Screening and Diagnosis of Diabetes Mellitus Using Glucose Testing

PATHCHAT Edition No. 46
January 2018
Please contact your local Ampath pathologist for more information.

Author: Dr. Marita du Plessis

Epidemiology of Diabetes

🔹 Diabetes is a major global health burden:

• 8.8% of the world's population (415 million people) were affected in 2015 (International Diabetes Federation).
• The incidence is rising in rural Africa, making diabetes a major non-communicable disease in Sub-Saharan Africa.

🔹 South African Statistics:

• South African National Health and Nutrition Examination Survey (SANHANES 2012) found:
  • 9.5% of people aged ≥15 years had diabetes based on HbA1c.
  • 45% of cases were previously undiagnosed.
  • Highest prevalence was in Asian Indian and Coloured populations.

📌 The increasing prevalence of obesity is a key modifiable risk factor for diabetes.

Screening & Diagnosis of Diabetes and Pre-Diabetes

✅ Diabetes diagnosis can be made using:

1. Fasting Plasma Glucose (FPG).
2. 2-hour Plasma Glucose (OGTT).
3. HbA1c levels.

🔹 Diagnostic Criteria for Diabetes & Pre-Diabetes:

✔ Normal:

• Fasting Plasma Glucose (FPG) < 6.1 mmol/L.
• 2-hour OGTT < 7.8 mmol/L.

✔ Pre-Diabetes (Intermediate Hyperglycaemia):

• Impaired Fasting Glucose (IFG):
  • FPG 6.1 – 6.9 mmol/L.
  • 2-hour OGTT < 7.8 mmol/L.
• Impaired Glucose Tolerance (IGT):
  • FPG < 7.0 mmol/L.
  • 2-hour OGTT 7.8 – 11.0 mmol/L.

✔ Diabetes:

• FPG ≥ 7.0 mmol/L and/or
• 2-hour OGTT ≥ 11.1 mmol/L and/or
• HbA1c ≥ 6.5%.

📌 No one test is preferred over another, but the OGTT detects more cases than FPG or HbA1c alone.

Key Diagnostic Considerations

✅ In symptomatic patients (polyuria, polydipsia, weight loss, blurred vision):

• A single abnormal test (including a random glucose > 11.1 mmol/L) confirms the diagnosis.

✅ In asymptomatic patients:

• Diagnosis should be confirmed by repeat testing of the same test on a separate day within 2 weeks.
• If HbA1c and a glucose-based test both meet diagnostic criteria, the diagnosis is confirmed without repeat testing.

✅ If the repeat test does not confirm diabetes:

• Initiate lifestyle interventions and re-test in 3–6 months.

✅ Hyperglycaemia detected in acute illness (infection, trauma, corticosteroid use) may be transient.

• Confirm diabetes diagnosis after resolution of the acute condition.

📌 HbA1c should be measured using NGSP-certified methods with strict quality control. Conditions that affect red blood cell turnover (e.g., haemolysis, iron deficiency, recent transfusion, pregnancy) may impact HbA1c accuracy.

Screening for Type 2 Diabetes

✅ Who Should Be Screened?

• All overweight adults (BMI >25 kg/m², or >23 kg/m² in Asian Indians) with one or more risk factors:
  • Physical inactivity.
  • Hypertension.
  • First-degree relative with diabetes.
  • Dyslipidaemia (low HDL, high triglycerides).
  • Polycystic ovarian syndrome (PCOS).
  • Previous gestational diabetes or baby >4 kg birth weight.
  • History of cardiovascular disease.
  • Previous impaired fasting glucose (IFG) or impaired glucose tolerance (IGT).
  • Other conditions associated with insulin resistance.

✅ When to Start Screening?

• At 45 years of age in the absence of risk factors.

✅ How Often to Rescreen?

• Depends on individual risk factors:
  • Every 3 months for high-risk patients (e.g., obese with IGT & multiple risk factors).
  • Every 3 years for low-risk patients (normal weight, no risk factors).

📌 OGTT is the most sensitive test and the only method to diagnose IGT.

Effect of Diurnal Variation on Glucose

🔹 Fasting glucose levels fluctuate due to circadian rhythms.

• Glucose & insulin levels are highest in the early morning, decreasing after 09:00.
• Afternoon glucose testing may miss diabetes diagnoses.

✅ Key Considerations for Accurate Glucose Testing:

• Fasting glucose should be collected before 09:00.
• Fasting duration should be 8–12 hours.
• OGTT should not be started after 09:00.

📌 Afternoon glucose levels may be 1.7–2.8 mmol/L higher than morning levels.

Factors Affecting OGTT Results

🔹 Medications That Affect Glucose Levels:
✅ Drugs That Elevate Blood Glucose:

• Corticosteroids.
• Thiazide diuretics.
• Beta-blockers.
• Nicotinic acid.
• Isoniazid.
• Theophylline.

✅ Drugs That Lower Blood Glucose:

• Aspirin.
• Biguanides (Metformin).
• Sulphonylureas.
• Ethanol.

✅ Drugs With Variable Effects:

• Caffeine.
• Lithium.

📌 Carbohydrate intake <150 g/day for several days before an OGTT may cause an abnormal result. Patients should follow normal dietary intake before testing.

Conclusion

✅ Diabetes prevalence is rising, making early screening & diagnosis crucial.
✅ HbA1c and glucose-based tests are both valid diagnostic methods.
✅ Fasting glucose samples should be collected before 09:00 due to diurnal variation.
✅ OGTT remains the most sensitive test, especially in high-risk patients.
✅ Correct specimen collection and interpretation are essential for accurate diagnosis.

📌 Accurate identification of pre-diabetes allows for early intervention to prevent progression to diabetes.