BRAIN INJURY, 1993, VOL. 7, NO. 4, 309-317

Glasgow Outcome Scale: an inter-rater reliability study

S H I R L E Y I . A N D E R S O N , A L M A M . H O U S L E Y , P A T R I C I A A . J O N E S , J A M E S S L A T T E R Y a n d J . D O U G L A S M I L L E R

Department of Clinical Neurosciences, University of Edinburgh, Western General Hospital, Edinburgh, Scotland

(Received 17 March 1992; accepted 30 July 1992)

This study was set up to test the reliability of the Glasgow Outcome Scale (GOS) when information was obtained &om different sources. Eighty assessments were carried out on a group of 58 patients at three different time intervals up to 24 months post-injury. Each assessment consisted of three independently obtained GOS scores for each patient; (i) a score by a research psychologist after interview and neuropsychological testing of the patient; (ii) a score, obtained by post, by the patient’s general practitioner (GP), and (iii) a score made by a research worker based on questionnaire information obtained from relatives by post. The agreement between the psychologist’s score and that based on the relatives’ information was high ( ~ 0 . 7 9 p=O.OOl) whereas the correlation between the psychologist’s score and that of the GP was low ( ~ 0 . 4 9 p=O.OOl). The GPs tended to make overoptimistic assessments and this was most notable at 6 months post-injury when only 50% of the GPs’ assessments agreed with those of the psychologist. We have shown that reliability of the GOS vanes with the method of obtaining data. Ideally patients should be interviewed and tested by staff who have not been involved in the acute care of the patient. Failing this, information should be obtained from relatives of the patient and used by staff, trained in the use of the GOS, to assign a GOS score.

Introduction

The Glasgow Outcome Scale (GOS) is a functionally based assessment tool for use with head-injured patients, which was devised primarily for use in epidemiological and early management research in order to provide reliable standardized categories of outcome [ 11. It was hoped that widespread acceptance would facilitate inter-centre comparisons [2] and, when used in conjunction with the Glasgow Coma Scale (GSC) [3], be helpful for predicting outcome after severe head injury. Criticism has been levelled at the GOS because it is insensitive to subtle improvements in the patient’s status, fails to reflect the multidimensional nature of a patient’s deficits and overemphasizes physical disability rather than cognitive and neurobehavioural problems [4, 51. However, the GOS was never intended as an instrument for documenting and describing individual patient’s recovery patterns but should be judged by the extent to which it can reliably categorize groups of patients in terms of their post-acute functional outcome.

The GOS has sacrificed detail in order to attain high reliability. The developers of the GOS found inter-rater reliability rates as high as 95% [6] but levels as low as 76% have been reported in studies involving trained raters having direct and simultaneous contact with patients [7] . The assessment technique has been widely adopted as the preferred tool

Address correspondence to: Professor J.D. Miller, Department of Clinical Neurosciences, Western General Hospital, Crewe Road South, Edinburgh EH4 2XU, Scotland.

0269-9052/93 $10.00 0 1993 Taylor & Francis Ltd

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S. I. Anderson et al. 310

Table 1 . Method 4-obtaining Glasgow Outcome Scale data in a cross-section of neurosurgical studies.

Authors Follow-up interval Assessment method

Bates L) et a/. (1977)(8)

Narayan RK et al. (1981) (21) hliller JU e ta! . (1981) (22)

Holliday PO et a/. (1982)(23) Braakman R et a/ . (1983)(24)

V a n I>ongen KJ et nl. (1983)(2i)

Lylc IIM et a / . (1986)(26) Pqanicolaou AC et nl. (1986)(27)

Alherico MA et al. (1987)(28) C:hangaris DG ct a/ . (1987)(29)

Bullock R et a/ . (1 990) (30) Jag@ JL et a/. (1990)(31)

