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Do you get irritable, tired or shaky when a meal is late? Crave something sweet or starchy every afternoon? These can be signs that your blood sugar regulation needs some attention, and you're far from alone. The International Diabetes Federation estimates that 589 million adults worldwide now live with diabetes,1 and an estimated 635 million have impaired glucose tolerance, an earlier stage of glucose dysregulation.2 Even people who look healthy on standard tests can have bigger glucose swings than expected: when researchers gave continuous glucose monitors to adults considered non-diabetic, many showed spikes reaching prediabetic or diabetic levels.3 This guide covers six blood markers that help you and your healthcare provider see how well your body handles glucose, with both the conventional and the optimal ranges for each.
Which blood tests show blood sugar health? The most useful markers are fasting glucose, HbA1c and post-meal glucose, which show your blood sugar directly, plus fasting triglycerides, fasting insulin and the ALT:AST ratio, which can reveal insulin resistance years before glucose rises. No single marker tells the whole story, so they're best read together and tracked over time.
A note on ranges
For each marker below, I show two kinds of range. Conventional ranges are the ones used for diagnosis, mostly from the American Diabetes Association (ADA). Optimal ranges are narrower targets, and I show two kinds. "Optimal (studies)" are the levels where a specific study found the lowest risk. "Optimal (ODX)" are the ranges set by Optimal DX (ODX), the functional blood chemistry platform I use in practice, which bases each range on a review of the wider research plus clinical experience, and cites its sources. The two don't always match, so I've shown both. If you'd like the background on how these two kinds of range differ, read my guide to optimal vs conventional reference ranges.
One important caveat: no single marker should be treated as a stand-alone diagnostic test. Each needs to be read as part of a pattern, and ideally over time, looking for trends toward or away from dysfunction.
Fasting glucose
Fasting glucose measures the sugar in your blood after at least 8 hours without food.4 It's part of most routine blood panels. Glucose is your cells' main fuel: what isn't used straight away is stored as glycogen in the liver and muscles.
The ADA considers a fasting glucose below 100 mg/dL (5.6 mmol/L) normal.4 But risk doesn't start at 100. In a 22-year follow-up of healthy men without diabetes, those with fasting glucose above about 85 mg/dL (4.7 mmol/L) had a significantly higher rate of cardiovascular death.5 A study of nearly 20,000 adults without cardiovascular disease found the lowest long-term risk below 90 mg/dL (5.0 mmol/L),6 and another linked high-normal fasting glucose with worse cardiovascular outcomes.7
| Fasting glucose | mg/dL | mmol/L | Source |
|---|---|---|---|
| Optimal (ODX) | 75 to 86 | 4.2 to 4.8 | ODX8 |
| Optimal (studies) | Below 85 to 90 | Below 4.7 to 5.0 | Lowest cardiovascular risk5,6 |
| Normal | Below 100 | Below 5.6 | ADA4 |
| Prediabetes | 100 to 125 | 5.6 to 6.9 | ADA (WHO uses 110 to 125) |
| Diabetes | 126 or higher | 7.0 or higher | ADA |
HbA1c
HbA1c, or glycated hemoglobin, reflects your average blood sugar over the past two to three months. It measures the percentage of hemoglobin in your red blood cells that has glucose attached. The longer glucose stays high in the bloodstream, the more it binds to proteins, which is part of how high blood sugar damages tissues over time.
