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Quantifying the Value of Stroke Disability Outcomes: WHO GlobalBurden of Disease Project Disability Weights for Each Level of theModified Rankin Scale

Keun-Sik Hong, MD, PhD1 and Jeffrey L. Saver, MD21 Department of Neurology, Clinical Research Center, Ilsan Paik Hospital, Inje University, Goyang,Korea2 Department of Neurology, UCLA Stroke Center, University of California, Los Angeles, California,USA

AbstractBackground—The modified Rankin Scale (mRS) categorizes poststroke disability among 7 broad,ordinal grades, but the interval distances between these levels are spaced along the disability spectrumhas not been previously investigated.

Methods—We used the person trade-off procedure developed by the World Health OrganizationGlobal Burden of Disease Project (WHO-GBDP) to generate disability weights (DWs) ranging fromfrom 0 [normal] to 1 [dead] for each 7 mRS grade. The ratings of an international, 9 member panelof stroke experts were combined by a modified Delphi process.

Results—DWs (95% confidence interval) were 0 for mRS 0, 0.046 (0.004–0.088) for mRS 1, 0.212(0.175–0.250) for mRS 2, 0.331 (0.292–0.371) for mRS 3, 0.652 (0.625–0.678) for mRS 4, 0.944(0.873–1.015) for mRS 5, and 1.0 for mRS 6. DWs of adjacent mRS levels were significantly different(p<0.001 for all). Coefficient of variations showed high degree of consensus for DWs among panelmembers. DWs placed each of the five intermediate mRS states in different disability class levels ofthe WHO-GBDP anchor conditions, and identified natural clusters to employ when reducing themRS to fewer categories.

Conclusion—Formal DW assignment confirms that the mRS is an ordered, but unequally spacedscale. The availability of DWs for each mRS level now permits direct comparison of each poststrokeoutcome state with the outcomes of hundreds of other diseases in the WHO-GBDP and the expressionof stroke burden in different populations using the uniform metric of disability-adjusted life yearslost.

Keywordsdisability weight; person trade-off; modified Rankin Scale; stroke outcome

IntroductionResidual disability among stroke survivors spans a wide functional range. A variety of ordinalscales have been developed to classify patients along this spectrum, arraying individuals amonga few broad, ordered levels. The most comprehensive and widely employed ordinal poststroke

Correspondence to Jeffrey L. Saver, MD, UCLA Stroke Center, 710 Westwood Plaza, Los Angeles, CA 90095. [email protected] of interest: The authors have no conflicts of interest.

NIH Public AccessAuthor ManuscriptStroke. Author manuscript; available in PMC 2010 December 1.

Published in final edited form as:Stroke. 2009 December ; 40(12): 3828–3833. doi:10.1161/STROKEAHA.109.561365.

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global disability scale is the modified Rankin Scale (mRS), which assigns stroke survivorsamong seven levels, including the extremes of normal and dead and five intermediate degreesof disability.

However, while ordinal scales are substantially more informative than binary outcomemeasures, they are not as sensitive to change as continuous outcome measures. Moreover,unlike interval scales, in which the distance between each scale step is uniform, ordinal scalesarray patients among unevenly spaced steps along the outcome range. Without knowing howfar apart each step of an ordinal scale is along the disability spectrum, it is difficult to assessand compare therapies that exert differential beneficial and/or harmful effects at various scalestate transitions.

Several methods have been developed to quantify how far apart on a health outcome dimensionare the ranked levels of an ordinal scale. Most studies have employed patient preferencetechniques, including standard gamble, time trade-off, and visual analogue scale methods.These techniques elicit utilities - how much value patients or healthy individuals place onspecified outcomes,1–3 and permit analysis of outcomes in terms of quality-adjusted life years(QALYs) gained or lost. However, when ratings are required for multiple, rather than a single,outcome states, these methods have the substantial weakness that patients and healthyindividuals have limited direct experience of different diseases and disease intensities. Patientsexperience disease in depth, but not in breadth.

