An HbA1c level of 6.5% or higher; the test should be performed in a laboratory using a method that is certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized or traceable to the Diabetes Control and Complications Trial (DCCT) reference assay, or
A fasting plasma glucose (FPG) level of 126 mg/dL (7.0 mmol/L) or higher; fasting is defined as no caloric intake for at least 8 hours, or
A 2-hour plasma glucose level of 200 mg/dL (11.1 mmol/L) or higher during a 75-g oral glucose tolerance test (OGTT), or
A random plasma glucose of 200 mg/dL (11.1 mmol/L) or higher in a patient with classic symptoms of hyperglycemia (ie, polyuria, polydipsia, polyphagia, weight loss) or hyperglycemic crisis
The American Association of Clinical Endocrinologists, however, recommends that HbA1c be considered an additional optional diagnostic criterion, rather than a primary criterion for diagnosis of diabetes. 
If unequivocal hyperglycemia is absent, then HbA1c, FPG, and OGTT results should be confirmed by repeat testing. The ADA recommends repeating the same test for confirmation, since there will be a greater likelihood of concurrence. However, the diagnosis of diabetes is also confirmed if the results of 2 different tests are above the diagnostic thresholds. 
If a patient has had 2 different tests and the results are discordant, the test that has a result above the diagnostic threshold should be repeated. A second abnormal result on this test will confirm the diagnosis. 
In asymptomatic patients whose random serum glucose level suggests diabetes (>140 mg/dL), an FPG or HbA1c level should be measured. An FPG level of 100-125 mg/dL is considered an impaired fasting glucose (IFG), and an FPG level of less than 100 mg/dL is considered a normal fasting glucose. However, an FPG of 91-99 mg/dL is a strong independent predictor of future type 2 diabetes. 
An HbA1c below 6%is considered normal glucose tolerance (using an assay that has been standardized to the DCCT normal range of 4-6%). An HbA1C of 6-6.4% is neither normal glucose tolerance nor diabetes. With current assays, an HbA1c of less than 5.7% is considered normal and an HbA1c of greater than 6.4% is considered diagnostic for diabetes mellitus (DM). A value between 5.7% and 6.4% is considered diagnostic of prediabetes.
In the emergency department, a fingerstick glucose test is appropriate for virtually all patients with diabetes. All other laboratory studies should be individualized to the clinical situation. 
Plasma glucose is determined using blood drawn into a gray-top (sodium fluoride) tube, which inhibits red blood cell glycolysis immediately. A serum glucose measurement (commonly obtained on chemistry panels, using a red- or speckled-top tube) may be significantly lower than a plasma glucose measurement. Capillary whole blood measurements are not recommended for the diagnosis of diabetes mellitus, but they are valuable for assessment of patients in acute care situations.
The noted values for fasting glucose measurements are based on the level of glycemia at which retinopathy, a fairly pathognomonic diabetic complication, appears. (However, evidence suggests that retinopathy may occur even in prediabetes.) Fasting glucose measurements are not as predictive for indicating macrovascular risk as are post–glucose load values. However, there are no formal recommendations for using glucose tolerance tests for this purpose.
Impaired glucose tolerance
The World Health Organization (WHO) criteria for impaired glucose tolerance (IGT) are an FPG of less than 126 mg/dL (7 mmol/L), if measured, and a venous plasma glucose of 140 mg/dL to just below 200 mg/dL (≥7.8 to <11.1mmol/L) 2 hours after a 75-g glucose load with one intervening plasma glucose value at or above 200 mg/dL.  The WHO notes that IGT is not a clinical entity but a risk factor for future diabetes and/or adverse outcomes and that the risk of future diabetes, premature death, and cardiovascular disease begins to increase at 2-hour plasma glucose levels below the IGT range.
These criteria are a better predictor of increased macrovascular risk than the ADA's current intermediate category of IFG or prediabetes. Presumably, patients with IFG are at increased risk for development of diabetes mellitus, but their risk for macrovascular disease does not appear to be the same as for patients with IGT (which is about the same as for patients with frank type 2 diabetes mellitus).
Glycated Hemoglobin Studies
Binding of glucose to hemoglobin A is a nonenzymatic process that occurs over the lifespan of a red blood cell, which averages 120 days. Measurement of glycated hemoglobin thus reflects plasma glucose levels over the preceding 2-3 months.
HbA1c measurements are the criterion standard for monitoring long-term glycemic control. In the past, HbA1c measurements were not considered useful for the diagnosis of diabetes mellitus, because of a lack of international standardization and insensitivity for the detection of milder forms of glucose intolerance.
In a 2009 report, however, an international expert committee appointed by the ADA, the European Association for the Study of Diabetes, and the International Diabetes Association recommended the HbA1c assay for diagnosing type 1 and type 2 diabetes mellitus.  The committee noted the improvement in standardization and cited the following advantages of HbA1c testing over glucose measurement:
Captures long-term glucose exposure
Has less biologic variability
Does not require fasting or timed samples
Is currently used to guide management decisions
Consequently, since 2010 the ADA has included an HbA1c level of 6.5% or higher as a criterion for diabetes diagnosis, with confirmation from repeat testing (unless clinical symptoms are present and the glucose level is >200 mg/dL). A target HbA1c level of less than 8% is supported for older patients (>60 y). levels below 6% are associated with increased mortality.  HbA1c testing cannot be used in patients with abnormal red cell turnover (eg, hemolytic or iron-deficiency anemia). 
