Screenings for diabetes: fasting glucose and A1C ranges explained

A small moment at my kitchen table started this post: I was staring at two numbers from a routine checkup—one called “fasting glucose” and another called “A1C”—and wondering why they didn’t match perfectly. I went down the rabbit hole so you don’t have to. What I learned is simple but reassuring: these tests are like two camera angles on the same story. One shows your blood sugar at a specific fasting moment; the other shows an average over months. Put them together and you get a clearer, steadier picture.

The one-page summary I wish I had first

Here’s the straight-to-the-point version I now keep in my notes. These cutoffs are widely used in U.S. clinical practice (see the ADA Standards of Care 2025):

  • Fasting plasma glucose (FPG): Normal <100 mg/dL (5.6 mmol/L). Prediabetes 100–125 mg/dL (5.6–6.9 mmol/L). Diabetes ≥126 mg/dL (≥7.0 mmol/L) on two separate tests.
  • Hemoglobin A1C: Normal <5.7%. Prediabetes 5.7–6.4%. Diabetes ≥6.5% on two separate tests.
  • Two-hour oral glucose tolerance test (OGTT) (less common for screening but sometimes used): Diabetes at ≥200 mg/dL (≥11.1 mmol/L) at 2 hours.
  • One high random plasma glucose ≥200 mg/dL with symptoms can also indicate diabetes and needs urgent follow-up.

High-value takeaway: A1C captures an average of the last ~3 months; fasting glucose captures one morning after an 8-hour fast. When they disagree a little, context and repeat testing matter more than a single number. If you’re unsure which test fits you, the U.S. Preventive Services Task Force (USPSTF) explains when to screen and how often; their plain-language recommendation is handy here: USPSTF screening guidance.

Why we use two angles on the same story

I used to think A1C must be “the better test.” Not quite. I see them as complementary:

  • FPG is sensitive to the moment. It reflects liver glucose release overnight and your insulin response right now. A poor night’s sleep, a cold, or missed meds can nudge it.
  • A1C is the long game. It reflects the percentage of hemoglobin with sugar attached, averaging roughly 8–12 weeks. Good for big-picture trends, less jittery day to day.
  • Real life is messy. Conditions that change red blood cell turnover (iron deficiency, recent blood loss, certain hemoglobin variants) can distort A1C. When that’s suspected, clinicians may rely more on plasma glucose tests or use alternatives like fructosamine. The NIH’s NIDDK has a clear explainer: A1C test basics.

So if your A1C looks borderline but your fasting number is squarely normal (or vice versa), it’s not a failure of testing; it’s a prompt to zoom out, repeat, and interpret alongside risk factors and symptoms.

Ranges in plain English that I can actually remember

I wrote this down like a fridge note—mg/dL first (the common U.S. unit), with mmol/L in parentheses:

  • Normal FPG: 70–99 mg/dL (3.9–5.5 mmol/L). Many labs round “normal” as <100.
  • Prediabetes FPG: 100–125 mg/dL (5.6–6.9 mmol/L). This is a yellow light, not a life sentence. Lifestyle steps here can make a big difference—CDC has practical guides: CDC prediabetes.
  • Diabetes FPG: ≥126 mg/dL (≥7.0 mmol/L) on two mornings.
  • Normal A1C: <5.7%.
  • Prediabetes A1C: 5.7–6.4%.
  • Diabetes A1C: ≥6.5% on two tests.

Outside of fasting numbers and A1C, there’s also the OGTT (the sweet drink test) where a 2-hour value ≥200 mg/dL signals diabetes; between 140–199 mg/dL suggests impaired glucose tolerance. It’s helpful when fasting and A1C don’t line up, or during pregnancy (with different protocols).

How often to check and who actually needs screening

I used to assume annual checks were mandatory for everyone. The nuance matters:

  • Adults 35–70 with overweight or obesity should be screened, typically every 3 years if results are normal, per the USPSTF (this can start earlier if you have higher risk). See their summary: USPSTF recommendation.
  • Earlier or more frequent screening may be reasonable if you have risk factors: a first-degree relative with diabetes, history of gestational diabetes, polycystic ovary syndrome, higher-risk ethnicity, hypertension, dyslipidemia, or signs of insulin resistance (e.g., acanthosis nigricans). The ADA summarizes risk-based screening: ADA Standards of Care 2025.
  • If you already have prediabetes, annual testing is common to track trends and re-assess.

Pregnancy uses different playbooks (one-step or two-step glucose testing at 24–28 weeks in most cases), so I keep that topic separate when I’m reading—same destination (safer outcomes) but different roads.

