Establishing the Cross‐Cultural Validity of the Immune Status Questionnaire Using Rasch Analysis

Reliable assessment of self-reported immune function is essential for identifying at-risk populations, monitoring public health interventions and informing global health strategies. Yet, current instruments are imprecise and lack cross-cultural validity. This study aimed to assess the cross-cultural validity of the 7-item Immune Status Questionnaire (ISQ). The Partial Credit Rasch model was applied to analyse responses to the ISQ (N = 1250) from five nations, including India (n = 250), the Netherlands (n = 250), New Zealand (n = 250), Saudi Arabia (n = 250) and the United States (n = 250). Initial analysis revealed a poor fit to the Rasch model. Using sub-test methodology, ISQ items were combined into sub-tests, resulting in an acceptable fit to the Rasch model, evidence of strict unidimensionality and measurement invariance across nations. Acceptable fit permitted the production of an ordinal-to-interval conversion table included here. While physiological assessments of immune functioning provide accurate and objective evaluations of immune functioning, self-report measures remain a practical method for their ease of administration and lower cost. Findings support the reliability and cross-cultural validity of the ISQ. By applying interval conversion, physicians and researchers can improve the measurement accuracy of self-reported immune status and conduct valid comparisons across nations and to interval-level data.