Proinsulin, the precursor for insulin, is secreted in higher concentrations when β-cells are under stress and previous studies have shown that elevated proinsulin could be used as a marker for individuals in a pre-diabetic state. The aim of this study was to assess the stability of proinsulin across a wide concentration range (3-882 and 2-187pmol/L; total and intact proinsulin respectively) in whole blood to determine whether it could be used in routine clinical care. 51 subjects (26 normal glucose tolerance, 17 impaired glucose tolerance and 8 type 2 diabetes) had blood taken into EDTA tubes at 0, 60 & 120min following a glucose load. The samples were kept at room temperature (~20°C) with aliquots taken, centrifuged and frozen at 0, 24, 48 and 72h. Comparison of the combined data (pre and post-glucose load) of baseline with 72h as a percentage of baseline gave an average of 123% (95% CI: 119-127) and 107% (95% CI: 105-109) for total and intact proinsulin respectively. A small change in the stability of total proinsulin was observed whilst there was no clinical difference over the 72h period for intact proinsulin.