Diagnostic Testing for Diabetes: A1C and/or Fasting Glucose
Either A1C or fasting plasma glucose (FPG) alone is sufficient for initial diabetes screening, but if the result is abnormal (but not in the unequivocal hyperglycemia range), you need confirmatory testing with a second test—which can be a repeat of the same test or a different test performed at the same time or on a different day 1.
Which Tests to Order
Standard Approach: Choose One Initial Test
You do not need both A1C and FPG drawn simultaneously for initial screening. According to the 2025 American Diabetes Association guidelines, diabetes can be diagnosed using either:
- A1C ≥ 6.5%, OR
- Fasting plasma glucose ≥ 126 mg/dL, OR
- 2-hour plasma glucose ≥ 200 mg/dL during a 75-g OGTT, OR
- Random glucose ≥ 200 mg/dL with classic symptoms 1
When Confirmatory Testing is Required
In the absence of unequivocal hyperglycemia (like diabetic ketoacidosis or hyperglycemic crisis), diagnosis requires two abnormal results. These can be:
- Two different tests at the same time (e.g., A1C and FPG both abnormal on the same blood draw), OR
- The same test repeated at two different time points 1
Should Your Doctor Order an OGTT?
An OGTT is generally not necessary for routine diabetes screening unless specific circumstances apply 1. Here's when to consider it:
OGTT is Most Useful When:
- You have conditions that make A1C unreliable: hemoglobin variants, pregnancy (2nd/3rd trimester), G6PD deficiency, HIV, hemodialysis, recent blood loss/transfusion, or erythropoietin therapy 2
- Detecting prediabetes for prevention programs: The OGTT identifies more people with prediabetes than A1C or FPG alone, and diabetes prevention interventions have been proven most effective in people with impaired glucose tolerance (IGT) detected by OGTT 1
- There's significant discordance between A1C and FPG results that needs explanation 2
OGTT is Generally NOT Needed When:
- You're doing routine screening in otherwise healthy adults
- A1C or FPG clearly shows diabetes (≥ 6.5% or ≥ 126 mg/dL respectively)
- You have unequivocal hyperglycemia with symptoms
Important caveat: Research shows the OGTT detects significantly more cases of diabetes than A1C or FPG alone—in one study, 22.5% were diabetic by OGTT vs. only 5.0% by A1C and 2.7% by FPG 3. However, the OGTT is more cumbersome, requires fasting and a 2-hour wait, and has higher biological variability 4. The ADA guidelines prioritize practical screening with A1C or FPG for most patients 1.
Practical Algorithm for Your Doctor
Step 1: Initial Screening
Order either A1C or FPG (your doctor's choice based on convenience and your medical conditions)
Step 2: If Initial Test is Abnormal
- If clearly diabetic (A1C ≥ 6.5% or FPG ≥ 126 mg/dL): Confirm with a second test (can be the same test repeated or a different test)
- If borderline/prediabetic (A1C 5.7-6.4% or FPG 100-125 mg/dL): Consider risk factors and whether OGTT would change management
Step 3: Consider OGTT Only If:
- You have conditions making A1C unreliable (see list above)
- Results are discordant or borderline and you're considering intensive diabetes prevention interventions
- Your doctor suspects diabetes despite normal A1C/FPG
Sample Script for Your Doctor
"I would like to be screened for diabetes. Based on the 2025 ADA guidelines, please order either a hemoglobin A1C or fasting plasma glucose test. If the result is abnormal, I understand we'll need confirmatory testing. Please also let me know if you think an oral glucose tolerance test would be appropriate based on my specific medical history and risk factors."
Key Points to Prepare for the Test:
- For FPG: Fast for at least 8 hours (no caloric intake)
- For OGTT (if ordered): Consume at least 150g of carbohydrates daily for 3 days before the test; avoid fasting or carbohydrate restriction, as this can falsely elevate results 2
The bottom line: Start with A1C or FPG—not both simultaneously. Add confirmatory testing only when needed. Reserve OGTT for specific clinical situations where it will meaningfully change your management.