2.3. Calculating disability weights

Disability weights (DWs) are associated with health states and not diseases. For every disease, an average DW, taking into account the different health states and possible disease sequelae as well as a severity distribution, have been computed. This methodology is primarily based on the approach proposed by IHME in [GBD 2016 Disease and Injury Incidence and Prevalence Collaborators et al., 2017 [72]].

A full list of DWs can be found here Table 2.1. Specific information for each disease can be found using the links below.

• Diabetes

• Stroke

• Ischaemic heart disease

• Chronic obstructive pulmonary disease

• Rheumatoid arthritis

• Low back pain

• Cirrhosis

• Chronic kidney disease

• Depression

• Dementia

• Cancers

• Injuries

2.3.1. Diabetes

In the IHME 2015 dataset we have access to the prevalence of different sequelae (“Diabetic foot”, “Amputation due to diabetes mellitus”, “Diabetic neuropathy”, “Vision impairment due to diabetes mellitus”) available by country, gender and age-group, which have been used to compute the average DW associated with diabetes.

It was assumed that people could only have one sequela. Therefore, where sequela data was available, it was assigned with the remaining lumped into ‘Uncomplicated diabetes mellitus’.

Table 2.3 Disability weight for diabetes

Sequela/Severity level	Health state	DW
Uncomplicated diabetes mellitus	Generic uncomplicated disease: worry and daily medication	0.049
Diabetic neuropathy	Diabetic neuropathy	0.133
Diabetic neuropathy with Diabetic foot	Multiple (specific to IHME)	0.02
Diabetic neuropathy with amputation	Amputation of one leg: long term	0.039
Moderate vision loss due to diabetes mellitus	Distance vision: moderate impairment	0.089
Severe vision loss due to diabetes mellitus	Distance vision: severe impairment	0.314
Blindness due to diabetes mellitus	Distance vision: Blindness	0.338

2.3.2. Stroke

The mid-level of severity: “Stroke, long‐term consequences, moderate plus cognition problems” is used as the average DW associated with long-term disability due to long-term consequences following an acute episode of stroke.

Table 2.4 Disability weight for stroke by severity level

Severity level	Health state	DW
Stroke, long-term consequences, mild	Stroke, long-term consequences, mild	0.019
Stroke, long-term consequences, moderate	Stroke, long-term consequences, moderate	0.07
Stroke, long-term consequences, moderate plus cognition problems	Stroke, long-term consequences, moderate plus cognition problems	0.316
Stroke, long-term consequences, severe	Stroke, long-term consequences, severe	0.552
Stroke, long-term consequences, severe plus cognition problems	Stroke, long-term consequences, severe plus cognition problems	0.588

2.3.3. Ischaemic heart disease

During the acute phase of myocardial infarction, a weighted average between the two DWs associated with “Acute myocardial infarction, days 1-2” and “Acute myocardial infarction, days 3-28” is used.

The disability weight associated with “Angina pectoris: moderate “is used as the average DW associated with long-term disability due to long-term consequences associated with acute myocardial infarction.

Table 2.5 Disability weight for IHD by severity level

Sequela/Severity level	Health state	DW
Acute myocardial infarction, days 1/2	Acute myocardial infarction, days 1-2	0.432
Acute myocardial infarction, days 3/28	Acute myocardial infarction, days 3-28	0.074
Mild angina	Angina pectoris: mild	0.033
Moderate angina	Angina pectoris: moderate	0.08
Severe angina	Angina pectoris: severe	0.167

2.3.4. Chronic obstructive pulmonary disease

The 2015 prevalence rate of COPD by stage (III-V) are available by country, gender and age-group in the IHME dataset. These data have been used to compute the average disability weight associated with COPD.

Table 2.6 Disability weight for COPD by severity level

Severity level	Health state	DW
COPD stage III	COPD and other chronic respiratory problems, mild	0.019
COPD stage IV	COPD and other chronic respiratory problems, moderate	0.225
COPD stage V	COPD and other chronic respiratory problems, severe	0.408

2.3.5. Rheumatoid arthritis

Using the severity split and the associated DWs, the weighted average is 0.24717 and was used for rheumatoid arthritis disability weight.

Table 2.7 Disability weight for rheumatoid arthritis by severity level

Severity level	Health state	DW	Severity Distribution
Mild	Musculoskeletal problems, upper limbs, moderate	0.117	48.8% (37.9-59.6%)
Moderate	Musculoskeletal problems, generalized, moderate	0.317	37.6% (29.3-46.2%)
Severe	Musculoskeletal problems, generalized, severe	0.581	12.2% (7.8-17.4%).

