A1C Versus Glucose Testing: A Comparison | Diabetes Care | American Diabetes Association

Diabetes was originally identified by the presence of glucose in the urine. Almost 2,500 years ago it was noticed that ants were attracted to the urine of some individuals. In the 18th and 19th centuries the sweetness taste of urine was used for diagnosis ahead chemical methods became available to detect sugars in the urine. Tests to measure glucose in the rake were developed over 100 years ago, and hyperglycemia subsequently became the lone criterion recommended for the diagnosis of diabetes. Initial diagnostic criteria relied on the reaction to an oral glucose challenge, while later measurement of blood glucose in an individual who was fasting besides became acceptable. The most widely accepted glucose-based criteria for diagnosis are fasting plasma glucose ( FPG ) ≥126 mg/dL or a 2-h plasma glucose ≥200 mg/dL during an oral glucose tolerance test ( OGTT ) on more than one occasion ( 1, 2 ). In a affected role with classical symptoms of diabetes, a single random plasma glucose ≥200 mg/dL is considered diagnostic ( 1 ). Before 2010 virtually all diabetes societies recommended blood glucose analysis as the single method acting to diagnose diabetes. Notwithstanding these guidelines, over the last few years many physicians have been using hemoglobin A1C to screen for and diagnose diabetes ( 3 ). Although considered the “ gold standard ” for diagnosis, measurement of glucose in the blood is subject to several limitations, many of which are not widely appreciated. Measurement of A1C for diagnosis is appealing but has some implicit in limitations. These issues have become the focus of considerable attention with the holocene publication of the Report of the International Expert Committee that recommended the use of A1C for diagnosis of diabetes ( 4 ), a put that has been endorsed ( at the time of writing ) by the American Diabetes Association ( ADA ) ( 1 ), the Endocrine Society, and in a more limited fashion by American Association of Clinical Endocrinologists/American College of Endocrinology ( 5 ). This review will provide an overview of the factors that influence glucose and A1C test. Before addressing glucose and A1C, it is crucial to consider the factors that impact the results of any lineage test. While testing ground medicine journals have devoted some discussion to the sources of unevenness in results of blood tests, this topic has received small attention in the clinical literature. Factors that lend to variation can handily be divided into three categories, namely biological, preanalytical, and analytic. biological variation comprises both differences within a individual person ( termed intraindividual ) and between two or more people ( termed interindividual ). Preanalytical issues pertain to the specimen before it is measured. analytic differences result from the measurement operation itself. The influence of these factors on both glucose and A1C results will be addressed in more detail below. measurement of glucose in plasma of fasting subjects is widely accepted as a diagnostic standard for diabetes ( 1, 2 ). Advantages include cheap assays on automatize instruments that are available in most laboratories worldwide ( table 1 ). Nevertheless, FPG is subject to some limitations. One report that analyzed repeated measurements from 685 fasting participants without diagnosed diabetes from the Third National Health and Nutrition Examination Survey ( NHANES III ) revealed that lone 70.4 % of people with FPG ≥126 mg/dL on the first test had FPG ≥126 mg/dL when analysis was repeated ∼2 weeks late ( 6 ). numerous factors may contribute to this lack of reproducibility. These are elaborated below. Fasting glucose concentrations vary well both in a individual person from day to day and besides between different subjects. Intraindividual variation in a healthy person is reported to be 5.7–8.3 %, whereas interindividual magnetic declination of up to 12.5 % has been observed ( 6, 7 ). Based on a CV ( coefficient of variation ) of 5.7 %, FPG can range from 112–140 mg/dL in an individual with an FPG of 126 mg/dL. ( It is significant to realize that these values encompass the 95 % confidence interval, and 5 % of values will be outside this range. )

numerous factors that occur before a sample is measured can influence results of blood tests. Examples include medications, venous stasis, position, and sample distribution handling. The concentration of glucose in the blood can be altered by food consumption, prolonged fast, or drill ( 8 ). It is besides important that measurements are performed in subjects in the absence of intercurrent illness, which frequently produces ephemeral hyperglycemia ( 9 ). similarly, acute try ( for example, not being able to find parking or having to wait ) can alter blood glucose concentrations.