HbA1c is a great medium-term measure, but it has limits. It doesn't show day-to-day swings,3 and conditions that change how long red blood cells live, such as some anemias, can make it misleading. In those cases, the ADA recommends diagnosing from glucose instead.4 For a shorter window of one to three weeks, tests such as fructosamine, glycated albumin and 1,5-anhydroglucitol can be used.9
While the ADA defines normal as below 5.7%, a study in adults without diagnosed diabetes found that an HbA1c below 5.3% was a better cut-off for ruling out prediabetes and other glucose abnormalities.10
| HbA1c | % | Source |
|---|---|---|
| Optimal (ODX) | 4.6 to 5.3 | ODX8 |
| Optimal (study) | Below 5.3 | Rules out glucose abnormalities10 |
| Normal | Below 5.7 | ADA4 |
| Prediabetes | 5.7 to 6.4 | ADA |
| Diabetes | 6.5 or higher | ADA |
Post-meal glucose and CGMs
Post-meal (postprandial) glucose shows how your body handles the food you actually eat. Fasting glucose and HbA1c can't capture it. The easiest way to see it is with a continuous glucose monitor (CGM), a small sensor worn on the skin that reads glucose in the fluid around your cells every few minutes for up to two weeks. It shows how meals, exercise, stress and sleep affect you in real life. Some CGMs are now sold without a prescription in some countries, so it's worth checking what's available where you live.
Two things are worth knowing before you start. First, CGMs aren't used to diagnose diabetes or prediabetes: the ADA says there isn't enough evidence for that yet.4 Second, the official numbers you may have seen, normal below 140 mg/dL, prediabetes 140 to 199 and diabetes 200 or higher, come from the oral glucose tolerance test (OGTT). That's a two-hour test after drinking a standard 75 g glucose drink, not a reading after an everyday meal.4
So what should a healthy post-meal response look like? There's no official target for people without diabetes, but a common goal is to stay below 140 mg/dL (7.8 mmol/L).8 Optimal DX suggests keeping the rise to no more than 40 mg/dL (2.2 mmol/L) above your fasting level,8 and one study suggests a tighter rise of no more than 30 mg/dL (1.7 mmol/L).11 What matters most is avoiding large, frequent swings. In people with type 2 diabetes, glucose swings were linked to more oxidative stress than steadily high glucose,12 and oscillating glucose has been found to affect blood vessel function more than average glucose alone.13
The example below shows what a rise of about 30 mg/dL above baseline looks like on a CGM.
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If you're curious about CGMs, Jessie Inchauspé, known as the Glucose Goddess, is a great place to start. She shares CGM graphs and practical, easy-to-follow tips for flattening glucose spikes.
| Post-meal glucose | Target | Source |
|---|---|---|
| Optimal (study) | Rise of no more than 30 mg/dL (1.7 mmol/L) above fasting | Study11 |
| Optimal (ODX) | Rise of no more than 40 mg/dL (2.2 mmol/L) above fasting | ODX8 |
| Normal | Below 140 mg/dL (7.8 mmol/L) | Common target8 |
Fasting triglycerides
Triglycerides are the main form of fat in food, in your fat cells and in your blood. When you eat more carbohydrate than your body can use or store as glycogen, the liver converts the extra into triglycerides. That's why triglycerides aren't only a heart marker: rising levels can also be an early sign of insulin resistance.
The conventional cut-off is 150 mg/dL (1.69 mmol/L), but the American Heart Association considers below 100 mg/dL (1.13 mmol/L) optimal.14 In a study of 8,068 adults followed for up to 10 years, cardiovascular risk kept rising with triglycerides up to about 100 mg/dL in men and 200 mg/dL in women,15 and Optimal DX notes that the lowest quartile, below about 83 mg/dL (0.93 mmol/L), had the lowest risk of a cardiovascular event.16
| Triglycerides | mg/dL | mmol/L | Source |
|---|---|---|---|
| Optimal (ODX) | 70 to 80 | 0.79 to 0.90 | ODX16 |
| Optimal (AHA) | Below 100 | Below 1.13 | AHA14 |
| Normal | Below 150 | Below 1.69 | Conventional |
Fasting insulin
Insulin is the hormone that moves glucose from your blood into your cells. When cells become less responsive to it, called insulin resistance, the pancreas makes more to compensate, so fasting insulin rises while glucose still looks normal. In a large study that tracked people for years before they developed diabetes, insulin sensitivity was already declining years before diagnosis.17 That makes fasting insulin one of the most useful early markers, and one that's rarely included in routine panels.
