SO004TEST-RETEST RELIABILITY, VALIDATION, AND ‘MINIMAL DETECTABLE CHANGE’ SCORES FOR A RANGE OF COMMON PHYSICAL FUNCTION AND STRENGTH TESTS IN NON-DIALYSIS CKD

Abstract

INTRODUCTION AND AIMS: Physical function is an important outcome in CKD, and poor functional capacity is strongly related to mortality and reduced quality of life. In rehabilitation and research settings, physical function can be assessed using a range of field tests. Frequently used tests include: the incremental shuttle walk test (ISWT) to assess exercise capacity, sit-to-stand (STS) chair tests to assess muscle endurance (STS- 60 second) and strength (STS-5 repetition), isokinetic quadriceps strength testing by estimated 1RM (e1RM) to measure lower limb function, and cardiopulmonary exercise testing (CPET) to assess aerobic capacity (VO2peak). Reliability indicates the degree to which scores of a test are free of measurement errors. Recognising the error inherent in outcome measures is imperative to the understanding of changes and interpretation of any research or rehabilitative intervention. A clinically useful means of interpreting reliability is the 'minimal detectable change' (MDC), defined as the 'smallest amount of change in a measurement necessary to conclude that the difference is not attributable to error'. We aimed to establish the reliability and MDC of these tests in CKD. We also examined the validity of the ISWT and STS-5 as measures of aerobic capacity and strength, respectively. METHODS: 43 CKD patients (23 females, mean age, 61 (SD: 11) years, eGFR, 24 (SD: 8) ml/min/1.73 m2) were assessed on two visits separated by 6 weeks of habitual activity. All tests (apart from CPET) had a familiarisation previously to remove learning effects. Test-retest reliability was assessed using intraclass correlation coefficient (ICC), and absolute reliability by standard error of measurement and MDC. Validity was assessed using linear regression modelling. RESULTS: The ISWT, STS-60, e1RM, and CPET had 'good' to 'excellent' reliability (ICC r = .973, .927, .927, and .866). The STS-5 reliability was 'poor' (ICC r = .676). The MDC (i.e. inherent error) for each test is as follows: ISWT = 20 m; STS-5 = 7.5 seconds; STS-60 = 4 repetitions; e1RM = 6.4 kg; VO2peak = 2.8 ml/kg/min. There was 'strong' agreement between the ISWT and VO2peak (r = 0.73 and 0.74), but 'poor' agreement between the STS-5 and quadriceps strength (via the e1RM) (r = 0.14 and 0.47). CONCLUSIONS: This study is the first investigation into the test-retest reliability and identification of the MDC for tests of physical function in a non-dialysis CKD population. Our results show the ISWT, STS-60, e1RM, and CPET are reliable tests of function in CKD with little intrinsic measurement error. The ISWT is a valid measure of aerobic capacity versus 'gold standard' VO2peak. Accordingly, these tests should be employed with confidence in clinical CKD settings. Knowledge of the MDC is essential to researchers and rehabilitation professionals to help interpret 'true' changes, above the inherent measurement error, caused by an intervention in their patients.