The rate of unused kidney donations has more than doubled in Australia and New Zealand but it is unclear why, researchers say.

A study led by the South Australian Health and Medical Research Institute examined de-identified Australia and New Zealand Organ Donor Registry (ANZOD) data on just over 7800 kidneys retrieved from deceased donors from 2005-17.

By comparing the non-utilisation rates in the period 2005-12 with the period 2013-17, the team aimed to determine if the era in which kidneys were donated was an independent predictor for them not being transplanted.

They controlled for donor characteristics, including age, BMI, ABO blood group, diabetes, hypertension, hepatitis B or C, and whether donation followed brain death or controlled circulatory death.