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The landscape of diabetes in pregnancy has shifted dramatically over the past 100 years with historical reports limited to descriptions of women with type 1 diabetes and a very poor prognosis [11]. Nowadays, diabetes is the most common clinical condition in the pregnant population, and it is estimated that in 2017, some form of hyperglycemia in pregnancy affected around 21.3 million live births [12, 13]. Within pregnancy, women affected by diabetes may be grouped into 2 broad categories (Fig. 1). The first category or pregestational diabetes refers to diabetes diagnosed before the pregnancy and includes women with type 1 diabetes, type 2 diabetes, and monogenic and other specific types of diabetes. The diagnosis of established diabetes may be made in any one of the following ways: [6, 14]

(i) Random plasma glucose concentration ≥11.1 mmol/L in the presence of classical symptoms of hyperglycemia (polydipsia, polyuria, weight loss).

Fig. 1. Classification of diabetes in pregnancy. GDM, gestational diabetes mellitus; Type 1, type 1 diabetes mellitus; Type 2, type 2 diabetes mellitus; Other, other forms of diabetes including monogenic diabetes.

(ii) Fasting plasma glucose ≥7.0 mmol/L. Fasting is defined as no caloric intake for at least 8 h.

(iii) Plasma glucose ≥11.1 mmol/L, 2 h after a glucose load containing the equivalent of 75 g anhydrous glucose dissolved in water (i.e., the 2 h oral glucose tolerance test [OGTT]).

(iv) Glycated hemoglobin ≥48 mmol/mol (6.5%).

If symptoms are absent, diagnosis should be confirmed and not just based on a single glucose determination [6]. The criteria based on plasma glucose concentrations were established by an expert committee of the American Diabetes Association (ADA) in 1997 and later ratified by the World Health Organization (WHO) in 2009 [15, 16]. In July 2009, the use of hemoglobin as a diagnostic tool was recommended by an international expert committee, and in 2010, the ADA also made this recommendation followed by the WHO in 2011 [14, 17, 18].

Within this category of diabetes, it is important to consider to which subgroup the patient belongs. Typically, this is based on family history, personal medical history, and clinical examination; however, further investigation is necessary where uncertainty surrounds the diagnosis [19]. For example, measurement of antibody titers may be necessary to confirm autoimmune type 1 diabetes or genetic testing to identify monogenic diabetes [6, 19].

The second category of diabetes affecting the pregnant population is GDM. The diagnosis of GDM is controversial, and screening and diagnostic criteria vary significantly [20]. Guidelines vary from recommending universal screening with a diagnostic OGTT or a glucose challenge test to risk factor-based screening [21, 23]. Even the amount of glucose recommended for the OGTT varies between 75 and 100 g, and there is variation in the postprandial glucose levels above which the diagnosis is established [10, 21–23]. For example, the WHO, the Endocrine Society in the United States, the European Board and College of Obstetrics and Gynaecology, and the International Federation of Gynecology and Obstetrics all endorse the 2008 International Association of Diabetes and Pregnancy Study Groups (IADPSG) 2008 criteria [10, 20, 24, 25]. The cutoff points chosen convey an OR for adverse outcomes of at least 1.75 compared to women with mean glucose levels at 24–28 weeks in the Hyperglycemia and Neonatal Outcomes study [21, 26]. The specific adverse outcomes include large for gestational age, neonatal c-peptide >90th percentile, and neonatal percentage body fat >90th percentile. These are the first criteria for GDM based on pregnancy outcomes, with prior diagnostic criteria most often based on the risk of developing type 2 diabetes after the delivery [27]. These criteria advise a 1-step approach by making a diagnosis of GDM if fasting glucose is ≥5.1 mmol/L, or 1 h postprandial is ≥10.0 mmol/L or 2-hour postprandial is ≥8.5 mmol/L after a 75 g OGTT. The ADA endorses this approach but states that an alternative “2-step” approach is also acceptable. This involves a 1-h, 50 g glucose challenge in the nonfasting state followed by a 3-h 100 g OGTT for those who screen positive as defined by a glucose of ≥7.8 mmol/L [23]. GDM is then diagnosed if at least 2 out of 4 glucose levels are met or exceeded on the OGTT. The Carpenter/Coustan (fasting glucose 5.3 mmol/L, 1-h 10.0 mmol/L, 2-h 8.6 mmol/L, 3-h 7.7 mmol/L) [28] or National Diabetes Data Group (fasting 5.8 mmol/L, 1-h 10.6 mmol/L, 2-h 9.2 mmol/L, 3-h 8.0 mmol/L) criteria may be used [29]. An alternative approach is taken by the National Institute for Health and Care Excellence in the United Kingdom who advise a diagnosis of GDM if the woman has either a fasting glucose of ≥5.6 mmol/L or a 2-h glucose level of ≥7.8 mmol/L after a 75 g glucose load [22]. Economic analyses suggest that these criteria are more cost-effective in the UK context than IADPSG 2008 criteria [30].