Outcome Measures for Patients with Lower Limb Amputations

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Aim:

1. To enable the reader to select appropriate outcome measures to demonstrate effective intervention. (See CSP Outcome Measures Toolbox)

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• Evidence based
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General introduction

Outcome measures can be used for many different purposes. A predictive measure should be able to classify individuals according to a set of pre-defined categories either concurrently or prospectively e.g. whether an amputee will use a prosthesis successfully   .Detecting differences between people or groups demonstrates the discriminative value of an outcome measure e.g. being able to determine the different abilities of a trans-tibial or trans-femoral amputee or differences between prosthetic components from scores or times recorded . Whereas an evaluative measure should be able to detect changes, usually over a period of time in an individual or group. An evaluative outcome measure may also detect changes occurring following some kind of intervention, e.g a therapy programme or provision of a prosthetic component. Some outcome measures are designed to do only one of the above, while others may do a combination, though some of the requirements of these different types of outcome measures are competing . Whichever purpose it is designed for, the psychometric properties of the outcome measure need to be reported to satisfy the user that it is fit for purpose with the population they wish to use it . The psychometric properties of an outcome measure are the characteristics that express it’s adequacy in terms of reliability, validity and responsiveness. Another term often used is clinimetric properties. While being developed from similar origins as psychometrics, clinimetricshas been described as the practice of assessing or describing symptoms, signs, and laboratory findings by means of scales, indices, and other quantitative instruments, all of which should have adequate psychometric properties .

Considerations before choosing an outcome measure

If you are considering using an outcome measure with an amputee it is worth asking yourself the questions posed on the Outcome Measures page here in Physiopedia (Guide_to_Selecting_Outcome_Measures).At the very least you should consider these questions with your amputee patient or group in mind.

Why am I using an outcome measure?

•  Am I trying to establish a baseline measure from which I can monitor changes over time for an individual patient?
• Am I trying to predicthow my patient is going to perform? 
• Am I trying to evaluate the impact of a treatment programme or prosthetic component on an individual or a group?
• Am I trying to evaluate the needs of the amputee attending my service?
• Am I trying to evaluate how my service is responding to needs of the amputee?
  

What am I aiming to measure?

• Impairments of body structure and function?
• Activity limitations?
• Participation restrictions?
• Quality of life?
• Something else?
  

When you think you may have an outcome measure in mind you should also consider these questions.

Have the clinimetric properties of the outcome measure I am considering been measured in a population similar to mine?

• Is the outcome measure reliable?

1.  Do I know the rate of error detected with scores?
2.  Do I know the minimum detectable change?

• Is the outcome measure valid?

1.  Does it measure what I want it to measure?

• Is the outcome measure responsive to change?

1.  Is there a known minimum clinically important difference?
   

Here are some examples of studies where the clinimetric, sometimes called psychometric, properties have been reported in an amputee population and what the results may tell you.

Reliability

Reliability is usually measured by Intra-class correlation coefficients (ICC) and is presented as a number between 0 (no consistency) to 1 (complete consistency)
Intra-rater Reliability: This indicates how consistently a rater administers and scores an outcome measure.
Inter-rater Reliability: This indicates how well two raters agree in the way they administer and score an outcome measure.
Test-retest reliability:If an individual completes a self-report survey and then repeats the survey on a second occasion when no change is expected, the results should be similar.

• Brooks, Hunter et al (2002) examined the reliability of the 2MWT . Participants completed 2 successive timed walks measured by 2 different raters on 2 consecutive days. Intra class correlations (ICC) were >0 .98 showing excellent intra- and inter-rater reliability.

Measurement error:This is the degree to which scores or ratings are identical irrespective of who performs or scores the test and can be reported using the standard error of measurement (SEM)or minimal detectable change (MDC), which is the same as smallest detectable change (SDC) .

• Deathe& Miller (2005) reported the SEM in absolute values, which was 3sec for the L-Test .
• Resnik& Borgia (2011) also reported MDCin absolute values for all the measures they studied: 2MWT (34.3m), 6MWT (45m), TUG (3.6s) and AMP (3.4pts)  

Internal Consistency:This reliability property is reserved for outcome measures that are designed to test only one concept. Internal consistency assesses the extent to which all items or question in an outcome measure address the same underlying concept, e.g. in a mobility scale, all the items should deal with mobility .
There are two main methods used to report internal consistency,the Classical Test theory uses Cronbachs alpha (α) to indicate the reliability of an outcome measure as a whole. And the Item Response Theory uses Rasch Analysis to assess internal consistency by looking at each item within the outcome measure .

• The internal consistency of the ABC scale was considered excellent as measured by Cronbachs alpha (0.93) in a study by Milleret al (2003) . 

Rasch analysis was used to examine all the items in the Berg Balance Scale which confirmed that it was able to test a range of difficulty and identify four levels of ability  .

Validity

Content /Face validity: This is the degree to which the content of an outcome measure is an adequate reflection of the construct or concept to be measured(5). It is usually considered and agreed by consensus of an expert group of clinicians and can and should include patient representatives. For example an instrument measuring activity limitation in young athletic individuals should include not only walking but also running, jumping, and climbing.
Structural validity: This refers to the degree to which the scores of an outcome measure are an adequate reflection of the dimension or factor of the construct being measured . It can be measured by performing factor analysis where the result demonstrate that if >50 % of data refer to one factor this confirms that the outcome measure is measuring one factor / dimension. Anything less indicates more than one factor is being assessed. Rasch Analysis may also be used it investigate where the outcome measure it’s unidimensionality, i.e. whether it is measuring one or more factors or dimensions.

