The United States FDA defines PROMs as "any report of the patient's health condition that comes directly from the patient, without interpretation of the patient’s response by a clinician or anyone else” [29]. A comprehensive assessment of patient health status should, consequently, combine objective clinical and biological data with the patient’s subjective opinion. The PROMs allow clinically focused health professionals to reliably assess the impact of interventions and effectively select optimal therapy solutions and interventions [30, 31].
Each of the proposed hypothesis were proven. The LLFI-PL had high but suitable and not excessive internal consistency, and test-retest reliability. The criterion validity was stronger between the LLFI-PL and WOMAC than between the LLFI-PL and the generic EQ-5D-5L. The EFA of the LLFI-PL confirmed a single-factor structure, though the presence of a possible inflection at point-2 in the scree plot suggests that a modification to the questionnaire may be necessary, such as shortening to remove potential redundant items. This is consistent with the recommendations of previous authors [11, 13, 14].
The cultural and linguistic adaptation that produced the LLFI-PL complied with recognized standards [15] and ensured the linguistic proportionality of the concepts used and accounted for the slight discrepancies from a number of synonyms for individual words. The process confirmed the strong psychometric properties of the original-English, Spanish, and Turkish versions. The decision to complete this study was justified as it provided a regional lower limb PROM in the Polish language that could be applied to a wide range of patients with various functional problems in the lower limbs of varying severity and duration.
This study’s results demonstrated the LLFI-PL has comparable psychometric properties to the original-English, Spanish, and Turkish versions [11, 13, 14]. The internal consistency (α = 0.94) is slightly higher than the original and Spanish versions (α = 0.91) [11, 13] and notably more than the Turkish (α = 0.82) [14].
The test-retest reliability (ICC2.1=0.96, at 6-days average) is identical to the Spanish version (ICC2.1=0.96, at 7-days) [13] and comparable to both the original and Turkish versions (at three-days, ICC2.1=0.97) [11, 14].
The LLFI-PL error scores (SEM = 0.9 points or 3.5%, MDC90 = 2.0 points or 8.1%) are slightly higher than the three published versions, the original English (SEM = 2.8%, MDC90 = 6.6%) [11], Turkish (SEM = 3.2%, MDC90 = 5.8%) [14], and Spanish (SEM = 3.1%, MDC90 = 7.1%) [13]. This finding suggests that changes obtained at the level > 6–8% can be interpreted as real change in functional status.
The criterion validity, assessed by the PCC, was higher with the joint and condition specific WOMAC (r = 0.81) than the generic EQ-5D-5L, (r = 0.63) and the EQ-5D-5L-VAS (r = 0.57). These correlation differences were expected with the higher level due to the greater relevance and specificity of a joint/condition-related PROM compared to a general health and quality of life PROM. These are mildly higher than the Spanish findings for the WOMAC (PCC, r = 0.77), EQ-5D-3L (r = 0.62), and EQ-5D-3L-VAS (r = 0.58) [13] and similar to the Turkish findings where the SF-36 subscales were used and with a high-moderate finding for the physical dimensions (from r = 0.43 to r = 0.76) but moderate-low (from r = 0.20 to r = 0.66 ) for the mental dimension [14].
The finding in all four versions of the LLFI (English, Spanish, Turkish, and Polish) recommend a preferred single-factor structure. This was achieved consistently with the recommended MLE and Varimax rotation format [11, 13, 14]. From the perspective of parsimony this confirms that the questionnaire items do measure the construct of lower limb functional status as a single kinetic chain and can be calculated with a single-summated score. However, each previous study also found multiple factor structures were potentially possible from the raw data analysis. This suggests a shortened version is preferred and an eight-item preference has been recommended and is currently under journal review and submission [personal communication, Gabel et al, ‘A Shortened eight-item version of the LLFI retains the psychometric properties while improving factor structure and practicality’, under submission].
The LLFI-PL questionnaire is easy and quick to complete and score. The questions are simple and clearly defined, so the patient and therapist burden is minimized. The times to complete (172 ± 33 seconds) and score (20 ± 9 seconds) are marginally longer than determined by the original study (131 ± 23 and 17 ± 5 seconds respectively) [11].
Limitations and Strengths
The current study limitations include a lack of assessment of responsiveness of the LLFI-PL. Further, the test-retest period of three to seven days may have been too long for the acute participants (32.0%) as change can occur within a shorter period for these patients and this may increase the change scores. However, the high ICC2.1 value reflects that of previous versions and, consequently, this difference may not have greatly affected the psychometric properties. Ideally, a regional criterion such as the LEFS [Binkley et al. 1999] should be used but, as there is none available in Polish, this was not possible. The substitution of the WOMAC was not ideal but provided a regional indication.
The study strengths include the use of standardized methods for both cross-cultural adaptation and assessment of the psychometric properties. Further strengths are the prospective nature and diversity of conditions affecting each lower limb sub-region with varied degrees of severity and duration.
Future Considerations
The lack of determination of the responsiveness suggests that future studies need to consider the ability of the LLFI-PL to detect minimal clinically important differences (MCIDs) over a period of time longer than two weeks. Also, that a larger study population (~ 1000) or data pooling be used to definitively clarify the factor structure through the use of confirmatory factor analysis (CFA). This has already been completed with the ideal shoened version being eight items [personal communication, Gabel et al, ‘A Shortened eight-item version of the LLFI retains the psychometric properties while improving factor structure and practicality’, under submission].