This is a great article below which goes through best practices to diagnose Diabetes.
JAMA Diagnostic Test Interpretation
September 12, 2019
Point-of-Care Hemoglobin A1c
JAMA. Published online September 12, 2019. doi:10.1001/jama.2019.14063
Hemoglobin A comprises approximately 97% of total hemoglobin and undergoes glycation with the nonenzymatic attachment of a sugar to its amino groups.
The quantity of HbA1c is directly related to the glucose concentration that erythrocytes are exposed to over their life span, making HbA1c a clinically useful measure of mean glycemia during the preceding 3 months.
1. Type 2 diabetes is defined by an HbA1c value of at least 6.5%
2. prediabetes is defined by an HbA1c value of 5.7% to 6.4%.
Individuals with prediabetes have an increased risk of developing type 2 diabetes, estimated at 5% to 10% annually and 70% in a lifetime.
A. Point-of-care (POC) HbA1c testing may be performed during outpatient visits using a benchtop analyzer and capillary blood samples obtained by fingerstick; done in office to get treatment asap: some MDs do this thought not standard of care yet.
Some authors have proposed acceptable levels of accuracy (ie, within 0.2 percentage points of the true value) and precision (coefficient of variation <3%) for POC HbA1c testing devices.2,3 A 2017 meta-analysis of 13 commercially available POC instruments reported accuracy ranging from −0.9 to 0.7 percentage points from the laboratory value, with 9 devices exhibiting lower values and 4 devices demonstrating higher values.4 Individual studies of the same POC HbA1c device also report wide variability relative to the laboratory measure.4 Minimal evidence is available regarding the analytic performance of POC HbA1c test results obtained in clinical settings.
A 2019 study reported that clinic-collected POC HbA1c test values were a mean of 0.2 percentage points lower than the laboratory test values.5 POC HbA1c testing should not be used to establish a diagnosis of diabetes unless the test is validated as accurate and is subject to regular monitoring of its accuracy in clinical settings.1,6
What Are Alternative Testing Approaches?
Laboratory-based criteria for diagnosing diabetes include HbA1c (≥6.5%), fasting glucose (≥126 mg/dL), and 2-hour glucose following a 75 g oral glucose load (≥200 mg/dL). In the absence of hyperglycemic symptoms, 2 abnormal glycemic results are required to establish a diagnosis of diabetes.1 Importantly, the 3 glycemic tests recommended for diagnosing diabetes may not yield consistent conclusions regarding the presence of diabetes. Because there is not 1 widely accepted criterion standard test, the clinical performance (eg, sensitivity and specificity) of HbA1c is unknown. No large published studies have evaluated the clinical performance of POC HbA1c testing compared with laboratory-based HbA1c testing.
Using laboratory-based HbA1c test results to diagnose diabetes has several advantages over glucose tests. It is more convenient because it does not require fasting, exhibits less intraindividual variation, and is not affected by acute fluctuations in blood glucose concentration.8 However, HbA1c may be affected by conditions with altered red blood cell turnover, iron deficiency anemia, or pregnancy. In addition, black patients may have slightly higher HbA1c values than white patients with the same glucose level. The reason for this difference is unknown.9
Clinical Bottom Line
Laboratory-based HbA1c testing is recommended as 1 of 3 tests (including fasting glucose and 2-hour glucose after a 75 g oral glucose load) for diagnosing diabetes, monitoring glycemic status, and predicting the risk for microvascular complications in patients with known diabetes.
There is wide variability in the performance of POC HbA1c testing devices among individual studies of the same device and across commercially available models, which limits its use for diabetes screening and diagnosis.
POC HbA1c should not be used to establish a diagnosis of diabetes unless the test is regularly validated as accurate.
According to clinical guidelines, POC HbA1c testing may be used to inform timely medical management of previously diagnosed diabetes.
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Corresponding Author: Matthew J. O’Brien, MD, MSc, Northwestern University, Feinberg School of Medicine, 750 N Lake Shore Dr, Sixth Floor, Chicago, IL 60611 (firstname.lastname@example.org).