Reproducibility of impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) classification: a systematic review.

Background: The classifications of impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) represent glucose levels above normal, but below the decision threshold for diabetes. We sought to determine what the reproducibility of these classifications was when repeat tests were performed by conducting a systematic review of the literature.
Methods: All primary studies published in English of any study design were included. Studies were excluded if they did not follow the World Health Organization or American Diabetes Association diagnostic criteria, used whole blood as the specimen type, a glucose meter for analysis, or performed repeat testing greater than 8 weeks apart.
Results: Five papers had reproducibility data for IGT or IFG, two of which where from the same population but sampled differently. The kappa coefficients, indicating agreement between repeat tests that exceeded chance, indicated poor to fair agreement for IGT (0.04, 0.22, 0.38, 0.42) and moderate agreement for IFG (0.44 and 0.56). Similarly, the observed reproducibility was slightly lower for IGT (33%, 44%, 47%, 48%) compared to IFG (51%, 64%). In two studies for which data were available for both IGT and IFG, the average reproducibility was lower (49%) for the prediabetes group compared to the diabetes group (73%) or the normal group (93%).
Conclusions: Poor reproducibility of IGT and IFG classification suggests caution should be exercised when interpreting a single test result.