Development and validation of the immunosuppressant therapy adherence instrument (ITAS).

A five-item scale was developed that asked respondents to indicate how often they were non-adherent to immunosuppressant therapy (IST) given a particular circumstance. Two hundred and twenty-two recipients completed the instrument. Validity of the scale was assessed by correlating composite item scores with refill record adherence rates (RRARs), serum immunosuppressant (IS) concentrations, graft rejection, and increased serum creatinine (SCr) levels. One scale item was deleted due to lack of response variability. Cronbach's alpha coefficient for the four-item scale was 0.81, demonstrating that the scale has acceptable reliability. All items loaded on a single principal component, suggesting that the scale measures a single adherence construct, which accounted for 64% of the scale items' variance. The four-item scale, adherence measured by IS RRARs, and "target" IS serum concentrations had positive correlations (p < 0.01). Item scores were shown to be negatively related to rejection occurrence and increased SCr (p < 0.05). The immunosuppressant therapy adherence scale is the first published, valid and reliable instrument that measures recipients IST adherence.