What is the initial workup for a patient suspected of having diabetes?

Initial Workup for Suspected Diabetes

The initial workup for a patient suspected of having diabetes should include fasting plasma glucose (FPG), hemoglobin A1C, and possibly a 2-hour oral glucose tolerance test (OGTT), with confirmation of abnormal results through repeat testing. 1, 2

Diagnostic Testing Algorithm

Step 1: Initial Laboratory Tests

• Hemoglobin A1C: Preferred initial test (no fasting required)

  • Diagnostic threshold: ≥6.5%
  • Must be performed using NGSP-certified method standardized to DCCT reference assay
  • Not valid in certain conditions (hemoglobinopathies, pregnancy, recent blood loss/transfusion)

• Fasting Plasma Glucose (FPG):

  • Diagnostic threshold: ≥126 mg/dL (7.0 mmol/L)
  • Requires at least 8 hours of fasting
  • Sample should be processed promptly to prevent glycolysis

• Random Plasma Glucose:

  • Diagnostic threshold: ≥200 mg/dL (11.1 mmol/L)
  • Only diagnostic when accompanied by classic symptoms (polyuria, polydipsia, unexplained weight loss)

• 2-hour Oral Glucose Tolerance Test (OGTT):

  • Diagnostic threshold: ≥200 mg/dL (11.1 mmol/L) at 2 hours
  • Uses 75g anhydrous glucose dissolved in water
  • More time-consuming but useful when other tests are borderline

Step 2: Confirmation Testing

• In asymptomatic patients: Repeat abnormal test on a different day to confirm diagnosis 1, 2
• In symptomatic patients with classic hyperglycemia or crisis: Confirmation may not be necessary 1
• When two different tests are available: If both above diagnostic threshold, diagnosis is confirmed
• When test results are discordant: Repeat the test that was above diagnostic threshold 1

Risk Assessment

Identify patients who should be screened for diabetes 1, 2:

• Age ≥45 years
• BMI ≥25 kg/m² (≥23 kg/m² in Asian Americans)
• First-degree relative with diabetes
• High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
• History of gestational diabetes or delivery of baby >9 lb
• Hypertension (≥140/90 mmHg)
• HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
• Physical inactivity
• Conditions associated with insulin resistance (PCOS, acanthosis nigricans)
• History of cardiovascular disease

Additional Workup Components

Once diabetes is diagnosed, the initial workup should include:

1. Classification of diabetes type:

   • Clinical presentation (acute vs. gradual onset)
   • Age at onset
   • Body habitus (obesity more common in type 2)
   • Family history
   • Consider autoantibody testing if type unclear

2. Assessment for complications:

   • Comprehensive eye examination
   • Comprehensive foot examination
   • Screening for albuminuria
   • Lipid profile
   • Blood pressure measurement

3. Evaluation of comorbidities:

   • Cardiovascular risk assessment
   • Screening for depression
   • Thyroid function tests (especially for type 1)

Common Pitfalls to Avoid

• Using point-of-care A1C tests for diagnosis - these lack sufficient accuracy 2
• Relying on a single test without confirmation in asymptomatic individuals 1
• Improper sample handling leading to falsely low glucose values due to glycolysis 2
• Failing to consider conditions that affect A1C interpretation (hemoglobinopathies, pregnancy, etc.) 2
• Not following proper OGTT protocol (requires proper preparation with adequate carbohydrate intake) 2

Prediabetes Assessment

Include screening for prediabetes as part of the workup 1, 2:

• A1C: 5.7-6.4%
• FPG: 100-125 mg/dL (impaired fasting glucose)
• 2-hour OGTT: 140-199 mg/dL (impaired glucose tolerance)

Early identification of prediabetes allows for interventions that can prevent or delay progression to diabetes, significantly reducing morbidity and mortality.