1,3,6 and 12 months

3,6 and 12 months 3,6 and 12 months

6 weeks to 26 months 6 months

1,3,6 and 12 months

>24 months >6 months

3 months to 8 years At least l y r from injury

>3 months 6 months

Direct examination of patient or interview with family or physician

Not specified Personal interview or tele- phone contact with patient or relative

Not specified Not specified

Not specified

Patient interviewed Not specified

Not specified Review of records and tele- phone contact with patient or relative

Not specified Two independent raters, blind to acute findings

h r describing outcome after head injury in large-scale neurosurglcal studies. Occasionally, comprehensive details of the method of judging outcome are provided [S] but many of these studies can be criticized for providing little information on how the GOS figures were obtained (see Table 1) . This contrasts with the meticulous way in which the authors have dcxcribed their patient population and the acute injury details, and casts some doubt on the reliability of the outcome data. Different methods of collecting information on which to base the GOS may affect the reliability of the categories assigned.

Most studies o n Inter-rater reliability and validity of the GOS, in common with testing reliability and validity of other outcome measures, involve comparison of ratings made by two or more trained assessors after personal contact with the patient [ S , 91. In other studies some patients are seen personally, and indirect information is obtained for others, although rcliability levels vary for direct and indirect contacts [7]. It is unlikely that direct patient contact can be universally achieved in large-scale studies of head-injury outcome. Patient compliance with appointment attendance is poor, making it difficult t o standardize the time of assessment. It may be necessary to obtain indirect information from which to assign a (;OS score. This may involve contact with relatives or other professionals, either by structured telephone interview or by correspondence. It is therefore important to examine which indirect methods can provide data of acceptable reliability.

T h e time interval after injury at which the GOS is assessed is not always clearly specified. Outcome is frequently reported at 6 months post-injury on the basis that Jennett t’t ‘7). [ 1 O] demonstrated that few patients improve sufficiently after this period to result in a change of GOS category, but there is little doubt that changes do occur well beyond this

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Glasgow Outcome Scale: rater reliability 31 1

time and this reflects the lack of sensitivity of the GOS. The time interval between injury and GOS assessment may be another factor influencing its reliability.

This study was set up to test the inter-rater reliability of the GOS when based on information from three sources obtained at three different time intervals after injury. The GOS obtained after interviewing and testing of the patient by a psychologist was taken as the gold standard, against which two indirect methods of obtaining GOS data were tested.

Method

Patients

Fifty-eight patients were included in the study, all of whom had been admitted to a regional neurosurgcal unit with a diagnosis of closed head injury. They were being followed up as part of a larger on-going study [ l l ] . Twenty-nine patients had sustained severe head injury (GSC 5 8 after resuscitation with no eye opening), 20 had moderate head injuries (GCS 9 to 12 or GCS 8 with no eye opening) and the remaining nine had minor head injuries (GCS 13 to 15 on admission). The age range of the patient group was 14 to 84 years with a mean age of 32. There were five females and 53 males.

Assessment procedures

The effect of the head injury on patients was assessed using the five-point Glasgow Outcome Scale with the following categories: death, persistent vegetative state, severe disability, moderate disability and good recovery. As all the patients in this follow-up study could be interviewed and tested, we were only concerned with the last three categories. Patients described as severely disabled were conscious but dependent on others for at least one of their activities of daily living; their dependency may have been due to physical, behavioural or psychological problems. Moderately disabled patients could live independently but were functioning at a reduced level either in occupational or social roles. Good recovery signified a return to pre-morbid level of functioning, although there may be minor residual physical or mental deficits [l].

AU patients included in the study had three GOS scores allocated. For each assessment the three GOS scores were obtained independently, based on information from the following sources:

(i) A letter was sent to the patient’s general practitioner (GP) giving details about the GOS categories, and aslung him/her to rate the patient accordingly. (ii) A 60-item questionnaire (RQ), based on a questionnaire previously used with this patient group [12], was mailed to a relative or friend of the patient asking for details about the patient’s recovery. The questionnaire covered physical disability, cognitive, affective and behavioural problems, and the patient’s current functional status. The information obtained was used by a member of the research team to allocate a GOS score. This research worker was blind to the test scores, interview report and subsequent GOS score allocated by the psychologist. (iii) A score was allocated by a research psychologist after interviewing the patient and carrying out neuropsychological tests. If necessary this was supplemented by information given by a close relative. This assessment was considered to provide optimal data for the purposes of assigning a GOS score and the other two GOS scores were compared with this score.

