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• Does Tubach’s Shortened WOMAC Function Scale Overcome the Full Length Version’s Shortcomings?
  Paul W Stratford
  Published on: 17 January 2005

• Published on: 17 January 2005

  Does Tubach’s Shortened WOMAC Function Scale Overcome the Full Length Version’s Shortcomings?

  • Paul W Stratford, Professor

  Dear Editor,

  Tubach and colleagues [1] propose an 8-item shortened version of the WOMAC physical function subscale which they consider promising for clinical use and large scale surveys and clinical trials.

  The authors report that the shortened measure’s test retest reliability and responsiveness are similar to those of the complete physical function subscale; the internal consistency of the shortened...

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  Dear Editor,

  Tubach and colleagues [1] propose an 8-item shortened version of the WOMAC physical function subscale which they consider promising for clinical use and large scale surveys and clinical trials.

  The authors report that the shortened measure’s test retest reliability and responsiveness are similar to those of the complete physical function subscale; the internal consistency of the shortened measure (0.84) is less than that of the full length measure (0.93). I have several questions concerning the methods:
  1. Was the entire WOMAC, including pain and stiffness subscales, administered?
  2. Were responses to the shortened version abstracted from the lengthened version?
  3. If the answer to the previous question is “yes,” was the difference in sample size for the responsiveness analysis (shortened version n=1169; full length n=1048) a result of missing item values?
  4. Would it be possible to provide descriptive statistics (mean, standard deviation) for the shortened version?
  5. Could the authors present the standard error of measurement associated with the internal consistency estimates?

  The answer to the last question would be helpful in labeling the measurement error at an instant in time and in ascertaining the consequence of using the shortened version to make confident—in a reliability sense—decisions concerning an individual patient’s functional status at an instant in time.

  Although the WOMAC has been the principal self-report functional status measure for patients with osteoarthritis of the hip or knee for 2- decades, there is consistent evidence in the literature refuting the measure’s factorial validity [2-5] The WOMAC purports three subscales— pain, stiffness, physical function—yet the pain and function subscales have been repeatedly shown to overlap [2-5]. That pain and function are not completely distinct is to be expected; however, the OMERACT III group deemed them distinct enough to warrant independent assessment [6]. At issue is the extent to which the overlap in responses to the pain and function subscales is real or biased due to a common item phrasing in pain and function items [4]. It would be informative to know whether the proposed shortened function scale represented a factor distinct from pain in the study sample.

  As a matter of interest I ran a factor analysis on the pain and shortened function scale proposed by the authors on a sample of 310 patients with osteoarthritis of the hip or knee reported previously in the peer reviewed literature [4]. I ran two analyses with oblique rotation, one used the eigenvalue greater than 1 rule and the other restricted the analysis to two factors. The eigenvalue greater than 1 rule identified three factors. Neither analysis grouped the items by purported subscales of pain and function.

  The consequence of poor factorial validity is that one may not be assessing what she/he is believed to be assessing. To examine the consequence of the apparently poor factorial validity of the proposed shortened version of the WOMAC function scale, I examined the measure’s ability to detect deterioration in a group of patients undergoing total hip or knee arthroplasty. This sample is described in a previous paper [7]. The rationale being that physical function deteriorates between preoperative and postoperative assessment performed within 16 days of surgery. I repeated the analysis reported previously in a peer- reviewed paper [7] (n=104) and found the following for the proposed shortened physical function subscale: mean pre-operation 15.7 (sd=5.2); mean post-operation (median assessment interval was 8 days, 1st, 3rd quartiles were 7, 10 days) 15.9 (sd=5.6); and the standardized response mean was –0.02 (95% CL: -0.23, 0.17). That the confidence interval for the standardized response mean includes the value zero indicates the measure was not able to detect deterioration in this group.

  Tubach and colleagues acknowledge that further evaluation of their shortened version is required. I believe an important next step is to ascertain whether the measure possesses factorial validity.

  References

  (1). Tubach F, Baron G, Logeart I, Dougados M, Bellamy N, Ravaud P. Using patients' and rheumatologists' opinions to specify a short form of the WOMAC function subscale. Ann Rheum Dis 2005;64:75-79.

  (2). Faucher M, Poiraudeau S, Lefevre-Colau MM, Rannou F, Fermanian J, Revel M. Algo-functional assessment of knee osteoarthritis: comparison of the test-retest reliability and construct validity of the WOMAC and Lequesne indexes. Osteoarthritis Cartilage 2002;10:602 -10.

  (3). Thumboo J, Chew LH, Soh CH. Validation of the Western Ontario and Mcmaster University osteoarthritis index in Asians with osteoarthritis in Singapore. Osteoarthritis Cartilage 2001;9:440-6.

  (4). Stratford PW, Kennedy DM. Does parallel item content on WOMAC's Pain and Function Subscales limits its ability to detect change in functional status? BMC Musculoskelet Disord 2004;5:17.

  (5). Guermazi M, Poiraudeau S, Yahia M, Mezganni M, Fermanian J, Habib Elleuch M, et al. Translation, adaptation and validation of the Western Ontario and McMaster Universities osteoarthritis index (WOMAC) for an Arab population: the Sfax modified WOMAC. Osteoarthritis Cartilage 2004;12:459-68.

  (6). Bellamy N, Kirwan J, Boers M, Brooks P, Strand V, Tugwell P, et al. Recommendations for a core set of outcome measures for future phase III clinical trials in knee, hip, and hand osteoarthritis. Consensus development at OMERACT III. J Rheumatol 1997;24:799-802.

  (7). Stratford PW, Kennedy DM, Hanna SE. Condition-specific Western Ontario McMaster Osteoarthritis Index was not superior to region- specific Lower Extremity Functional Scale at detecting change. J Clin Epidemiol 2004;57:1025-1032.

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  Conflict of Interest:
  None declared.

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