Employing as raters physicians and allied health professionals, who have direct encounterswith multiple diseases and disease severities, overcomes the unfamiliarity problem of patient-based quality weighting techniques. Healthcare professionals observe disease both in breadthand in depth, albeit externally.4,5 Accordingly, tasked with deriving health outcome stateratings for all human diseases, the World Health Organization Global Burden of Disease Project(WHO-GBDP) developed the person trade-off (PTO) method.6 In the PTO framework, healthprofessionals judge health states from a broad public health viewpoint that ensures equityacross health states.7 This technique yields disability weights (DWs) for each disease,permitting analysis of treatment and policy outcomes in terms of disability-adjusted life years(DALYs) gained or lost. The WHO’s PTO approach requires raters to undertake interpersonalcomparisons of utility for individuals with different disease states providing greater contentvalidity to the resulting relative weights.6,8 In addition, the approach minimizes theconfounders arising from adjustment to disease bias and lack of knowledge of a health state.6,9 The WHO’s DALY analyses have clarified understanding of the relative contribution ofindividual diseases to the total burden of disease in developed and underdeveloped regions andworldwide.6

The PTO method can facilitate comparison of health outcomes not only across differentdiseases, but also across varying severities of one disease.10 Consequently, the PTO methodis well-suited to assign quantified DWs for each of the ordinal levels of poststroke outcomescales.

MethodsWe convened an international expert panel of stroke neurologists (7), a rehabilitationneurologist (1), and a double-boarded emergency physician and neurologist (1) from the US(4), South Korea (3), mainland China (1), and Taiwan (1).

DWs range from 0 (normal health without disability) to 1.0 (dead or as bad as being dead).DWs for mRS 0 and 6 were assigned 0 and 1.0, respectively. To determine the DWs for thefive intermediate mRS categories, the panel members each individually performed the WHO-GBD PTO1 exercise,6 in which they were asked to assume they were allocating health system

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resources and had to decide what number would make them indifferent between the choicesof extending the lives of 1,000 healthy persons for 1 year versus extending lives of N personswith the mRS health state for 1 year. As a reference framework during this task, the panelmembers were provided with the WHO-GBDP framework table, which displays sevendisability classes and 22 anchoring example conditions (Table 1).6

In the first round of the modified Delphi process,11 each expert individually provided ratingsfor the 5 intermediate mRS levels. The panel then met as a group and discussed each mRSlevel in turn. At the start of each level discussion, each panelist received a report showing: 1)the group’s first round mean and median PTO and disability class values for that mRS level,2) the PTO and disability class values given by all respondents (anonymized) in the first round,and 3) the panelist’s own individual PTO rating in the first round. The group discussed themost appropriate PTO value for each of the 5 mRS states, with the moderator emphasizing fullparticipation of all panel members, respect for all opinions, and continuation of discussion untildistinctive thematic considerations were no longer elicited. Each panel member then again,independently and confidentially, assigned a PTO value for the discussed mRS state. Thecoefficient of variation (CV) was calculated to determine the need for additional Delphi rounds,with CV ≤ 0.5 indicating achievement of substantial consensus.12

PTO values were converted to DWs by the formula: DW=1–1000/PTO. As PTO values areexponentially distributed, their central tendency was assessed by calculating geometric meanand 95% confidence interval. A few experts assigned mRS 5 state a PTO value of infinity(equal to death). In these instances, the extreme PTO value of 1,000,000,000 was substitutedto permit calculation of geometric means. DWs are linearly distributed and were described bylinear means and 95% confidence intervals.

Normalization to existing WHO-GBDP rating for poststroke disabilityThe WHO-GBDP has generated DWs for acute stroke (0.920) and chronic poststroke disability(0.266).13 Although these ratings appropriately distinguish between acute and chronicpoststroke health states, they fail to distinguish among chronic poststroke states of varyingseverity. The multispecialty WHO panel was well-suited to compare disability across diverseorgan-specific conditions, but not to make fine distinctions among different stroke outcomes.In contrast, the PTO ratings of our stroke expert panel may be expected to be most reliable indelineating the relative value of different stroke outcomes compared with each other, and lessprecise in determining the relative position of the disease category of stroke compared withhundreds of other, especially non-neurologic, diseases. To compare the DW ratings of the fiveintermediate mRS ranks generated by our stroke expert panel with the unitary chronicpoststroke DW rating of WHO-GBDP, we generated a composite DW for all stroke survivorsfrom the DWs of individual mRS levels, using frequency-weighted averaging. For each mRSlevel, the panel-assigned DW was multiplied by the frequency of occurrence of that levelamong stroke survivors. To make the panel’s DWs for individual mRS levels fully consistentwith the WHO-GBDP framework, an adjustment formula was derived to normalize the DWsfor each mRS level to the WHO-GBDP unitary DW for all stroke survivors (supplementalMathematical Appendix, available online at http://stroke.ahajournals.org).