The American Association of Clinical Endocrinologists recommends that HbA1C be considered an additional, optional diagnostic criterion, rather than the primary criterion for diagnosis of diabetes.  Using HbA1c alone in initial diabetes screening identifies approximately 20% fewer cases of diabetes than diagnosis based on fasting and 2-hour postload plasma glucose levels. 
Glucose measurement should remain the choice for diagnosing pregnant women or if HbA1c assay is unavailable. In addition, a study by Nowicka et al stated that used on its own, HbA1c is not effective in detecting prediabetes and diabetes in obese children and adolescents. 
However, a study by Vijayakumar et al suggested, in contrast, that the evaluation of HbA1c levels is as effective as FPG and 2-hour postload plasma glucose tests in predicting the development of type 2 diabetes in children and adolescents. The study determined that among the report's subjects, the incidence of diabetes at follow-up was fourfold higher in male children and adolescents belonging to the highest HbA1c category (5.7-6.4%) at baseline than in those in the lowest category (5.3% or lower), while the incidence of diabetes in female children and adolescents in the highest category was sevenfold greater than in those belonging to the lowest category. [109, 110]
Lu et al found evidence that a screening HbA1c of 5.5% or below predicts the absence of type 2 diabetes, while an HbA1c of 7% or greater predicts its presence, and levels of 6.5-6.9% indicate a high probability that diabetes is present. The investigators derived these cutoffs from a clinical group of 2494 patients, 34.6% of whom had undiagnosed diabetes, and then evaluated the cutoffs in a population-based sample of 6015 patients, 4.6% of whom had undiagnosed diabetes.
In the population-based group, HbA1c at 5.5% had a sensitivity of 83.5%, while HbA1c at 7% had a specificity of 100%. In both groups, many (61.9-69.3%) individuals with HbA1c levels of 5.6-6.9% had an abnormal glucose status. 
A community-based study by Lerner et al of 10,201 patients not previously diagnosed with diabetes found that patients whose baseline HbA1c levels were at 5.5% or higher but less than 6.5% had an increased risk of developing diabetes over the subsequent 5-8 years. Moreover, the risk of developing diabetes doubled with every increase of 0.5% in HbA1c level. The risk of diabetes was not associated with age or low socioeconomic status in this study. [112, 113]
An analysis of 15,780 patients from the INTERHEART study determined that HbA1c levels were more suggestive of myocardial infarction risk than were self-reported diabetes status or many other established risk factors.  Each 1% increase independently predicts a 19% increase in odds of experiencing a myocardial infarction after accounting for other risk factors, including diabetes.
HbA1c cannot be used as an indicator of glycemic control in patients with neonatal diabetes mellitus because of the high levels of fetal hemoglobin (HbF) remaining in the blood. A study by Suzuki et al found that glycated albumin, which is not affected by HbF, more strongly correlated with 1-month average postprandial blood glucose and was therefore a better marker of diabetes in neonates. This finding is important to neonatologist and those caring for newborns. 
A study by Wilson et al (in patients with type 1 diabetes) found persistent individual variations in the rate at which individuals glycate hemoglobin. These variations may contribute to inaccuracy in estimating mean glucose concentration from the HbA1c level. 
Hemoglobin A1c versus total glycated hemoglobin
Glycated hemoglobin can be measured as HbA1c, HbA1 (HbA1a, b, and c), or total glycated hemoglobin (GHb). HbA1c constitutes approximately 80% of GHb.
Whether HbA1c or GHb assays are superior for measuring glycemic control is debatable. Hemoglobinopathies can affect both measurements. Because the Diabetes Control and Complications Trial (DCCT) and the United Kingdom Prospective Diabetes Study (UKPDS), as well as the ADA Standards of Care, use HbA1c measurements, this article refers to HbA1c as the standard for glycemic control.
Using GHb measurements is acceptable, but these values are 1-2% higher than HbA1c concentrations. When using GHb, a conversion factor to HbA1c for the assay utilized is helpful.
International Federation of Clinical Chemistry standardization
The International Federation of Clinical Chemistry (IFCC) has developed a reference method for measurement of HbA1c. The IFCC method produces values lower than the HbA1c methods in current clinical use; conversion requires a complex formula. Although more accurate, the IFCC method is complicated, time consuming, and expensive. Consequently, there are no immediate plans for its adoption into clinical practice.
Urinary Albumin Studies
Annual screening for microalbuminuria (see Microalbumin) is recommended in all patients with diabetes. Measuring the albumin-to-creatinine ratio in a spot urine sample is probably the easiest method; the ratio, expressed in mg/g, is equivalent to albumin excretion in milligrams daily. A result greater than 30 mg/g indicates albuminuria, in which case a quantitation on a timed urine specimen (ie, overnight, 10 h, or 24 h) should be performed.
