Recommended Venous Lab Tests for Diabetes Diagnosis
For diagnosing diabetes, the recommended venous lab tests include fasting plasma glucose (FPG), hemoglobin A1c (HbA1c), and in some cases, a 2-hour oral glucose tolerance test (OGTT). 1
Primary Diagnostic Tests
Fasting Plasma Glucose (FPG): Should be measured in venous plasma with a value ≥7.0 mmol/L (≥126 mg/dL) diagnostic of diabetes. Blood should be drawn after fasting overnight for at least 8 hours. 1
Hemoglobin A1c (HbA1c): Laboratory-based HbA1c testing can diagnose diabetes with a value ≥6.5% (≥48 mmol/mol). The test should be performed using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) reference assay. 1
2-hour Oral Glucose Tolerance Test (OGTT): A 2-hour plasma glucose ≥11.1 mmol/L (≥200 mg/dL) during an OGTT is diagnostic of diabetes. The test should be performed using a glucose load containing the equivalent of 75g anhydrous glucose dissolved in water. 1
Test Selection and Considerations
The FPG is the preferred screening test in clinical settings because it is easier and faster to perform, more convenient, more acceptable to patients, and less expensive than OGTT. 1
HbA1c offers advantages including greater convenience (fasting not required), greater preanalytical stability, and less day-to-day perturbations during stress and illness. 1
To minimize glycolysis in FPG samples, a tube containing a rapidly effective glycolytic inhibitor such as granulated citrate buffer should be used. If this cannot be achieved, the sample tube should immediately be placed in an ice-water slurry and subjected to centrifugation within 15-30 minutes. 1
Point-of-care HbA1c testing for diabetes screening and diagnosis should be restricted to FDA-approved devices at CLIA-certified laboratories that perform testing of moderate complexity or higher. 1
Diagnostic Criteria and Confirmation
In the absence of unequivocal hyperglycemia, results should be confirmed by repeat testing. 1, 2
Prediabetes is defined as:
- FPG of 100-125 mg/dL (5.6-6.9 mmol/L) and/or
- 2-hour plasma glucose of 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT and/or
- HbA1c of 5.7-6.4% (39-46 mmol/mol) 1
Special Considerations
In conditions associated with increased red cell turnover (pregnancy, recent blood loss, transfusion, erythropoietin therapy, or hemolysis), only blood glucose criteria should be used to diagnose diabetes. 1, 2
For patients with hemoglobinopathies or certain anemias, an HbA1c assay without interference from abnormal hemoglobins should be used, or rely only on glucose criteria. 1, 2
Screening should be considered in adults who are overweight or obese (BMI ≥25 kg/m² or ≥23 kg/m² in Asian Americans) and who have one or more additional risk factors for diabetes. 1
Testing Frequency
If tests are normal in high-risk individuals, repeat testing should be carried out at a minimum of 3-year intervals. 1
For individuals with prediabetes, annual monitoring is recommended. 1
Common Pitfalls to Avoid
Relying solely on urine glucose testing is not recommended for routine care of patients with diabetes mellitus. 1
Using tubes with only enolase inhibitors such as sodium fluoride should not be relied on to prevent glycolysis in FPG samples. 1
Failing to confirm abnormal results with a repeat test can lead to misdiagnosis. 1, 2
Not considering factors that may affect HbA1c results, such as hemoglobinopathies, anemia, or conditions with increased red cell turnover. 1, 2