Statistical AnalysesThe Mann-Whitney test was used to compare the DWs of adjacent mRS states and DWsbetween the US and the Asian panel members. Wilcoxon signed rank test was used to comparefirst and second round DW CVs.

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ResultsThe mean DWs and geometric mean PTO values yielded by the panel members’ PTO ratingsare shown in Table 2. All five intermediate disability states were assigned DW values consonantwith their rank order on the mRS. DWs of adjacent mRS states were significantly different forall levels (p<0.0001). PTO values placed each of the five intermediate mRS states in differentdisability class levels of the WHO-GBDP anchor conditions, with mRS 1 assigned to WHOdisability class (WHO-DC) 1, mRS 2 to WHO-DC 3, mRS 3 to WHO-DC 4, mRS 4 to WHO-DC 6, and mRS 5 to WHO-DC 7.

For all mRS states, 95% confidence intervals of DWs were narrow, less than 0.15 (Table 2).In the first round, the CV values for the DWs were less than 0.5 for all mRS ranks except formRS 1 (Table 3). Inspection of individual rater values (supplemental Table A, available onlineat http://stroke.ahajournals.org) shows actual good agreement for DWs for mRS 1. Theextremely low value range of the DWs of mRS 1 magnifies the relative value of small absolutedifferences. The CVs decreased in the second Delphi round compared to those in the first round(p=0.043). As the overall CVs showed high degree of consensus, further rounds were notnecessary. Panel members from the US and Asia showed no difference in DW ratings (p>0.2for all DWs).

The distance between one step mRS increments on the DW scale varied, confirming that themRS is not an interval scale with equally spaced steps, but rather a rank order scale withunequally spaced steps (Figure 1). The closest mRS levels were mRS 0 and mRS 1, separatedby 0.046 units; furthest apart were mRS 3 and mRS 4, separated by 0.321 units, a 7-folddifference. Modified Rankin Scale steps clustered among 4 groups along the DW scale: nodisability, with or without symptoms – mRS 0–1; mild to moderate disability – mRS 2–3;severe disability – mRS 4; and extreme disability or death – mRS 5–6.

Medline review identified six studies in broad populations measuring the frequency of eachmRS level among stroke survivors, 2 studies each for ischemic stroke (IS), intracerebralhemorrhage (ICH), and subarachnoid hemorrhage (SAH) (supplemental References, availableonline at http://stroke.ahajournals.org). Since each study was performed among not fullycomparable race-ethnic groups, the mRS frequencies for each stroke subtype were derived byaveraging at the study level, rather than by weighting for sample size. Similarly Medline reviewidentified 2 population-based studies reporting the incidence of different stroke subtypes, onefrom the Cincinnati/Northern Kentucky region in the US and one from the Changsha regionin China.14,15 The frequency of ICH was substantially higher among the Asian than the NorthAmerican population. Based on the global perspective of the WHO-GBDP, these frequencieswere averaged at the study level. This yielded incidence frequencies of stroke subtypes of IS– 64.3%, ICH – 29.8%, and SAH – 2.1%. The relative frequency of each stroke subtype amongall stroke survivors was derived by correcting for differential stroke subtype 30-day case-fatality rates (10.2% for IS, 37.6% for ICH, and 31.3% for SAH)14 to yield frequencies of eachstroke subtype among stroke survivors of 74.3% IS, 23.9% ICH, and 1.9% SAH. Thefrequency-weighted average DWs for survivors of stroke subtypes were 0.223 for IS, 0.329for ICH, and 0.209 for SAH (Table 4). The frequency-weighted average DW for all strokesurvivors was 0.248, which was close to chronic poststroke DW of the WHO-GBDPmultispecialty panel (0.266).13 Adjusted individual mRS DWs that normalize the stroke expertpanel DWs to the WHO-GBDP global DW are shown in Table 2, and are close to the unadjustedvalues.