There's no consensus on an optimal level, and the conventional range is wide.8 In a long-term Iranian study, fasting insulin above about 5.5 µIU/mL in men and 7.5 µIU/mL in women helped predict who would later develop type 2 diabetes.18 Insulin tests vary between labs, so it's especially important to compare results from the same lab over time. Fasting insulin is also used with fasting glucose to calculate insulin resistance scores such as HOMA-IR.
| Fasting insulin | µIU/mL | pmol/L | Source |
|---|---|---|---|
| Optimal (ODX) | 2 to 5 | 13.9 to 34.7 | ODX8 |
| Risk threshold (study) | Above 5.5 (men), 7.5 (women) | Above 38 (men), 52 (women) | Predicted type 2 diabetes18 |
| Normal | About 2 to 18 | About 14 to 128 | Varies by lab8 |
ALT:AST ratio
ALT (alanine aminotransferase) and AST (aspartate aminotransferase) are enzymes best known as liver markers, but both are also involved in making new glucose. The ratio between them, ALT divided by AST, is emerging as a marker of metabolic health: a higher ratio has been linked to insulin resistance, poor glucose regulation and fatty liver.8,19,20 In a study of Chinese adults, an ALT:AST ratio of about 0.8 was a reliable cut-off for insulin resistance, slightly lower at 0.78 in people with excess belly fat.19
Two things to know: labs don't usually report this ratio, so you'll need to calculate it from your ALT and AST results. And many people know the reverse ratio, AST divided by ALT, which is used differently in liver medicine, so make sure you're looking at the right one.
| ALT:AST ratio | Ratio | Source |
|---|---|---|
| Optimal (ODX) | 0.1 to 0.78 | ODX8 |
| Insulin resistance threshold (study) | About 0.8 (0.78 with excess belly fat) | Study19 |
All ranges at a glance
| Marker | Conventional normal | Optimal (studies) | Optimal (ODX) |
|---|---|---|---|
| Fasting glucose | Below 100 mg/dL | Below 85 to 90 mg/dL | 75 to 86 mg/dL |
| HbA1c | Below 5.7% | Below 5.3% | 4.6 to 5.3% |
| Post-meal glucose | Below 140 mg/dL | Rise of 30 mg/dL or less | Rise of 40 mg/dL or less |
| Triglycerides | Below 150 mg/dL | Below 100 mg/dL (AHA) | 70 to 80 mg/dL |
| Fasting insulin | About 2 to 18 µIU/mL | Below 5.5 (men), 7.5 (women) | 2 to 5 µIU/mL |
| ALT:AST ratio | Not usually reported | Below about 0.8 | 0.1 to 0.78 |
Track all six markers over time
My Blood Test Tracker Plus Optimal and Plus Charts & Optimal let you record your lab's ranges and your chosen optimal ranges side by side, for Excel or Google Sheets. They also calculate insulin resistance scores like HOMA-IR, QUICKI and TyG automatically from your fasting glucose, insulin and triglycerides, so you can see how your metabolic markers are moving together.
Frequently asked questions
What is a healthy fasting blood sugar level?
The ADA considers below 100 mg/dL (5.6 mmol/L) normal. Studies have found the lowest cardiovascular risk below about 85 to 90 mg/dL, and Optimal DX uses 75 to 86 mg/dL as its optimal range. A single result is a snapshot, so it's best looked at alongside HbA1c and fasting insulin, and over time.
Can my blood sugar be a problem if my HbA1c is normal?
Yes. HbA1c is an average, so it can look normal even if your glucose swings up and down after meals. It can also be misleading in conditions that affect red blood cells. Fasting insulin and post-meal glucose can reveal problems that HbA1c misses.
Should I use a CGM if I don't have diabetes?
A CGM can be a useful short-term learning tool, showing how specific meals, exercise, sleep and stress affect your glucose. It isn't used to diagnose diabetes or prediabetes, though, and readings need context, so it's worth reviewing the results with a practitioner rather than reacting to every spike.
Why test fasting insulin if my glucose is normal?
Because insulin often rises first. When cells become resistant to insulin, the pancreas produces more to keep glucose normal, so fasting insulin can climb years before fasting glucose does. It's rarely part of a routine panel, so you may need to ask for it.