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312 S. I. Anderson et al.

Assessments were carried out at 6, 12 and 24 months after injury. Thirty-two assessments were undertaken at 6 months, 30 at 12 months and 18 at 24 months.

Statistical analysis

In addition to analysing percentage agreement levels and correlation coefficients between the different raters, the agreements were analysed using the weighted kappa coefficient (Kw). The kappa coefficient is a statistic used to assess nominal scale reliability [13-151, and the weighted version used in this study allows disagreements of varying gravity to be differentially weighted. A discrepancy of two categories difference, for example a severely disabled patient assessed as a good recovery, is judged to be more serious than if the patient had been assessed as moderately disabled. Kw reflects the level of agreement, from 0, which would be expected by chance alone, to 1 reflecting perfect agreement between raters.

Results

Reliability o f the RQ and the CP as sources ofinformationfor GOS assessment

The distribution of scores can be seen in Tables 2 and 3. In 81% of cases the GOS derived from RQ information coincided with the psychologst’s assessment, whereas the GP’s assessment agreed with the psychologst’s assessment in only 61% of cases (Kw 0.79 for RQ and 0.45 for GP assessment). While both RQ and GP assessments were significantly correlated with the psychologist’s GOS assessment, the correlation between the psychologist’s assessment and that based on the RQ was high ( ~ 0 . 7 9 p=0.001), whereas that between the psychologst and the GP was relatively low ( ~ 0 . 4 9 p=0.001). While agreement was mostly within one outcome category, five patients were categorized by their GPs as achieving good recoveries when the psychologist had placed than in the severely disabled group. Most of the disagreement between the psychologst and the GP occurred in the group of patients considered to be moderately disabled by the psychologst. Fourteen of these patients were assessed by the GP as having achieved good recoveries. Thus the tendency was for the GP to make an over-optimistic assessment.

Table 2. Distribution Of‘Glasgow Outcome Scale (GO57 scores assessed by psychologist and G P (6, 12 arid 24 months data combined; n=SO assessments).

Psychologist’s assessment OJ GOS

GP’s assessment .f GOS Severe disability Moderate disability Good recovery Total

Severc disability 4 Moderate disability 5 Good recovery 5

Totdl 14

2 9

14

25

0 5

36

41

6 19 55

80

Lcvd ofdgreemcnt = 61%; Kwr = 0.45 95% C.I. (0.254, 0.646)

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Glasgow Outcome Scale: rater reliability 313

Table 3. Distribution o f Glasgow Outcome Scale ( G O S ) scores assessed by psychologist and relatives’ questionnaire (RQ) (6, 12 and 24 months data combined: n=8O assessments).

Psychologist’s assessment o f GOS

RQ assessment o f cos Severe disability Moderate hsability Good recovery Total

Severe disability 9 Moderate disability 4 Good recovery 1

Total 14

1 20 4

25

0 10 5 29

36 41

41 80

Level of agreement = 81%; Kw = 0.79 95% C.I. (0.676, 0.904)

Table 4. Percentage agreement levels between psychologist’s rating o f Glasgow Outcome Scale (GOS) and relatives’ questionnaire (RQ) rating and GP rating at dflerent time intervals.

Time since injury 6 months 12 months 24 months (%I (%) (%)

RQ 78 83 83

GP 50 67 61

Table 5. Distribution o f Glasgow Outcome Scale ( G O S ) scores assessed by psychologist and GP ( 6 months data only; n=32 assessments).