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DiscussionThis study is the first to derive DWs for each level of the mRS of poststroke global disability.The internal validity of the study is supported by the low CVs among the raters and the derivedDWs conforming to the original rank order of the mRS. The external validity is supported byconsistency with 1) two prior DW studies of broader poststroke categories, 2) a survey studyof physician intuitions regarding differences in mRS levels, 3) patient-derived quality of life(QOL) utility ratings of poststroke states, and 4) concurrent validity studies of complementaryfunctional measures associated with different mRS levels.

Our results concur with and extend two prior studies of DWs for poststroke health states.10,13 The composite DW for all stroke survivors yielded by frequency-weighting individual DWsfor each mRS level in the current study (0.248) is quite close to the unitary DW for the chronicpoststroke state derived by the WHO-GBDP multispecialty panel (0.266).13 As a result,calibrating the fine-grained state distinctions made by stroke experts to the broad global stateassessment made by a multispeciality panel required minimal adjustment (Table 2).

A Dutch study used PTO to derive DWs for three categories of poststroke impairment: severe,moderate, and mild.10 The DW of 0.94 for mRS level 5 in this study is consistent with the DWof 0.8–1.0 for severe poststroke impairment in the Dutch study, and the DW of 0.65 for mRSlevel 4 in the present study comports with the Dutch DW of 0.5–0.65 for moderate poststrokeimpairment. In contrast, the DWs of 0.05, 0.21, and 0.33 for mRS levels 1, 2 and 3 in the currentstudy are generally lower than the Dutch DW for mild poststroke impairment of 0.3–0.4. Thismay reflect differences in the specialty composition of the expert panels, as the generalpractitioners in the Dutch study may have less familiarity and experience with mild poststrokeimpairments than the stroke physicians in our study. Cultural difference regarding equity, animportant consideration in determining PTO values, might also account for the differences inthe DWs ratings. However, as our panelists were composed of multinational experts (althoughnone of European countries), the cultural difference might not be an important determinantfactor. The current study goes beyond prior DW investigations to provide specific DWs for amore granular series of poststroke states, yielding valuations with greater sensitivity to changeover the full stroke outcome range.

The findings of the current study largely conform to a prior, informal questionnaire studydelineating stroke physician valuation of individual mRS levels.16 The survey found strokephysicians value health state transitions across all the mRS levels, with the frequent exceptionof transitions from extreme disability (mRS 5) to death (mRS 6), which are often collapsedinto a single worst outcome category.17 Using PTO methodology, the current study formallydemonstrated a monotonic, ordered separation of DWs for all mRS levels, but with the smallestadjusted DW separation between mRS 5 and 6.

No prior study has derived patient preference QOL utility ratings for each mRS level. However,many studies of broader poststroke categories have been undertaken, typically delineatingseparate utilities for two (minor and major) or three (minor, moderate, and major stroke)poststroke states. To compare these utility values with disability weights, it is important torecognize that QOL utility analyses assign a score of 0 for being as bad as dead and 1 for beingas good as normal, and thus yield inverse values compared with DWs. To permit directcomparison, QOL utility values may be converted to quality weights (QWs) by the formula:QW=1 utility. A systematic review of all studies deriving QOL utility ratings by patient timetradeoff (TTO) preferences for poststroke states found that, to minor stroke, stroke survivorsassigned a QW of 0.28 and nonstroke patients with vascular risk factors a QW of 0.45; whileto major stroke, stroke survivors assigned a QW of 0.59 and nonstroke patients with vascularrisk factors a QW of 0.74.18 The DWs in this study for the mRS levels comprising minor stroke

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(mRS 1, 2, possibly 3) and major stroke (4, 5, and possibly 3) are roughly consonant with theQWs previously derived using patient TTO techniques.

