Assessment of an asthma quality of life scale using item-response theory

Health-related quality of life (HRQOL) scores produced by simply summing individual item values have been criticized for lacking linearity and for not being equally discriminating across the range of scores. Differences in summed scores may depend more on their starting point on the scale rather than actual differences in the underlying dimension of HRQOL, making it difficult to judge what a particular score means. We sought to examine the usefulness of an alternative method of scoring questionnaires, item-response theory (IRT), for the clinical interpretation of a modified version of the Marks Asthma Quality of Life Questionnaire (MAQLQ-M).

Using the MAQLQ-M, we surveyed 293 adults with moderate to severe asthma, managed at two university teaching hospitals in Adelaide, South Australia. Scores obtained by usual summative Likert-type scores were compared to estimates using the partial credit method of IRT.

We found a non-linear relationship between raw, summative scores and the IRT estimates. The departure from linearity was marked for summative scores below 3.0 and above 5.0 (range 1.0-7.0), values that included half of the study patients. Summative scoring did not produce scores at the interval level of measurement.

For an equivalent difference measured in the underlying dimension of actual HRQOL using IRT, traditional summated scores showed a much smaller difference in scores at both the lower and upper end of HRQOL, than at the mid-range of HRQOL. Caution should be used when interpreting HRQOL surveys scored in the usual summative manner. Advantages may be gained by using IRT as an alternative method of scoring HRQOL questionnaires.