Samples for fasting glucose analysis should be drawn after an overnight fast ( no thermal consumption for at least 8 planck’s constant ), during which clock the topic may consume water ad lib ( 10 ). The necessity that the capable be fast is a considerable practical trouble as patients are normally not fasting when they visit the sophisticate, and it is frequently inconvenient to return for venesection. For exemplar, at an HMO affiliated with an academician medical center, 69 % ( 5,752 of 8,286 ) of eligible participants were screened for diabetes ( 11 ). however, FPG was performed on only 3 % ( 152 ) of these individuals. ninety-five percentage ( 5,452 ) of participants were screened by random plasma glucose measurements, a technique not reproducible with ADA recommendations. In addition, rake draw in the good morning as FPG has a diurnal variation. analysis of 12,882 participants aged 20 years or older in NHANES III who had no previously diagnosed diabetes revealed that mean FPG in the dawn was well higher than in the good afternoon ( 12 ). prevalence of diabetes ( FPG ≥126 mg/dL ) in afternoon-examined patients was half that of participants examined in the dawn. other patient-related factors that can influence the results include food consumption when supposed to be fasting and hypocaloric diet for a workweek or more prior to testing.

Glucose concentrations decrease in the test tube by 5–7 % per hour due to glycolysis ( 13 ). therefore, a sample distribution with a on-key blood glucose measure of 126 mg/dL would have a glucose concentration of ∼110 mg/dL after 2 planck’s constant at board temperature. Samples with increased concentrations of erythrocytes, white blood cells, or platelets have even greater rates of glycolysis. A coarse misconception is that sodium fluoride, an inhibitor of glycolysis, prevents glucose consumption. While fluoride does attenuate in vitro glycolysis, it has no effect on the rate of decay in glucose concentrations in the first 1 to 2 h after blood is collected, and glycolysis continues for up to 4 planck’s constant in samples containing fluoride ( 14 ). The delay in the glucose stabilizing impression of fluoride is most likely the solution of glucose metamorphosis proximal to the fluoride target enolase ( 15 ). After 4 hydrogen, fluoride maintains a stable glucose concentration for 72 h at board temperature ( 14 ). A holocene issue showed that acidification of the blood sample inhibits glycolysis in the beginning 2 henry after venesection ( 16 ), but the collection tubes used in that study are not commercially available. Placing tubes in ice rink urine immediately after collection may be the best method to stabilize glucose initially ( 2, 16 ), but this is not a practical solution in most clinical situations. Separating cells from plasma within minutes is besides effective, but impractical .
The nature of the specimen analyzed can have a big influence on the glucose concentration. Glucose can be measured in unharmed blood, serum, or plasma, but plasma is recommended by both the ADA and World Health Organization ( WHO ) for diagnosis ( 1, 2 ). however, many laboratories measure glucose in serum, and these values may differ from those in plasma. There is a miss of consensus in the publish literature, with glucose concentrations in plasma reported to be lower than ( 17 ), higher than ( 16, 18, 19 ), or the same as ( 20 ) those in serum. importantly, glucose concentrations in solid blood are 11 % lower than those in plasma because erythrocytes have a lower water content than plasma ( 13 ). The magnitude of the difference in glucose between wholly blood and plasma changes with hematocrit. Most devices ( normally handheld meters ) that bill glucose in capillary blood use hale blood. While the majority of these report a plasma equivalent glucose value ( 21 ), this leave is not accurate in patients with anemia ( 22 ) ( unless the meter measures hematocrit ) .
The beginning of the blood is another variable. Although not a substantial problem in the fast state, capillary glucose concentrations can be 20–25 % higher ( mean of 30 mg/dL ) than venous glucose during an OGTT ( 23 ). This find has practical implications for the OGTT, particularly because the WHO deems capillary blood samples satisfactory for the diagnosis of diabetes ( 2 ) .

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