What are the recommended screening labs for diabetes?

Screening Labs for Diabetes

Primary Screening Test

Fasting plasma glucose (FPG) is the recommended screening test for diabetes because it is easier and faster to perform, more convenient and acceptable to patients, less expensive, and more reproducible than other screening tests. 1, 2

Available Screening Tests

Three tests can be used for diabetes screening, each with distinct advantages and limitations:

Fasting Plasma Glucose (FPG) - Preferred Test

• Diagnostic threshold: ≥126 mg/dL (≥7.0 mmol/L) 1
• Superior reproducibility with less intraindividual variation compared to oral glucose tolerance testing 1
• Similar predictive value for microvascular complications as the 2-hour post-load test 1
• Must be performed after at least 8 hours of fasting 1

Hemoglobin A1c (HbA1c)

• Diagnostic threshold: ≥6.5% (≥48 mmol/mol) 1
• Can be used for screening alongside FPG per updated guidelines 1, 2
• Less sensitive for detecting lower levels of hyperglycemia at usual cut-points 1
• Must be performed in an NGSP-certified laboratory standardized to the DCCT assay; point-of-care assays should not be used for diagnosis 1

2-Hour Oral Glucose Tolerance Test (OGTT)

• Diagnostic threshold: ≥200 mg/dL (≥11.1 mmol/L) 1
• May identify more individuals as diabetic compared to FPG 1
• Major disadvantages: more expensive, time-consuming, less convenient, and highest intraindividual variability 2
• Should be reserved for cases where FPG is normal but clinical suspicion remains high 1

Who Should Be Screened

Primary Screening Criteria

• Adults aged 40-70 years who are overweight or obese (BMI ≥25 kg/m²) 2
• All adults beginning at age 45 years 1, 2
• Repeat screening every 3 years if results are normal 1

High-Risk Individuals Requiring Earlier or More Frequent Screening

• Physical inactivity 2
• First-degree relative with diabetes 2
• High-risk ethnicity (American Indians, African Americans, Hispanics, Asian Americans) 1
• History of gestational diabetes or delivering a baby >9 lb 2
• Hypertension (blood pressure ≥140/90 mmHg or on antihypertensive therapy) 1, 2
• HDL cholesterol <35 mg/dL or triglycerides >250 mg/dL 2
• Previous HbA1c ≥5.7%, impaired glucose tolerance, or impaired fasting glucose 2

Special Populations

• Patients with hypertension or hyperlipidemia should be screened as part of an integrated cardiovascular risk reduction approach 1
• Screening in these patients is particularly important because lower blood pressure targets (diastolic <80 mmHg) and lower LDL cholesterol targets benefit patients with diabetes 1

Critical Technical Requirements for Accurate Testing

Sample Collection and Handling

• Use tubes containing rapidly effective glycolytic inhibitors such as granulated citrate buffer 1, 3
• If proper tubes unavailable, immediately place sample in ice-water slurry and centrifuge within 15-30 minutes 1, 3
• Do not rely on tubes with only enolase inhibitors (sodium fluoride) to prevent glycolysis 1
• Measure glucose in venous plasma, not capillary blood or serum 1, 3

Laboratory Standards

• Testing must be performed in an accredited laboratory using enzymatic methods 1, 3
• Analytical performance should meet: imprecision ≤2.4%, bias ≤2.1%, total error ≤6.1% 1, 3

Confirmation of Diagnosis

Any positive screening result must be confirmed with repeat testing on a separate day 1, 2

Exceptions to Repeat Testing Requirement

• If two different tests (e.g., HbA1c and FPG) are both above diagnostic thresholds, diagnosis is confirmed without additional testing 2
• Random plasma glucose ≥200 mg/dL with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) confirms diabetes without repeat testing 1, 4

Common Pitfalls to Avoid

Testing Errors

• Using random capillary blood glucose for screening is less well standardized and should be avoided 1
• Delayed sample processing causes glycolysis and falsely low glucose values 1, 3
• Point-of-care glucose meters lack sufficient precision for diagnostic purposes 1

Clinical Considerations

• Be alert to symptoms suggestive of diabetes (polydipsia, polyuria) and test anyone with these symptoms regardless of screening criteria 1
• Certain medications (glucocorticoids, nicotinic acid) can produce hyperglycemia and should be considered when interpreting results 1
• Community screening outside healthcare settings is not recommended due to lack of evidence for benefit and potential for harm 1

Interpretation Issues

• Intraindividual variation in FPG can be 4.8-7.1%, which should be considered when results are near diagnostic thresholds 3
• Using HbA1c alone in initial screening identifies fewer cases than FPG, particularly in populations with high diabetes prevalence 5
• Neither FPG nor HbA1c alone is effective for screening impaired glucose tolerance; OGTT is required for high-risk individuals 6, 7

Prediabetes Identification

Impaired Fasting Glucose (IFG)

• FPG 100-125 mg/dL (5.6-6.9 mmol/L) 1, 2

Impaired Glucose Tolerance (IGT)

• 2-hour OGTT result 140-199 mg/dL (7.8-11.0 mmol/L) 1

Prediabetes by HbA1c

• HbA1c 5.7-6.4% (39-47 mmol/mol) 1, 2

Individuals with prediabetes should receive intensive lifestyle modification interventions (diet, exercise, behavior modification), as large trials demonstrate these programs significantly reduce progression to diabetes 1