Normal urine albumin excretion is defined as less than 30 mg daily. Microalbuminuria is defined as 30-300 mg daily (20-200 mcg/min). Because of wide variability among patients, microalbuminuria should be found on at least 2 of 3 samples over 3-6 months. Higher values can be detected by standard protein dipstick screening and are considered macroproteinuria.
Unlike type 1 diabetes mellitus, in which microalbuminuria is a good indicator of early kidney damage, microalbuminuria is a common finding (even at diagnosis) in type 2 diabetes mellitus and is a risk factor for macrovascular (especially coronary heart) disease. It is a weaker predictor of future kidney disease in type 2 diabetes mellitus.
Diabetes Testing in Asymptomatic Patients
The U.S. Preventive Services Task Force recommends screening for type 2 diabetes in asymptomatic adults with sustained blood pressure (either treated or untreated) greater than 135/80 mm Hg (grade B recommendation). 
The ADA recommends considering testing for prediabetes and diabetes in asymptomatic adults who are overweight (body mass index [BMI] ≥25 kg/m2; may be lower in at-risk ethnic groups) and have 1 or more of the following additional risk factors  :
First-degree relative with diabetes
Member of a high-risk ethnic population (eg, African American, Latino, Native American, Asian American, Pacific Islander)
Delivered a baby weighing over 9 lb or diagnosed with gestational diabetes mellitus
Hypertension (≥140/90 mm Hg or on therapy for hypertension)
Polycystic ovary disease
IGT or IFG on previous testing
Other clinical conditions associated with insulin resistance (eg, severe obesity, acanthosis nigricans)
History of cardiovascular disease
In the absence of the above criteria, the ADA recommends testing for prediabetes and diabetes beginning at age 45 years. If results are normal, testing should be repeated at least every 3 years. More frequent testing may be considered, depending on initial results and risk status.
Tests to Differentiate Type 2 and Type 1 Diabetes
Measuring concentrations of insulin or C-peptide (a fragment of proinsulin that serves as a marker for insulin secretion) rarely is necessary to diagnose type 2 diabetes mellitus or differentiate type 2 diabetes from type 1 diabetes mellitus. Insulin levels generally are high early in the course of type 2 diabetes mellitus and gradually wane over time.
A fasting C-peptide level of more than 1 ng/dL in a patient who has had diabetes for more than 1-2 years is suggestive of type 2 diabetes (ie, residual beta-cell function). Stimulated C-peptide concentrations (after a standard meal challenge such as Sustacal or after glucagon) are somewhat preserved until late in the course of type 2 diabetes mellitus. Absence of a C-peptide response to carbohydrate ingestion may indicate total beta-cell failure.
Latent autoimmune diabetes of adults (LADA) is a form of slow-onset type 1 diabetes that occurs in middle-aged (usually white) adults. It can be differentiated from type 2 diabetes by confirming the presence of antibodies against the 65-kd isoform of glutamic acid decarboxylase (GAD65), an enzyme found in pancreatic beta cells. Such patients may respond to insulin secretagogues for a brief period (months).
Autoantibodies can be useful in differentiating between type 1 and type 2 diabetes. Islet-cell (IA2), anti-GAD65, and anti-insulin autoantibodies can be present in early type 1 diabetes, but not in type 2 disease. IA2 autoantibodies titers typically decrease after 6 months. Anti-GAD65 antibodies can be present at diagnosis of type 1 diabetes and are more likely to be persistently positive over time.
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- Approach Considerations
- Pharmacologic Therapy
- Management of Glycemia
- Dietary Modifications
- Activity Modifications
- Bariatric Surgery
- Laboratory Monitoring
- Monitoring for Diabetic Complications
- Management of Hypertension
- Management of Dyslipidemia
- Management of Coronary Heart Disease
- Management of Ophthalmologic Complications
- Management of Diabetic Neuropathy
- Management of Infections
- Management of Intercurrent Medical Illness
- Management of Critical Illness
- Pharmacologic Considerations in Surgery
- Prevention of Type 2 Diabetes Mellitus
- Stroke Prevention in Diabetes
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- Medication Summary
- Antidiabetics, Biguanides
- Antidiabetics, Sulfonylureas
- Antidiabetics, Meglitinide Derivatives
- Antidiabetics, Alpha-Glucosidase Inhibitors
- Antidiabetics, Thiazolidinediones
- Antidiabetics, Glucagonlike Peptide-1 Agonists
- Antidiabetics, Dipeptidyl Peptidase IV Inhibitors
- Antidiabetics, Amylinomimetics
- Selective Sodium-Glucose Transporter-2 Inhibitors
- Bile Acid Sequestrants
- Antidiabetics, Rapid-Acting Insulins
- Antidiabetics, Short-Acting Insulins
- Antidiabetics, Intermediate-Acting Insulins
- Antidiabetics, Long-Acting Insulins
- Antiparkinson Agents, Dopamine Agonists
- Antidiabetics, Glucagon-like Peptide-1 Agonists
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