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Trio of Scales Deemed Valid, Reliable for Use in Prurigo Nodularis

Article

The Skin Pain Numeric Rating Scale, Worst-Itch Numeric Rating Scale, and Dermatology Life Quality Index were evaluated for their content validity, fit-for-purpose, and psychometric and measurement properties among patients with the chronic skin disease being treated with dupilumab.

Meaningful within-patient improvement thresholds of 4 (range, 3.5-4.5), 4 (range, 3.0-4.5), and 9 (range, 8.0-10.0) were seen following evaluation of the Skin Pain Numeric Rating Scale (Skin Pain-NRS), Worst-Itch Numeric Rating Scale (WI-NRS), and Dermatology Life Quality Index (DLQI), respectively, for their utility among individuals with prurigo nodularis (PN) being treated with dupilumab.

These findings indicate that the 3 scales are fit-for-purpose instruments for evaluating outcomes among adult patients living with PN uncontrolled on topical therapies.

The international team of investigators presented their findings in 3 posters at this year’s American Academy of Dermatology Annual Meeting in New Orleans.

Skin Pain Numeric Rating Scale1

This scale measures maximum 24-hour pain, from 0 (no pain) to 10 (worst pain). Data from the randomized, double-blind, placebo-controlled, phase 3 PRIME and PRIME2 trials were analyzed, and the investigators incorporated anchor- and distribution-based methods for the Skin Pain-NRS to determine within-patient improvement. Their target anchors were a 1-category improvement in the Patient Global Impression of Severity (PGIS) and “moderately better” on the Patient Global Impression of Change (PGIC).

At baseline, the mean (SD) weekly Skin Pain-NRS score was 7.2 (2.41).

Findings from cognitive debriefing interviews showed that 89.5% thought this scale was relevant to their PN experience, and a majority noted they understood the item and response scale. Intraclass correlations (ICC) of 0.81 (95% CI, 0.66-0.88) from baseline to week 4 per PGIS, 0.92 (95% CI, 0.87-0.95) from weeks 8 to 12 per PGIS, and 0.90 (95% CI, 0.81-0.95) at week 4 that equated to disease stability were reported in patients. Consecutive group mean changes were also seen in the improved patients compared with the patients with stable disease: 0.5 to 0.8 vs above 0.8, respectively.

At baseline and week 12, the scale was also shown to differentiate well between groups, with correlations above the recommended threshold of 0.37 (absolute r = 0.46-0.52); this threshold also exceeded the distribution-based results (at baseline, 0.5 [1.21]; SEM, 1.05).

Worst-Itch Numeric Rating Scale2

As with the Skin Pain-NRS, the WI-NRS uses a 24-hour period to evaluate the worst itch and on a 0 (no itch) to 10 (worst imaginable itch) scale. The investigators again used PRIME and PRIME2 study data, anchor- and distribution-based methods, and 1-category improvement in the PGIS and moderately better results for the PGIC.

Cognitive debriefing interviews were incorporated for content-validity of the WI-NRS, and the results were even better. All patients who had these interviews thought the WI-NRS was relevant to their PN experience, along with the items it evaluated and the response scale; in addition, 95% thought the recall period was relevant.

The patients’ mean (SD) weekly WI-NRS score at baseline was 8.5 (1.0).

An ICC of 0.70 or greater indicated test-retest reliability, and results of 0.72 (95% CI, 0.63-0.79) between screening to baseline and 0.88 (95% CI, 0.82-0.92) from weeks 8 to 12 were reported in stable patients. Further, changes in Skin Pain-NRS, Sleep-NRS, DLQI, PGIS, and PGIC from baseline to week 24 had moderate/strong correlations with change in the WI-NRS from baseline to week 24 (absolute r = 0.40-0.76).

Overall, patients with different severities of PN were easily distinguishable using the WI-NR, with the groups defined by large PGIS scores demonstrating large effect sizes (>0.8) at baseline and week 12 between consecutive groups: severe disease vs none and mild or moderate (baseline) and moderate vs none or mild severe vs moderate (week 12).

Dermatology Life Quality Index3

This scale has not been previously validated among patients with PN; health-related quality of life (HRQOL) outcomes in dermatology clinical trials are its main backdrop. Also, unlike the Skin Pain-NRS and the WI-NRS, the DLQI evaluates HRQOL over the previous week and on a scale of 0 (no effect on HRQOL) to 30 (extremely large effect on HRQOL).

PRIME and PRIME2 study data, anchor- and distribution-based methods, and 1-category improvement in the PGIS and moderately better results for the PGIC again were incorporated.

Internal consistency was good to excellent at baseline (Cronbach α = 0.88; 95% CI, 0.85-0.90) and week 12 (Cronbach α = 0.92; 95% CI, 0.90-0.93). Among stable patients, defined per PGIS score between weeks 8 and 12 and PGIC score at week 4, the ICCs for test-retest reliability were reported at 0.87 (95% CI, 0.84-0.91) and 0.70 (95% CI, 0.31-0.86), respectively.

Good convergent and divergent validity was also seen, with moderate to strong correlation observed for conceptually related measures (absolute r = 0.33-0.60) and low to moderate correlations seen for less-related measures (absolute r = 0.03 to 0.45). Further, there were moderate to strong correlations seen between DLQI score changes and changes in PGIS, PGIC, WI-NRS, Sleep NRS, and Skin Pain-NRS from baseline to week 24 (absolute r = 0.38 to 0.60).

Disease severity was also shown to be easily distinguishable among the patients evaluated (all P < .001).

References

1. Bahloul D, Thomas RB, Rhoten S, et al. Validation of the Skin Pain Numeric Rating Scale (NRS) in prurigo nodularis (PN) based on clinical studies of dupilumab in adults with PN. Presented at: American Academy of Dermatology Annual Meeting; March 17-21, 2023; New Orleans, Louisiana. https://bit.ly/3LpQsg0

2. Bahloul D, Thomas RB, Rhoten S, et al. Validation of the Worst-Itch Numeric Rating Scale (WI-NRS) in prurigo nodularis (PN) based on clinical studies of dupilumab in adults with PN. Presented at: American Academy of Dermatology Annual Meeting; March 17-21, 2023; New Orleans, Louisiana. https://bit.ly/3JkMHFX

3. Validation of the Dermatology Life Quality Index (DLQI) in prurigo nodularis (PN) Based on clinical studies of dupilumab in adults with PN. Presented at: American Academy of Dermatology Annual Meeting; March 17-21, 2023; New Orleans, Louisiana. https://bit.ly/3ZNJu8R

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