analysis of the factors affecting the interval between blood donations using log-normal hazard model with gamma correlated frailties

background : time to donating blood plays a major role in a regular donor to becoming continues one. the aim of this study was to determine the effective factors on the interval between the blood donations. methods : in a longitudinal study in 2008, 864 samples of first-time donors in shahrekord blood transfusion center,  capital city of chaharmahal and bakhtiari province, iran were selected by a systematic sampling and were followed up for five years. among these samples, a subset of 424 donors who had at least two successful blood donations were chosen for this study and the time intervals between their donations were measured as response variable. sex, body weight, age, marital status, education, stay and job were recorded as independent variables. data analysis was performed based on log-normal hazard model with gamma correlated frailty. in this model, the frailties are sum of two independent components assumed a gamma distribution. the analysis was done via bayesian approach using markov chain monte carlo algorithm by openbugs. convergence was checked via gelman-rubin criteria using boa program in r. results : age, job and education were significant on chance to donate blood ( p <0.05). the chances of blood donation for the higher-aged donors, clericals, workers, free job, students and educated donors were higher and in return, time intervals between their blood donations were shorter. conclusions : due to the significance effect of some variables in the log-normal correlated frailty model, it is necessary to plan educational and cultural program to encourage the people with longer inter-donation intervals to donate more frequently.