Rasch measurement in the Assessment of Growth Hormone Deficiency in adult patients.

The Assessment of Growth Hormone Deficiency in Adults (AGHDA) questionnaire was previously designed, translated and validated in several European countries to evaluate the impact of the disease on Quality of Life. This study aimed to test the metric properties of the Spanish version by means of Rasch analysis. A sample of 356 consecutive adult patients with untreated GHD was included in the study. Patients responded to the questionnaire at baseline and 12 months apart. Answers were analyzed following the dichotomous logistic response model. Parameter estimates, model-data fit and separation statistics were computed. The invariance of the item parameters across time was tested in the follow-up. Rasch results were additionally employed to ascertain score changes through the calculation of the Reliable Change Index (RCI). Items varied in severity from 8.3 -16.8 units (SE= 0.4-0.5) and fit to define a unidimensional variable. The item separation index (SI)(5.2) indicates a good and reliable (0.9) separation of items along the variable that they define. Moreover, results showed the AGHDA conforms to the model expectation of item parameter invariance between administrations. The substantial (2.3) and reliable (0.8) person SI also suggests the sample was well targeted by the questionnaire. According to the RCI, 84% of the patients did not show a significant transition in their measures. Results denote the items of the AGHDA succeeded in defining a scale characterized by the interval-level of its measures, suggesting the questionnaire could be a useful complement of the clinical evaluation of GHD patients at both group and individual level.