Psychologist’s assessment o f GOS

GP’r assessment o f GOS Severe disability Moderate hsability Good recovery Total

Severe disability 1 0 0 1 Moderate disability 2 5 1 8 Good recovery 2 11 10 23

Total 5 16 11 32

Level of agreement = 50%; Kw = 0.31 95% C.I. (0.003, 0.62)

Efects oftime since injury on assessment

Table 4 shows the percentage agreement levels attained by the GP’s and the RQ’s with the psychologist’s assessment at 6, 12 and 24 months. The GOS scores based on RQ information achieved a high level of agreement with the psychologist’s rating at all three time intervals. The least reliable assessment procedure (when measured by lack of agreement with the psychologist’s rating) was the GP’s rating at 6 months post-injury when only half of the assessment scores coincide (Kw 0.31). In 15 out of these 16 discordant assessments the GP had overestimated the patients’ level of recovery (see Table 5). Figure 1 shows the distribution of the 32 GOS assessments of the psychologist and the GP at 6 months post-injury.

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314 S. I . Anderson et al.

= Psychologist's GP's rating rating

20 -

10 -

Severe disab. Moderate disab. Geed Recovery Glasgow Outcome Scale

Fipm 1. Psycholofist 's arid CP'; ratirigs of' Glaspu~ Oictconie Scale a t 6 months post-head injiwy.

Discussion

Previous studies suggest that the GOS is a reliable assessment method for use in large-scale studies of late outcome after head injury, and that this niay compensate for its lack of scrisitivity to subtle differences in recovery patterns. The present study shows that the reliability of the GOS varies according to the method used for obtaining data.

O u r results suggest that it i5 preferable to gather as much information as is practically possible from a relative or close friend of the patient and for this data to be used by a research worker Experienced in the use of the GOS to allocate a score, if interviewing and testing the patient IS not possible. The alternative of providing less experienced fellow professionals with details of the scoring system and asking them to allocate a score based on contact with the patient seems to be less reliable. The importance of training raters in the use of such scales has been emphasized (91 and is endorsed here. No guidelines are gven by the authors of the scale about how to deal with certain problems which are prcvallcnt in the head-injured population. These include the effects of other bodily injuries sustained at the same time as the head injury, epilepsy or the risk thereof consequent to the head injury, the influence of preexisting personality disorder, and chronic illness on subsequent outcome. A high proportion of the head-injured population are unemployed prior to their injury. This may also create difficulties when trying to assess functional outcome: non-work social roles are more difficult to assess, and the GOS has been shown t o correlate highly with rates of return to work [16].

The letter to the GP eniphasized the importance of considering only the effect of the head injury on outcome rather than other bodily injuries sustained concurrently, or chronic conditions which had pre-existed the head injury. However, it became apparent that in certain cases the GP's GOS rating reflected such extraneous factors. In several cases,

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Glasgow Outcome Scale: rater reliability 315

patients who had made a good recovery &om their head injury were assessed by the GP as being moderately disabled due to physical disability which was independent of the head injury. Patients who had sustained a major spinal injury at the time of the head injury were excluded &om this study, as a GOS score, reflecting only the effects of the head injury, would be difficult to assign in this particular patient group. A large proportion of the head-injured population sustain other injuries at the time of their head injury [17], so the confounding effects of these on GOS scores is important. The nature of the consultation undertaken by the GP may result in an emphasis on medical and neurological symptoms, rather than psychological or behavioural problems. In some cases a good physical recovery was equated with a good outcome overall, even though the patient was clearly functioning at a reduced level due to behavioural problems and/or cognitive impairment. For example one patient, who had previously run his own business, had had to sell his business, was never outdoors unattended and could not be left alone in the home for more than a couple of hours because of the severity of h s cognitive impairment. The patient was physically very well, bright in mood and superficially presented as being unimpaired; his lack of insight into his problems meant that he had few complaints and made light of his changed situation. The patient’s GP assessed him as a good recovery at both 6 and 12 months fiom the time of injury whereas the psychologist had rated him as severely disabled. Pre-morbid personality and social problems, such as chronic alcoholism or drug abuse, which are frequent in the head-injured population, must also be taken into account when assessing outcome. An attempt must be made to isolate such features fiom those aspects of the patient’s behaviour and symptomatology that are consequential to the head injury. Five per cent of patients suffering head injury wdl develop post-traumatic epilepsy [18] and a further proportion are treated prophylactically with anti-convulsant medication. This has implications for their social and occupational functioning but it is not clear how the GOS should categorize patients who are well recovered but prevented from driving and working due to the risk of epileptic seizure.

