The impact of new guidelines for glucose tolerance testing on clinical practice and laboratory services

The Canadian Diabetes Association's 2003 clinical practice guidelines for diabetes mellitus were published in mid-December 2003.1 These guidelines retain the fasting plasma glucose test for the diagnosis of diabetes, and the criterion for diagnosis (fasting plasma glucose level of 7.0 mmol/L or more) remains unchanged. However, the guidelines now include a new recommendation: that anyone with a risk factor for diabetes and a fasting plasma glucose level between 5.7 and 6.9 mmol/L should be considered for an oral glucose tolerance test (OGTT).1 The previous version of the guidelines,2 published in 1998, lacked management guidelines for the diagnostic category of impaired fasting glucose (fasting plasma glucose level of 6.1 to 6.9 mmol/L). The move to recommend more OGTTs responds to consistent reporting of a significant number of people with fasting plasma glucose values below the diagnostic threshold for diabetes but with postload glucose values exceeding the diagnostic cut-off of 11.1 mmol/L. If the new recommendation is followed, the prevalence of diagnosed diabetes should increase, and the significant number of undiagnosed cases should be correspondingly reduced.3,4 Strong evidence exists that interventions can reduce the progression of impaired glucose tolerance to diabetes mellitus over time,8,9 and this evidence has also driven the recommendation to promote formal glucose tolerance testing over a wider range of fasting plasma glucose levels than impaired fasting glucose alone.1 It should be recognized, however, that the cited intervention trials examined people with impaired glucose tolerance precisely because this group is known to have a high event rate. Impaired glucose tolerance is a phase in the natural history between "at risk" and the development of diabetes. It is reasonable to assume that "at-risk" patients with normal glucose tolerance or impaired fasting glucose would benefit from lifestyle interventions, and the need for definitive determination of impaired glucose tolerance is moot. Complete adherence to the new guidelines would have a large impact on laboratory services that, in the absence of additional financial resources, could be detrimental to health service delivery. We gathered data on fasting plasma glucose tests for different patients (repeat tests excluded) from the main laboratory service provider in the Calgary region (Calgary Laboratory Services) for the period Sept. 1 to Nov. 30, 2003. The distribution of the 57 357 fastin