Assessing damage in individual joints in rheumatoid arthritis: a new method based on the Larsen system

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Assessing damage in individual joints in rheumatoid arthritis: a new method

based on the Larsen system

P. Zangger, , a, J. R. Kachurab, C. Bombardierc, D. A. Redelmeierd, E. M.

Badleye and E. R. Bogochf

Received 29 April 2003; accepted 29 July 2003. ; Available online 13

November 2003.

Abstract

Objectives: To evaluate observer agreement using the Larsen system (LS) and

a Modified Larsen system (ML) when assessing individual joints of the hands

and wrists in rheumatoid arthritis, and to compare the two systems. To

determine the minimally important difference (MID) for the ML. Patients and

methods: Thirty radiographs of hands and wrists from 10 patients who

presented with RA were graded by two blinded observers, using the LS and

then the ML. Patients were followed for a mean of 7.2 years (range: 4­10

years). Inter- and intra-observer agreement were calculated using the kappa

statistic with linear incremental weights. Inter-observer agreement was also

computed for the summed score, using an intraclass correlation coefficient.

Inter-observer error was estimated by calculating the mean and standard

deviation of the grading differences between the two observers. Prevalence

of damage was calculated as a ratio of damage: no damage and expressed as a

percentage. Pairs of radiographs were comparatively graded using a

seven-point Likert scale. Results: The kappa statistic for inter-observer

agreement was 0.38 (marginal reproducibility) for the LS and 0.52 (good

reproducibility) for the ML (P = 0.004). Using a difference of one grade as

perfect agreement, it was 0.56 (good reproducibility) for the LS and 0.87

(excellent reproducibility) for the ML (P = 0.001). Intra-observer agreement

was high in both systems. The distribution of ML-grade differences varied

according to the level of the Likert scale: for " a little bit worse " ,

representing the smallest amount of detectable damage progression, the

distribution differences peaked around two grades. This value represented a

MID 87% of the time. Conclusions: The LS lacks precision for individual

joints. The ML, it is proposed, has more detailed definitions of grades, and

is more reliable. When pairs of radiographs were compared, a two-grade

difference on the ML was the MID.