Lack of Concordance between the 75-g and 100-g Glucose Load Tests for the Diagnosis of Gestational Diabetes Mellitus Giorgio Mello, 1 Parretti Elena, 1 Agostino Ognibene, 2* Riccardo Cioni, 1 Filippo Tondi, 1 Paola Pezzati, 2 Monica Pratesi, 3 Gianfranco Scarselli, 1 and Gianni Messeri 2 Background: Gestational diabetes mellitus (GDM) is common and can have a substantial impact on fetal growth, birth weight, and morbidity. The American Diabetes Association recommends GDM testing with either a 3-h, 100-g glucose load (100g) (criteria according to Am J Obstet Gynecol 1982;144:768 –73) or a 2-h, 75-g glucose load (75g). We investigated the comparability of the 75g and the 100g tests in the diagnosis of GDM. Methods: From January 1997 to December 1999, in 1061 consecutive Caucasian nonobese and nondiabetic preg- nant women who attended the Maternal-Fetal Medicine Unit, we performed GDM testing with a 75-g load during 2 periods of pregnancy: early (16 –20 weeks) and late (26 –30 weeks). Because we assumed there would be few GBM cases in women with a 1-h plasma glucose <1300 mg/L in the 75g test, we did not retest these women. We retested the remaining women with possi- ble or diagnosed GDM with a 100-g load within a week. Results: GDM was diagnosed in 41 of 227 women with the 100-g load and 15 of 227 with the 75-g load (11 concordant); the  index was 0.21. At 26 –31 weeks of pregnancy, 484 of 976 women (49.9%) underwent both tests. GDM was diagnosed in 60 of 484 woman with the 100-g load and in 26 of 484 with the 75-g load (13 concordant); the  index was 0.18. Conclusions: Among women with possible GDM in both early and late periods of pregnancy, there was only weak diagnostic agreement between results determined with 75-g and 100-g glucose loads. © 2006 American Association for Clinical Chemistry Gestational diabetes mellitus (GDM) 4 is a common distur- bance that can have a substantial impact on pregnancy outcomes such as fetal growth, birth weight, and morbid- ity. For this reason, most pregnant women in developed countries undergo GDM testing as part of routine ante- natal care. The Clinical Practice Recommendations issued by the American Diabetes Association (ADA) in 2003 (1) state that GDM is diagnosed on the basis of the oral glucose tolerance test (OGTT). The OGTT can consist of either a 3-h, 100-g glucose load (100g) [criteria of Carpen- ter and Coustan (2)] at fasting and 1, 2, and 3 h after glucose load, or a 2-h, 75-g glucose load (75g), with the same criteria at fasting and 1 and 2 h after glucose load. This recommendation derives from the conclusions of the ADA Fourth International Workshop Conference on Ges- tational Diabetes Mellitus, held in March 1997, where it was first suggested that both tests, at the same cutoff values, could be used to diagnose GDM (3). Despite their supposed equivalence, the 3 tests clearly have many relevant differences and have not been com- pared thus far as to their usefulness for identifying women with GDM in the same population. The aim of this study was to compare the performance of the 75g and 100g tests in pregnant women with possible or diagnosed GDM according to the 75g test. 1 Department of Gynecology, Perinatology and Human Reproduction, University of Florence, Firenze, Italy. 2 Department of Laboratory, Clinical Chemistry Laboratory, Azienda Os- pedaliera–Universitaria Careggi, Firenze, Italy. 3 Department of Statistics and Mathematics Applied to Economics, Uni- versity of Pisa, Pisa, Italy *Address correspondence to this author at: Department of Laboratory, Clinical Chemistry Laboratory, Piastra dei Servizi, Azienda Ospedaliera– Universitaria Careggi, Viale Morgagni 85, 50139 Firenze, Italy. Fax: 39-55- 4279390; e-mail: a.ognibene@tin.it, a.ognibene@med.unifi.it. Received July 22, 2005; accepted June 30, 2006. Previously published online at DOI: 10.1373/clinchem.2005.058040 4 Nonstandard abbreviations: GDM, gestational diabetes mellitus; ADA, American Diabetes Association; OGTT, oral glucose tolerance test; 100g, 3-h, 100-g glucose load; 75g, 2-h, 75-g glucose load; AUC, area under the curve. Clinical Chemistry 52:9 000 – 000 (2006) Evidence-Based Laboratory Medicine and Test Utilization 1 Papers in Press. First published July 27, 2006 as doi:10.1373/clinchem.2005.058040 Copyright © 2006 by The American Association for Clinical Chemistry