The proposed use of a more precise standard for glycated (A_1c) and non-glycated haemoglobin would lead to an A_1c value, when expressed as a percentage, that is lower than that currently in use. One approach advocated to address the potential confusion that would ensue is to replace ‘HbA_1c’ with a new term, ‘A_1c-derived average glucose.’ We review evidence from several sources suggesting that A_1c is, in fact, inherently imprecise as a measure of average glucose, so that the proposed terminology should not be adopted.