Many of the problems reported by the head injured and their relatives in the post-acute phase are of a neurobehavioural, cognitive and affective nature. Relatives may describe the patient as being lethargic, impatient and laclung in insight; difficulties which may have devastating effects on the patient’s level of functioning. Because the problems are not medical in nature, many patients and families may not even approach the GP for help. Earlier work has shown that these neurobehavioural problems are reported with increasing frequency as time goes on [19]. This apparent deterioration in the patient’s condition may be due to a reaction to his or her changed circumstances or to psychological factors predominating after progressive physical and neurological recovery. Brooks et al. reported that these personality and behavioural changes are associated both with stress in caring relatives and with difficulty in returning to pre-morbid work level [12, 201. Such problems are readily reported by relatives if they are asked, and this information can be used by a research worker to assign a GOS score. It has been suggested that the frequency and severity of reported problems may vary with the personality of the reporting relative; however the high level of agreement in this study between the GOS based on the relatives report and the assessment undertaken by the psychologist, suggested that this factor is not sufficient to substantially affect the reliability of GOS scores obtained by this method.

Both methods of assessment were less reliable at 6 months than at the two later time intervals, although this is the most common interval for follow-up to be undertaken. In half of the assessments undertaken at 6 months after injury, the GP’s rating differed from that of the psychologist. This may be related to the tendency, emphasized by Jennett et al.

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316 S. I . Anderson et al.

[6] for the patient to be assessed in relation to the severity of the initial damage: this factor is less likely to operate with increasing time from injury. The GP knew the details of the severity of injury whereas the research worker using the RQ was blind to this information. At 6 months post-injury the assessor may m i s s the subtle, yet disabling problems being experienced by these patients; problems readily elicited by the RQ. By 1 year after injury patients may either improve to a good recovery or the problems may have become much more overt.

Incomplete data is a problem in large-scale follow-up studies; it is therefore important to consider how methods of data collection differed. For inclusion in this study each patient needed to have data for all three GOS assessments carried out. We also looked at return rates for the two indirect methods of assessment in a group of 342 patients who were not included in the study and found that the rate of return for GP letters was more than 95%, whereas the return rate for RQ forms dropped to 59%. However, unlike the RQs for study patients, no attempts were made to retrieve RQ forms not returned after the first mailing and it is probable that a second mailing of R Q s would have substantially increased the data set. As the quahty of the data obtained by the RQ is so much higher, this is an important area for future research.

The reliability of the GOS has been shown to vary depending on the source of information used and the time interval at which assessment is completed. In ideal circumstances the patients should be tested and interviewed by personnel who have not been involved in the early care of the patient. Failing this, information should be obtained from relatives of the patient and used by staff, trained in the use of the GOS, to assign a score. The quality of the data so obtained is good and further work should be directed to increasing the return rate of questionnaires. A 6-month follow-up interval has been seen to be least reliable for GOS assessment and future work should consider a 12-month assessment period.

Although the present study was restricted to outcome assessment in patients aged 14 years and over, the GOS is frequently used to describe outcome in children. Because the scale was not designed with children in mind, many of the dfficulties inherent in its use are likely to be magnified when applied to this patient group. A reliability study of the GOS applied to children would therefore be a worthwhile undertaking.

Acknowledgements

This work was supported by Medical Research Council Special Project Grant (SPG 8809197).

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