2.3.6. Low back pain

The proportion of low back pain and leg pain was assumed to be 30%, which is a conservative estimated based on IHME (GBD) estimates [GBD 2016 Disease and Injury Incidence and Prevalence Collaborators et al., 2017 [72]]. The severity distribution and the associated DWs were combined to compute an average DW for low back pain of 0.125857.

Table 2.8 Disability weight for back pain by severity level

Severity level	Health state	DW
Mild	Low back pain, mild	0.02
Moderate	Low back pain, moderate	0.054
Severe without leg pain	Back pain, severe, without leg pain	0.272
Severe with leg pain	Back pain, severe, with leg pain	0.325
Most severe without leg pain	Back pain, most severe, without leg pain	0.372
Most severe with leg pain	Back pain, most severe, with leg pain	0.384

Table 2.9 Back pain severity prevalences

Severity level	Prevalence without leg pain	Prevalence with leg pain
Mild	39% (29-50%)	27% (18-37%)
Moderate	36% (26-44%)	37% (28.44%)
Severe	11% (9%- 0.12%)	13% (10%-16%)
Most Severe	15% (9-21%)	23% (15-32%)

2.3.7. Cirrhosis

The DW for Cirrhosis is shown in the table below.

Table 2.10 Disability weight for cirrhosis by severity and health status

Severity level	Health state	DW
Cirrhosis	Decompensated cirrhosis of the liver	0.178

2.3.8. Chronic kidney disease

As stage IV and V of chronic kidney disease are modelled, the two following disability weights were directly used in the simulation

Table 2.11 Disability weight for chronic kidney disease by severity level

Severity level	Health state	DW
CKD stage IV	CKD stage IV	0.104
CKD stage V	End-stage renal disease: on dialysis	0.571

2.3.9. Depression

The severity distribution and the associated DWs were combined to compute an average DW for major depressive disorder of 0.219.

Table 2.12 Disability weight for major depressive disorder by severity

Severity level	Health state	DW	Prevalence
Asymptomatic			13% (10-17%)
Mild	Major Depressive disorder: mild episode	0.145	59% (49-69%)
Moderate	Major Depressive disorder: moderate episode	0.396	17% (13-22%)
Severe	Major Depressive disorder: severe episode	0.658	10% (3-20%)

2.3.10. Dementia

The severity distribution and associated DWs for dementia were combined to compute an average DW for the following three age groups:

• <70 years: 0.218

• 70-79 years: 0.240

• 80+ years: 0.267

Table 2.13 Disability weight for dementia by severity level and age group

Severity	Health State	DW	Severity distribution by age
			<70	70-80	80+
Mild	Dementia: mild	0.165	79% (71-86%)	71% (63-78%)	61% (53-68%)
Moderate	Dementia: moderate	0.388	17% (11-23%)	19% (14-24%)	26% (22-30%)
Severe	Dementia: severe	0.545	4% (2-7%)	9% (5-13%)	12% (7-17%)

2.3.11. Cancers

DWs for cancers are linked to the various stages of cancer, of which there are three. As the stages in cancer are not modelled, the DW associated with “diagnosis and primary therapy” is used for all cancers.

Table 2.14 Disability weight for cancer by severity level

Severity level	Health state	DW
Primary stage	Cancer: diagnosis and primary therapy	0.288
Metastatic phase	Cancer: metastatic	0.451
Terminal phase	Terminal phase: with medication	0.54

2.3.12. Injuries

For injuries, average disability weights were computed using the framework developed by IHME and explained in the supplementary appendix of [Vos et al., 2012 [61]] using two specific matrixes.

The first matrix (see Fig. 2.5) maps ICD10 codes to “23 nature of injuries”. Each nature of injury refers to specific health states (long- and short-term disability weights). The second matrix (see Table 2.15) describes the proportion of the different natures of injuries in each cause of injury (Falls, Road accident, etc.).

These two matrices were combined to compute an average short/long term disability weight for every injury modelled. When an individual is injured, the short-term disability weight applies until recovery or death. The individual is then attached to an injury sequelae and the long-term disability weight is applied.

Fig. 2.5 Relationship between GBD Cause Names and Injury N-codes

Table 2.15 Disability weight for injuries by severity level

	Road injuries	Falls	Drowning	Self-harm	Interpersonal violence
Short Term	0.113	0.11	0.194	0.12	0.083
Long Term	0.141	0.119	0.157	0.018	0.11