• Wong et al (2013) reported results from factor analysis performed on the Berg Balance Score (BBS). The results showed that 70% of the data were explained in the model related to one dimension, i.e. balance capability .
• Franchignoni et al (2007) used Rasch modelling on a modified LocomotorCapability Index to confirm good structural validity when level 1 and 2 category responses were combined and 4 items were deleted due to either over or under-fitting . The resultant modified index is known as LCI-5 which many clinicians now use.

Construct Validity: This is the degree to which the scores of an outcome measure are consistent with pre-defined (apriori) hypotheses that outline relationships to the scores of other instruments, or differences between groups. If > 75% of hypotheses are proved this is an indication of good validity .
It can also be referred to as:Concurrent validity – showing the ability to distinguish between groups (e.g. older and younger LLAs)which is often measured by testing hypotheses, or; Convergent validity – showing that measures that should be related are relatedwhich can also be measured using ICC with high values indicating good validity.

• Major et al (2013) hypothesised positive relationships between the BBS scores and the Activities specific Balance Confidence (ABC) scale, the mobility scale of the Prosthetic Evaluation Questionnaire (PEQ(ms), the Frenchay Activities Index and the 2MWT and a negative relationship with the L-test score . These were all proved.
• To assess concurrent and convergent validity of the L-test Deathe& Miller (2005) asked subjects to complete walks test; Timed "Up & Go" Test (TUG),10-Meter Walk Test, and 2-Minute Walk Test, followed by self-reported measures; ABC scale, Frenchay Activities Index (FAI), PEQ (ms). Concurrent validity washigh (ICC = 0.86-0.97)between the L Test data and the other walk tests and fairto moderate ( ICC = 0.22 – 0.54) for self-report measures.Higher mean times were observed for thosesubjects who:

1. were older
2. used a walking aid
3. had to concentrate on each step they took
4. had a vascular amputation and
5.  had a TF amputation.

Therefore it was demonstrated that the L Test was able to discriminate between all groups as hypothesized .

Criterion validity: This is the degree to which the scores of an outcome measure are an adequate reflection of a ‘gold standard’. However there are very few situations in rehabilitation where such a gold standard test exists. If no gold standard is available then it may be appropriate to test hypothetical relationships with comparator measures.
The estimation of criterion validity depends on the type of data. Intra-class correlations are used if both instruments (outcome instrument and comparator) have continuous scores (e.g. time, distance etc) and the results should preferably be above 0.70. If the outcome instrument has a continuous score but the comparator has a dichotomous score (e.g. Yes / No) then area under the receiver operated characteristic (ROC) is the preferred method. Again, a criterion of 0.70 is suggested .

• Gremeauxet al (2012) presented ROC curves for the 2MWT. The modified Houghton Scale was used to stratify the patients into two groups; those with no mobility problems (scored 20/20) and those who scored less than 20 indicating a functional limitation. According to the ROC analysis cut off values of 130m or 150m were highly associated with the existence of functional limitations .

Responsiveness

Internal responsiveness is the ability of a measure to change over a specified time frame. It will depend on the particular population being studied, the treatment or intervention which occurs during the time frame and the outcome measure used to determine any changes(5).
Standard effect size is the difference between the mean baseline scores and the follow-up scores, divided by the baseline standard deviation (SD). If there is a high variability in baseline scores in relation to the mean change scores the effect size will be small and the ability of the OM to detect meaningful changes is also small. A small effect will be 0.2 representing a change of approx 1/5 that of the baseline SD, 0.5 is considered moderate and anything over 0.8, or a change of at least 4/5 of the baseline SD is considered large(21).
The paired-t-test is a statistical test that can be used to detect the change in the average scores at two time points, but is dependent on the sample size and variability / reliability of the outcome measure used(14).
In a study by Devlinet al (2004) the effect size calculated for the change in mean scores for the Houghton Scale, from discharge to follow-up was 0.60, indicating a moderate difference(22).

• Findings by Brooks et al (2001) indicated that the 2MWT was “responsive to change during rehabilitation”. Significant improvements were seen in means and SDs of the distances walked between baseline and discharge and follow-up(23). However, effect sizes were not calculated.

Other Considerations

Other considerations (link to main Physiopedia outcome measures page)may come into play when deciding which outcome measure to use:
Financial Considerations:

• What is the cost of this test?
• Is a licence required? 

Therapist Implementation

•  Is the measure easy for a clinician to conduct?
• Is special training required/available?
• Are there clear standardised instructions on how to carry out and score the measure?
• How long does it take to carry out the measure?
• How long does it take to record results?

Resources

• Is special equipment or are special forms required?
• Is space sufficient for this measure to be carried out?

Client

• How much time does it take for the person to complete?
• Is the task difficult?
• Is privacy required?

Patient-Reported Outcome Measures (PROQ)

• Is face-to-face contact required or can this measure be completed in the waiting room?
• Does the questionnaire cover sensitive personal issues?
• Is there a specific reading level required?
• Is the measure available in other languages?