In the Journals

Modified disease activity index using only 28 joint counts effective in PsA

Although the Disease Activity Index for Psoriatic Arthritis based on 66 swollen or 68 tender joint counts remains the preferred measure, a modified version using just 28 joint counts demonstrated good criterion, correlational and construct validity and sensitivity to change for patients with low disease activity, according to recent data in the Annals of the Rheumatic Diseases.

“By virtue of joint involvement in PsA, which often comprises swelling and tenderness of distal interphalangeal, ankle and foot joints, the 66/68 joint counts are traditionally used to assess patients with PsA during clinical examinations,” Brigitte Michelsen, MD, of the Hospital of Southern Norway Trust, and colleagues wrote. “However, many databases and registries solely collect 28 joint counts and not the comprehensive 66/68 joint counts, not only in patients with [rheumatoid arthritis] but also in patients with PsA, despite the lack of face validity of reduced joint counts outside of RA.”

To evaluate the psychometric performance of a modified Disease Activity Index for Psoriatic Arthritis (DAPSA) based on 28 joint counts, Michelsen and colleagues reviewed data from the Danish national quality registry, known as DANBIO, which includes information on the disease course of patients with RA, PsA and ankylosing spondylitis, regardless of treatment. Focusing on patients with PsA, the researchers divided the patients into two cohorts — 3,157 individuals, with 23,987 visits, in the examination group, and 3,154 patients, with 24,160 visits, in the validation group. According to the researchers, patients in the registry with PsA are routinely assessed using a 28-joint disease activity score.

The researchers defined DAPSA28 with the following equation: (28 tender joint count [TJC] × conversion factor1) + (28 swollen joint count [SJC] × conversion factor2) + patient’s global assessment (0–10VAS) + pain (0–10VAS) + C reactive protein (CRP) (mg/dL). Michelsen and colleagues determined the conversion factors by generalized estimating equations in the examination cohort, as well as an analysis of criterion, correlational and construct validity in the validation cohort.

According to the researchers, the final equation defining DAPSA28 was (28TJC × 1.6) + (28SJC × 1.6) + patient’s global assessment (0–10VAS) + pain (0–10VAS) + CRP (mg/dL).

Regarding criterion validity, the 28-joint count measure had comparable discriminative power to traditional index, with a standardized mean difference of 0.93 for DAPSA28 and 0.9 for DAPSA, to distinguish between patients in high and low disease activity. Kappa, with quadratic weighting of DAPSA and DAPSA28 disease activity states, was high at 0.92 (95%CI, 0.92-0.92), the researchers wrote. In addition, the standardized response mean for DAPSA28 was –0.92, compared with –0.96 for traditional DAPSA, for visits after the start of biological DMARD treatment.

Regarding correlational validity, baseline DAPSA and DAPSA28 demonstrated high correlation with 28-joint disease activity score with CRP (r = 0.87 for DAPSA, r = 0.93 for DAPSA28), simplified disease activity index (r = 0.92 for DAPSA, r = 0.99 for DAPSA28) (P < .001). In addition, a Bland-Altman plot demonstrated better agreement between DAPSA and DAPSA28 for low disease activity, compared with high disease activity.

In terms of construct validity, DAPSA and DAPSA28 were both similarly correlated to Health Assessment Questionnaire (r = 0.6 for DAPSA, r = 0.62 for DAPSA28) (P < .001). In addition, both DAPSA and DAPSA28 discriminated patients reporting their symptom state as acceptable versus not acceptable equally well, with mean 9.1 (SD = 8.7)/8.4 (SD = 8) and 24.2 (SD = 14.9)/22.5 (SD = 13.8), respectively.

“We recommend that the original DAPSA should be preferred and that 66/68 joint counts should be performed in patients with PsA to compute DAPSA,” Michelsen and colleagues wrote. “However, our study suggests that data sets with only 28-joint counts available can be used to calculate a modified DAPSA28, as a valid tool for database or outcomes research in the field of PsA, especially in patients with low disease activity.” – by Jason Laday

Disclosure: Michelsen reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.

