IRT in rheumatology

Liseth Siemons

The contribution of assessing forefoot joints in early rheumatoid arthritis patients

Insights from item response theory

Liseth Siemons

Introduction (1/5)

Rheumatoid arthritis (RA):

Non-curable rheumatic autoimmune disease

Disease outcome is unpredictable

Any age, mostly females

Chronic inflammations joints and joint tissues

Core measure for measuring disease activity is the DAS-28

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Introduction (2/5)

DAS-28, widely used index measure, including: 28-tender joint count (TJC-28)

28-swollen joint count (SJC-28)

Erythrocyte sedimentation rate (ESR)

General heath

.56 * √TJC28 + .28 * √SJC28 + .70* LN(ESR) + .014 * (GH) = DAS-28 score

Score 0-10, where <2.6 means the disease is in remission

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Introduction (3/5)

“The DAS-28 underestimates actual disease activity.”

Including or excluding foot joints?

Remains a topic of debate and research

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Introduction (4/5)

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Advocates including foot joints:

1. Commonly involved

2. Forefoot involvement during first years of RA

3. DAS-28 underestimates actual disease activity

4. Remission in DAS-28 might not be a true state of remission

Opponents including foot joints:

1. Administration time

2. Feet abnormalities may result from other processes than RA

3. Assessment difficulties

Introduction (5/5)

No studies used IRT to examine the contribution of foot joints.

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Study objective

To evaluate the contribution of assessing forefoot joints to the measurement range and measurement precision

of joint counts in early RA patients using IRT.

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Methods (1/2)

Patients

DREAM remission induction cohort

Measures (at baseline)

TJC-28 and TJC-38 SJC-28 and SJC-38 VAS-general health ESR

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Methods (2/2)

Statistics

IRT (generalized partial credit model)

We were particularly interested in:

- Information curves: measurement range, precision, local reliability

- Alignment between patient and joint distributions

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Results – Patients (1/3)

Variable Mean (SD) or n (%)

DAS-28 score 4.69 (1.31)

TJC-28 5.60 (5.55)

TJC-38 7.90 (7.26)

SJC-28 7.39 (5.60)

SJC-38 9.15 (6.73)

VAS general health 49.83 (25.03)

ESR 29.16 (20.89)

CRP 21.11 (35.80)

≥ 1 MTP joint tender, n (%) 233 (50.76%)

≥ 1 MTP joint swelling, n (%) 200 (43.57%)

> 1 MTP joint tender, n (%) 196 (42.70%)

> 1 MTP joint swelling, n (%) 170 (37.04%)

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459 patients: - 286 female - 173 male

mean age: 58 years

Results – Patients (2/3)

Variable Mean (SD) or n (%)

DAS-28 score 4.69 (1.31)

TJC-28 5.60 (5.55)

TJC-38 7.90 (7.26)

SJC-28 7.39 (5.60)

SJC-38 9.15 (6.73)

VAS general health 49.83 (25.03)

ESR 29.16 (20.89)

CRP 21.11 (35.80)

≥ 1 MTP joint tender, n (%) 233 (50.76%)

≥ 1 MTP joint swelling, n (%) 200 (43.57%)

> 1 MTP joint tender, n (%) 196 (42.70%)

> 1 MTP joint swelling, n (%) 170 (37.04%)

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459 patients: - 286 female - 173 male

mean age: 58 years

Results – Patients (3/3)

Variable Mean (SD) or n (%)

DAS-28 score 4.69 (1.31)

TJC-28 5.60 (5.55)

TJC-38 7.90 (7.26)

SJC-28 7.39 (5.60)

SJC-38 9.15 (6.73)

VAS general health 49.83 (25.03)

ESR 29.16 (20.89)

CRP 21.11 (35.80)

≥ 1 MTP joint tender, n (%) 233 (50.76%)

≥ 1 MTP joint swelling, n (%) 200 (43.57%)

> 1 MTP joint tender, n (%) 196 (42.70%)

> 1 MTP joint swelling, n (%) 170 (37.04%)

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459 patients: - 286 female - 173 male

mean age: 58 years

Results - Measurement range TJC

The range where the tender joint counts provide reliable measurements.

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TJC-28: r >.80 for -0.3 < ϴ < 2.8

TJC-38: r >.80 for -0.5 < ϴ < 3.1

Results – Item information curves TJC-38

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Most information was provided by small joints (MCP, PIP, MTP).

Foot joints mainly contained information along a range already covered by the joints of the TJC-28.

Results - Measurement range SJC

SJC-28: r >.80 for -0.7 < ϴ < 2.2

SJC-38: r >.80 for -0.8 < ϴ < 2.7

The range where the tender joint counts provide reliable measurements.

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Results – Item information curves SJC-38

Most information was provided by MCP and PIP joints.

Foot joints did not provide a high amount of information.

Foot joints mainly contained information along a range already covered by the joints of the SJC-28.

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Results – patient and joint distributions TJC-38

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Upper figure: The patient distribution of the 38-tender joint count. Lower figure: The joint distribution of the 38-tender joint count. The grey dots in the joint distribution represent the foot joints.

Results – patient and joint distributions TJC-38

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A discrepancy between the patient and joint distribution of the TJC-38.

Joint distribution: from 0.65 to 3.07 Patient distribution: from -1.48 to 3.51

The joints cluster together in a small range at the right half of the person distribution.

The elbow joints (most to the right) are only relevant for a small proportion of patients.

The foot joints are relevant for a larger proportion of the early RA sample, but they function along a range already covered by other joints of the instrument.

Results – patient and joint distributions SJC-38

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Upper figure: The patient distribution of the 38-swollen joint count. Lower figure: The joint distribution of the 38-swollen joint count. The grey dots in the joint distribution represent the foot joints.

Results – patient and joint distributions SJC-38

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A discrepancy between the patient and the joint distribution of the SJC-38.

Joint distribution: from -0.20 to 8.95 Patient distribution: from -1.78 to 4.03

5 joints fall outside the range covered by the patient distribution (i.e. elbows, shoulders, right knee), diminishing their relevance.

4 of the 10 foot joints function along a range not yet covered by other joints, but their relevance is limited.

The other foot joints function along a range already covered by other joints.

Conclusions (1/2)

Foot joints were frequently affected.

No significant improvements in measurement range or measurement precision.

Discrepancy between the joint and the patient distributions remained large for both the tender and the swollen joint count.

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Conclusions (2/2)

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From a measurement perspective, reduced joint counts are appropriate to use on a population level.

On an individual level foot joints often need attention.

Thank you for your attention!

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Questions?