In this study, we evaluated three aspects of construct validity: known-groups comparisons, convergent validity, and floor/ceiling effects. We observed statistically and clinically distinct differences between Skindex-16 domain scores and OPGA severity levels. Likewise, we observed the expected positive, moderate correlation hypothesized between all domains of Skindex-16 (PROM) and OPGA (CROM) both cross-sectionally and over time. These data support the construct validity of the Skindex-16 in routinely evaluated psoriasis patients.
While women did experience more severe impact on HRQL in this cohort, the relationship between OPGA and Skindex-16 domains was similar between sexes. This is similar to recent findings in hidradenitis suppurativa.(17)
Reliability, or the consistency of multiple test adminstrations in the same patient without clinical change, could not be assessed as Skindex-16 was tracked over time in a clinical setting. However, we hypothesized that given a reliable test, patient-level characteristics such as their own understanding of the questions, baseline HRQL levels, and pattern of answering should lead to at least a moderate degree of within-person correlation. Our findings suggest that person-level effects contributed importantly to the score variability over time, even after adjusting for changes in disease severity. This underscores the importance of tracking individual patient HRQL data and having personalized discussions.(28) Each patient’s characteristics are incorporated into their scores, thus deviations from in their own trends are more likely to be meaningful than established cut points of severity. Lastly, ICC’s offer insight into differences in domain measurement. For example, Skindex-16 functioning and emotions domains showed less within-person variability over time than the symptoms domain. Emotional and functioning-based HRQL may be more person-specific and may respond differently to changes in disease status than symptoms. For example, prior work has demonstrated that emotional effects and impact on social interactions can persist even after treatment.(29–31)
Similar to prior studies, domain-level floor and ceiling effects were generally rare and may be an important advantage of Skindex-16 over other general dermatology measures such as the commonly used Dermatology Quality of Life Index (DLQI).(32) Though floor effects > 20% were seen for the Skindex-16 functioning domain, most of these scores were in cleared disease (OPGA score 0) or very mild disease (OPGA score 1). Importantly, OPGA scores of 0 and 4 were much more common in the cross-sectional cohort than Skindex-16 scores of 0 or 100 (n = 279, 27.1% and n = 60, 5.82% respectively) illustrating how PROs can add information beyond CROs, regardless of disease severity.(9)
In this cohort, only about 1% of patients fell into discordant groups where PROM and CRO scores were highly mismatched. Most patients only had one discordant event. Together, our findings of good construct validity, apparently reliable detection of person-level impacts on HRQL scoring, with the findings of rare discordance suggest that these discordant events may be clinically important and not just artifacts of HRQL instrument error. cspHRQL may suggest a sudden worsening in HRQL without clinically observable disease change or a new skin condition that is now affecting patient HRQL. For example, Skindex-16 scores were elevated for one patient’s first two visits while OPGA was 0. At the third visit, Skindex-16 remained elevated and OPGA jumped to 3, potentially suggesting HRQL effects that occurred prior to a clinically observable psoriasis flare. However, discordances may also represent more complex phenomenon such as present HRQL impacts, such as anxiety of recurrence, side effects of medication, etc., which may persist despite good disease management and have more complicated time trends.(29–31, 33)
If collected prior to the visit, discordant Skindex-16 scores, may give context to the visit and trigger investigation for new skin findings, non-obvious symptoms, or impact from other comorbid or new disease. Pruritus, for example, is often a stronger determinant of depression than other visible processes in patients with psoriasis.(34) Another non-obvious issue that could affect functioning is sleep disturbance, which is common with psoriasis.(35) In this study, we found that discordance was more common in patients with other comorbid skin conditions or multiple diagnoses in addition to psoriasis being listed at the clinical visit. Thus, exploring discordance could result in improved understanding of the patient experience, aid shared decision making, and potentially alter management to better meet the global needs of the patient.(36–39)
Limitations
This study has important limitations. First, our data came from a single academic institution and PROM scores are known to vary by socioeconomic status or other demographic characteristics.(4, 6, 40) Also, data on treatment regimen were unavailable for this analysis. Treatment regimens could potentially add variability to correlations and especially could alter longitudinal relationships. Lastly, convergent and known-groups comparisons were limited to those defined by the OPGA. Though recently validated, it represents a composite of two simple and validated measures which is simple to interpret and is routinely collected.(11) The psoriasis area and severity index (PASI), though considered a gold-standard CRO, is not routinely collected routinely collected.(12) Simple measures such as investigator global scores or body surface area are more readily used in clinic, and since a composite of these (the OPGA) is routinely used in clinic, we sought to validate the Skindex-16 with a clinic-ready and easily calculated severity score that is routinely utilized in our institution. Though the OPGA itself is likely not widely used, the measures which it is based on are and it has been shown to be highly related to those. Future studies evaluating Skindrx-16 characteristics in other populations and with other outcome measurements will be important in further understanding how to best implement this measure.