International Expert Committee Report on the Role of the A1C Assay in the Diagnosis of Diabetes

CAN THE A1C TEST BE USED TO DIAGNOSE DIABETES ?

Glycated hemoglobin (HbA1c) is a form of Hb used primarily to identify the average plasma glucose concentration over prolonged periods of time. It is formed in a non-enzymatic pathway by hemoglobin's normal exposure to high plasma levels of glucose.
In the normal 120-day life span of the red blood cell, glucose molecules react with hemoglobin, forming glycated hemoglobin. In individuals with poorly controlled DM, the quantities of these glycated hemoglobins are much higher than in healthy people.
Once a hemoglobin molecule is glycated, it remains that way. A buildup of glycated hemoglobin within the red cell therefore reflects the average level of glucose to which the cell has been exposed during its life cycle.
An updated examination of the laboratory measurements of glucose and A1C by the current International Expert Committee indicates that with advances in instrumentation and standardization, the accuracy and precision of A1C assays at least match those of glucose assays. The measurement of glucose itself is less accurate and precise than most clinicians realize. A recent analysis of the performance of a variety of clinical laboratory instruments and methods that measure glucose revealed that 41% of instruments have a significant bias from the reference method that would result in potential misclassification of _12% of patients. There are also potential preanalytic errors owing to sample handling and the well-recognized lability of glucose in the collection tube at room temperature. Even when whole blood samples are collected in sodium fluoride to inhibit in vitro glycolysis, storage at room temperature for as little as 1 to 4 h before analysis may result in decreases in glucose levels by 3–10 mg/dl in nondiabetic individuals.
By contrast, A1C values are relatively stable after collection, and the recent introduction of a new reference method to calibrate all A1C assay instruments should further improve A1C assay standardization in most of the world. In addition, between- and within-subject coefficients of variation have been shown to be substantially lower for A1C than for glucose measurements. The variability of A1C values is also considerably less than that of FPG levels, with day-to-day within-person variance of _2% for A1C but 12–15% for FPG.


WHAT IS THE MOST APPROPRIATE A1C CUT POINT FOR THE DIAGNOSIS OF DIABETES ?

A recent analysis derived from DETECT-2 and including the 3 that were included in the 1997 report examined the association between A1C and retinopathy, objectively assessed and graded by fundus photography This analysis included _28,000 subjects from nine countries and showed that the glycemic level at which the prevalence of “any” retinopathy begins to rise above background levels (any retinopathy
includes minor changes that can be due to other conditions, such as hypertension), and for the more diabetes-specific “moderate” retinopathy, was 6.5% when

In summary, the large volume of data from diverse populations has now established an A1C level associated with an increase in the prevalence of moderate retinopathy and provides strong justification for assigning an A1C cut point of 6.5% for the diagnosis of diabetes. A recently published population-based study of 3,190 adults of Malay ethnicity independently concluded that A1C levels “in the range 6.6 to 7% were optimal for
detecting microvascular complications”

Destaco que realmente la A1c no reemplaza las glicemias y me parece que aún hay pocos estudios como los que se mencionan en el artículo. Muy importante es que a partir de 6.5% ya debiera considerarse anormal, según el estudio más grande con retinopatía...
Me parece que lo más valioso NO es el diagnóstico, eso ya está más que claro, lo más importante son el enorme grupo de pacientes en alto riesgo o pre-diabéticos...

Fuente: DIABETES CARE, VOLUME 32, NUMBER 7, JULY 2009