.557
Interaction between period 2006 and second/third quartile of income	0.028	0.715	0.813	0.013
Interaction between period 2011 and second/third quartile of income	0.030	0.554	0.671	0.137
Interaction between period 1996 and second/third quartile of income (reference)b

Notes:

a These are results after sex, age categories, causes of renal disease, duration of dialysis, period, and cohort were controlled and their coefficients were omitted from this table. “Unknown” categories of income were excluded from this table. Because the coefficients of the income quartiles did not have significant random effects by birth cohort, these results were omitted from this table.

b We analyzed categorical independent variable using dummy coding; we created K-1 dummy variables from a variable with K categories, omitted categories were named as reference categories, as a result, coefficients of reference categories were not calculated.

Figure 2 Mean differences by age categories or period for depressive symptoms between respondents below the first income quartile and ones over the fourth quartile.

Notes: (A) differences by age categories, and (B) differences by period. Values were estimated by substitutions of mean values for independent variables without income and age variables (without income and period variables in case of period effects), which were entered as fixed effects of a cross-classified random-effects model. Vertical lines indicate 95% confidence interval.