Small factors that can nudge your numbers

I used to panic over a single high fasting value. Now I troubleshoot first:

  • True fasting matters. It means ~8 hours with only water. Midnight snacks (even “just a bite”) count. So do sugary drinks, cream in coffee, or alcohol.
  • Short-term stressors—illness, poor sleep, intense exercise late at night—can push fasting glucose up temporarily.
  • Medications like steroids can raise glucose; iron therapy can shift A1C if it corrects anemia. Share your med list before testing.
  • Hemoglobin variants and conditions altering red blood cell lifespan (e.g., CKD, hemolysis) can make A1C read misleadingly low or high; that’s when clinicians may emphasize plasma glucose or alternative markers (fructosamine/glycated albumin).
  • Lab variability is real but usually small when labs use NGSP-certified A1C methods. Repeating borderline results is normal practice.

A simple way I compare and confirm

When my numbers sit near a cutoff, I follow a personal, non-medical checklist before I let worry run away with me:

  • Step 1 Re-read the range with the units in mind (mg/dL vs mmol/L) so I’m not misreading a chart from another country.
  • Step 2 Ask: did I truly fast? Was I fighting a cold? Did I have a rough, short night of sleep?
  • Step 3 Consider my risk context—family history, weight changes, blood pressure, lipids, and physical activity.
  • Step 4 If still uncertain, repeat the test or add the complementary one (A1C if I did fasting; FPG if I did A1C). The ADA explicitly recommends confirming abnormal results on a separate day: ADA Standards of Care 2025.
  • Step 5 If I’m in the “prediabetes” zone, I look for small, realistic habits rather than perfection. The CDC’s National DPP materials are down-to-earth: CDC lifestyle programs.

Little habits I’m testing that don’t feel like punishment

My rule is to pick things I can keep doing on a grumpy Tuesday, not just on a sunny weekend:

  • Walk after meals for even 10–15 minutes. It’s tiny but consistent; my post-dinner scroll turned into a lap around the block.
  • Early-evening dinner if possible. Long gaps before sleep smooth my fasting numbers more reliably than debating “perfect” carbs.
  • Protein anchor at breakfast (eggs, Greek yogurt, tofu, nut butter). It helps me avoid the 10 a.m. snack pinball.
  • Keep a one-page lab log with date, test type, and anything unusual (illness, travel). It makes patterns obvious without fancy apps.

None of these are a guarantee, but collectively they set up nicer trends—especially if your numbers live in the prediabetes range and you want to nudge them in a safer direction.

When I tap the brakes and ask for help

There are moments I don’t self-optimize—I escalate:

  • Symptoms plus a very high random glucose (extreme thirst, frequent urination, unexplained weight loss, blurred vision) are a same-day issue. Random values near or above 200 mg/dL warrant prompt evaluation.
  • Signs of low blood sugar (shakiness, sweating, confusion) in someone using insulin or sulfonylureas need urgent attention.
  • Pregnancy changes the playbook—screening methods and targets differ. Medical team > internet rabbit holes.

For easy-to-read, vetted info, I bookmark MedlinePlus on diabetes and the CDC pages above.

What I’m keeping and what I’m letting go

I’m keeping the idea that consistency beats perfection. I’m keeping a two-angle mindset—A1C for trend, fasting glucose for the snapshot. And I’m letting go of the drama around a single borderline number. The goal is not to pass a lab test; it’s to steer risk in a kinder direction over time.

FAQ

1) Do I need to fast for an A1C test?
Answer: No. A1C can be drawn any time of day without fasting. Fasting is required for the fasting plasma glucose test (about 8 hours with only water).

2) My A1C is 6.0% but my fasting glucose is 97 mg/dL. Which one is right?
Answer: They can both be “right” because they measure different things. A1C reflects the average over ~3 months; fasting glucose is one morning. In this situation, clinicians often confirm with a repeat and may consider an OGTT if needed. See ADA definitions: ADA Standards of Care 2025.

3) How often should I be screened if my results are normal?
Answer: If you’re 35–70 with overweight/obesity and have normal results, every 3 years is typical, per USPSTF. If you have prediabetes or higher risk, annual checks are common. Source: USPSTF.

4) Can anemia or blood disorders affect A1C?
Answer: Yes. Conditions that change red blood cell lifespan (iron deficiency, recent transfusion, some hemoglobin variants) can skew A1C. In such cases, plasma glucose–based tests or alternative markers may be preferred. See NIDDK: A1C test basics.

5) What lifestyle changes matter most for prediabetes?
Answer: There’s no single formula, but consistent physical activity, weight management if relevant, and balanced meals improve odds. CDC’s National DPP programs translate this into weekly, doable steps: CDC lifestyle programs.

Sources & References

This blog is a personal journal and for general information only. It is not a substitute for professional medical advice, diagnosis, or treatment, and it does not create a doctor–patient relationship. Always seek the advice of a licensed clinician for questions about your health. If you may be experiencing an emergency, call your local emergency number immediately (e.g., 911 [US], 119).