Although the Disease Activity Index for Psoriatic Arthritis based on 66 swollen or 68 tender joint counts remains the preferred measure, a modified version using just 28 joint counts demonstrated good criterion, correlational and construct validity and sensitivity to change for patients with low disease activity, according to recent data in the Annals of the Rheumatic Diseases.

“By virtue of joint involvement in PsA, which often comprises swelling and tenderness of distal interphalangeal, ankle and foot joints, the 66/68 joint counts are traditionally used to assess patients with PsA during clinical examinations,” Brigitte Michelsen, MD, of the Hospital of Southern Norway Trust, and colleagues wrote. “However, many databases and registries solely collect 28 joint counts and not the comprehensive 66/68 joint counts, not only in patients with [rheumatoid arthritis] but also in patients with PsA, despite the lack of face validity of reduced joint counts outside of RA.”

To evaluate the psychometric performance of a modified Disease Activity Index for Psoriatic Arthritis (DAPSA) based on 28 joint counts, Michelsen and colleagues reviewed data from the Danish national quality registry, known as DANBIO, which includes information on the disease course of patients with RA, PsA and ankylosing spondylitis, regardless of treatment. Focusing on patients with PsA, the researchers divided the patients into two cohorts — 3,157 individuals, with 23,987 visits, in the examination group, and 3,154 patients, with 24,160 visits, in the validation group. According to the researchers, patients in the registry with PsA are routinely assessed using a 28-joint disease activity score.

The researchers defined DAPSA28 with the following equation: (28 tender joint count [TJC] × conversion factor1) + (28 swollen joint count [SJC] × conversion factor2) + patient’s global assessment (0–10VAS) + pain (0–10VAS) + C reactive protein (CRP) (mg/dL). Michelsen and colleagues determined the conversion factors by generalized estimating equations in the examination cohort, as well as an analysis of criterion, correlational and construct validity in the validation cohort.

According to the researchers, the final equation defining DAPSA28 was (28TJC × 1.6) + (28SJC × 1.6) + patient’s global assessment (0–10VAS) + pain (0–10VAS) + CRP (mg/dL).

Regarding criterion validity, the 28-joint count measure had comparable discriminative power to traditional index, with a standardized mean difference of 0.93 for DAPSA28 and 0.9 for DAPSA, to distinguish between patients in high and low disease activity. Kappa, with quadratic weighting of DAPSA and DAPSA28 disease activity states, was high at 0.92 (95%CI, 0.92-0.92), the researchers wrote. In addition, the standardized response mean for DAPSA28 was –0.92, compared with –0.96 for traditional DAPSA, for visits after the start of biological DMARD treatment.

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Regarding correlational validity, baseline DAPSA and DAPSA28 demonstrated high correlation with 28-joint disease activity score with CRP (r = 0.87 for DAPSA, r = 0.93 for DAPSA28), simplified disease activity index (r = 0.92 for DAPSA, r = 0.99 for DAPSA28) (P < .001). In addition, a Bland-Altman plot demonstrated better agreement between DAPSA and DAPSA28 for low disease activity, compared with high disease activity.

In terms of construct validity, DAPSA and DAPSA28 were both similarly correlated to Health Assessment Questionnaire (r = 0.6 for DAPSA, r = 0.62 for DAPSA28) (P < .001). In addition, both DAPSA and DAPSA28 discriminated patients reporting their symptom state as acceptable versus not acceptable equally well, with mean 9.1 (SD = 8.7)/8.4 (SD = 8) and 24.2 (SD = 14.9)/22.5 (SD = 13.8), respectively.

“We recommend that the original DAPSA should be preferred and that 66/68 joint counts should be performed in patients with PsA to compute DAPSA,” Michelsen and colleagues wrote. “However, our study suggests that data sets with only 28-joint counts available can be used to calculate a modified DAPSA28, as a valid tool for database or outcomes research in the field of PsA, especially in patients with low disease activity.” – by Jason Laday

Disclosure: Michelsen reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.