It is noteworthy that the physician-derived DWs correspond closely with the stroke patient-derived QWs, in contrast with the QWs derived from nonstroke patients, which aresubstantially more severe. It is a common finding that patients rate their disability less thannon-patients.18 This pattern has been attributed to several factors, including adaptation todisease by affected patients (“adjustment to disease bias”), and non-patients’ fear ofunaccustomed conditions (“unfamiliarity with disease bias”). We hypothesize that physician-derived ratings resemble affected patient-derived ratings because physicians areknowledgeable regarding the disease state and have observed the substantial capacities of theirpatients to adjust to physical challenge.

This study’s findings of distinctive, separate DWs for each mRS level also correlates with priorstudies of the clinical and economic course of patients at different mRS grades.19,20 Theproportion of the first 3 poststroke months spent at home decreases as mRS level increases,longterm life expectancy decreases as mRS level increases, and cost of care increases as mRSlevel increases from 0–5.

That adjacent mRS levels are unequally spaced along the disability spectrum has importantimplications for interpreting the results of stroke treatment in clinical trials and in health policymodels. In the past, when evaluating socioeconomic consequences of trial results and policychanges, an equal-interval distance between mRS levels had to be assumed.21 Now, actual DWvalue distances can be employed, enabling more accurate assessment of the human impact ofstroke treatments and public health policies.

Sometimes it is desirable to reduce the full seven level mRS to fewer categories to simplifyanalysis. Hitherto, the assortment of the seven mRS levels into fewer categories was performedin an intuitive, somewhat arbitrary manner. Supraordinate category formation may now beguided by the natural clustering of the formally derived, quantitative DWs observed in thisstudy. For example, thrombolysis trialists have reduced the mRS to four categories: mRS 0–1, mRS 2–3, mRS 4–5, and mRS 6.22 However, the clustering of DWs derived in this studyindicates that a more natural tetrachotomization of the Rankin Scale, combining mRS levelswith similar DWs, would be mRS 0–1, mRS 2–3, mRS 4, and mRS 5–6. Another approach totetrachotomization was taken by the CLOTBUST trialists, who reduced the mRS to: 0–1, 2,3–5, and 6.23 The DWs here derived suggest the CLOTBUST approach yields a distortedtetrachotomy, with the single mRS 3–5 category spanning 70% of the disability spectrum, andthe remaining 3 categories subdividing only the remaining 30%. This might be useful for aclinical trial aimed at detecting only transitions to, from, and among a range of good outcomes,but will miss important changes in outcome in the moderate to severe range. An additionalapproach in stroke trial analysis is to reduce the mRS to 6 levels, rather than 7, by collapsingmRS 5 and mRS 6 into a single worst outcome category.17 This study supports thishexachotomization approach, indicating little difference along the disability dimension amongmRS 5 and 6 outcomes.

This study has limitations. The expert panel included stroke physicians from North Americaand Asia, but not other regions of the world. The panel included representatives of threespecialties/subspecialties (Stroke Neurology, Rehabilitation Neurology, EmergencyMedicine) but not additional specialties involved in stroke care, nor nurses or allied healthpersonnel. Some panelists who rated mRS 5 equivalent to death may have actually consideredextreme poststroke disability to be worse than death, but the PTO task does not permit scoringan outcome state as worse than death.

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Stroke is the only disease that is both a leading cause of death and a leading cause of disabilityworldwide. While stroke death rates are inherently easy to enumerate, the total burden ofpoststroke disability has been challenging to quantify. The availability of DWs for each mRSlevel now permits direct comparison of each poststroke outcome state with the outcomes ofhundreds of other diseases in the WHO-GBDP and the expression of stroke burden in differentpopulations using the uniform metric of DALYs lost. Since DALYs are a continuous outcomemeasure, stroke treatments and health policy effects can now be examined not only withdichotomized and ordinal statistical methods, but also with the often more powerful methodsappropriate to continuous data.

Supplementary MaterialRefer to Web version on PubMed Central for supplementary material.

AcknowledgmentsAcknowledgements and Funding

This work was supported by a grant (A060171) of the Korea Health 21 R&D project, Ministry of Health, Welfare andFamily Affairs, Republic of Korea (K.-S.H.), and NIH-NINDS Award P50 NS044378 (JLS). The authors are gratefulto the members of the Expert Panel (David Alexander, MD; Si-Ryung Han, MD; Keun-Sik Hong, MD; Meng Lee,MD; David S. Liebeskind, MD; Jaseong Koo, MD; Jeffrey L. Saver, MD; Sidney Starkman. MD; Liangfu Zhu, MD).

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Figure 1.Disability weights (means and 95% CIs) for each level of modified Rankin Scale

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Xsp

+ 0

.005

, whe

re D

WX

adj i

s the

adj

uste

d di

sabi

lilty

wei

ght f

or m

RS

leve

l X, D

WX

sp is

the

stro

ke e

xper

t pan

elge

nera

ted

disa

bilit

y w

eigh

t for

mR

S le

vel X

, 1.0

52 is

the

adju

stm

ent f

acto

r, an

d 0.

005

is th

e y-

axis

inte

rcep

tion.


NIH


NIH


NIH


Hong and Saver Ta

ble

3

Coe

ffic

ient

of v

aria

tions

of d

isab

ility

wei

ght f

or e

ach

mod

ified

Ran

kin

Scal

e st

atus

in th

e fir

st a

nd se

cond

roun

d

Rou

ndm

RS

1m

RS

2m

RS

3m

RS

4m

RS

5

11.

400.

410.

250.

130.

112

1.19

0.23

0.16

0.05

0.10

mR

S, m

odifi

ed R

anki

n Sc

ale.

P=0.

043

for c

ompa

rison

of t

he c

oeff

icie

nt o

f var

iatio

ns o

f dis

abili

ty w

eigh

ts b

etw

een

the

first

and

the

seco

nd D

elph

i rou

nd (W

ilcox

on si

gned

rank

test

)


NIH


NIH


NIH


Hong and Saver Ta

ble

4

Freq

uenc

ies o

f mod

ified

Ran

kin

Scal

e O

utco

mes

Am

ong

Stro

ke S

urvi

vors

and

Res

ultin

g Fr

eque

ncy-

Wei

ghte

d D

isab

ility

Wei

ghts

for S

troke

Isch

emic

Str

oke

Intr

acer

ebra

l Hem

orrh

age

Suba

rach

noid

Hem

orrh

age

All

Stro

ke*

Ger

man

stro

keD

ata

Ban

k(n

=245

5)C

OM

PASS

(n=1

142)

Isch

emic

Str

oke

Com

bine

dFA

ST p

lace

bo (n

=217

)O

XV

ASC

(n=1

4)IC

H C

ombi

ned

ISA

T (n

=196

9)JS

SRS

(n=3

67)

SAH

Com

bine

d

mR

S023

.330

.226

.75.

07.

26.

117

.644

.931

.221

.9m

RS1

30.2

24.2

27.2

20.0

21.6

20.8

29.6

21.8

25.7

25.7

mR

S217

.416

.016

.718

.828

.723

.726

.87.

717

.218

.4m

RS3

9.3

13.2

11.2

23.8

21.6

22.7

15.0

6.4

10.7

14.0

mR

S414

.09.

311

.627

.514

.420

.94.

17.

75.

913

.7m

RS5

5.8

7.3

6.6

5.0

7.2

6.1

6.8

11.5

9.2

6.5

Com

posi

te D

W0.

228

0.21

80.

223

0.35

40.

304

0.32

90.

211

0.20

70.

209

0.24

8

mR

S, m

odifi

ed R

anki

n Sc

ale;

DW

, dis

abili

ty w

eigh

t

* Freq

uenc

ies o

f mR

S ou

tcom

es a

mon

g al

l stro

ke su

rviv

ors w

ere

adju

sted

for t

he fr

eque

ncy

of e

ach

stro

ke su

btyp

e. N

umbe

rs a

re p

erce

ntile

exc

ept f

or D

Ws.

Ref

eren

ces o

f the

indi

vidu

al st

udie

s are

list

ed in

the

Supp

lem

enta

l Ref

eren

ces,

avai

labl

e on

line

at h

ttp://

stro

ke.a

hajo

urna

ls.o

rg.



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