What is the ALT:AST ratio and how do I calculate it?
Divide your ALT result by your AST result, using the same units. A ratio above about 0.8 has been linked to insulin resistance in research, and Optimal DX uses 0.78 as its upper optimal limit. Don't confuse it with the reverse ratio, AST:ALT, which is used differently in liver medicine.
Which blood sugar markers should I ask my doctor for?
Fasting glucose, HbA1c and triglycerides are usually part of routine panels. Fasting insulin often has to be requested separately. Your ALT and AST will be on a standard liver panel, so you can calculate the ratio yourself. Ask your doctor which tests suit your situation.
The bottom line
Blood sugar problems rarely appear overnight. Insulin, triglycerides and post-meal swings often change long before fasting glucose or HbA1c cross a diagnostic line. Looking at all six markers together, comparing them with both conventional and optimal ranges, and following the trend over time gives you and your healthcare provider a much earlier and clearer picture. Please consult a qualified healthcare professional before making significant changes to your diet or lifestyle.
References (20)
- 1Global estimatesInternational Diabetes Federation. IDF Diabetes Atlas, 11th edition. Brussels: IDF; 2025. Source ↗
- 2Global estimatesGlobal prevalence of impaired glucose tolerance and impaired fasting glucose: estimates for the IDF Diabetes Atlas, 11th edition. Diabetes Care. 2025;48(11):e142-e144. Source ↗
- 3Observational studyHall H, et al. Glucotypes reveal new patterns of glucose dysregulation. PLoS Biol. 2018;16(7):e2005143. Source ↗
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- 5Cohort studyBjørnholt JV, et al. Fasting blood glucose: an underestimated risk factor for cardiovascular death. Results from a 22-year follow-up of healthy nondiabetic men. Diabetes Care. 1999;22(1):45-49. Source ↗
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- 7Cohort studyShaye K, et al. Fasting glucose levels within the high normal range predict cardiovascular outcome. Am Heart J. 2012;164(1):111-116. Source ↗
- 8IndustryOptimal DX. The complete ODX biomarker guide: biomarkers of blood glucose regulation. Source ↗
- 9ReviewLee JE. Alternative biomarkers for assessing glycemic control in diabetes: fructosamine, glycated albumin, and 1,5-anhydroglucitol. Ann Pediatr Endocrinol Metab. 2015;20(2):74-78. Source ↗
- 10Diagnostic studyAdamska E, et al. The usefulness of glycated hemoglobin A1c (HbA1c) for identifying dysglycemic states in individuals without previously diagnosed diabetes. Adv Med Sci. 2012;57(2):296-301. Source ↗
- 11StudyJ Diabetes Sci Technol. 2007;1(5). Source ↗
- 12Clinical studyMonnier L, et al. Activation of oxidative stress by acute glucose fluctuations compared with sustained chronic hyperglycemia in patients with type 2 diabetes. JAMA. 2006;295(14):1681-1687. Source ↗
- 13Clinical studyCeriello A, et al. Oscillating glucose is more deleterious to endothelial function and oxidative stress than mean glucose in normal and type 2 diabetic patients. Diabetes. 2008;57(5):1349-1354. Source ↗
- 14Scientific statementMiller M, et al. Triglycerides and cardiovascular disease: a scientific statement from the American Heart Association. Circulation. 2011;123(20):2292-2333. Source ↗
- 15Cohort studyAberra T, et al. The association between triglycerides and incident cardiovascular disease: what is "optimal"? J Clin Lipidol. 2020;14(4):438-447.e3. Source ↗
- 16IndustryOptimal DX. Lipid biomarkers: triglycerides. Source ↗
- 17Cohort studyTabák AG, et al. Trajectories of glycaemia, insulin sensitivity, and insulin secretion before diagnosis of type 2 diabetes: an analysis from the Whitehall II study. Lancet. 2009;373(9682):2215-2221. Source ↗
- 18Cohort studyGhasemi A, et al. Cut-off points of homeostasis model assessment of insulin resistance, beta-cell function, and fasting serum insulin to identify future type 2 diabetes: Tehran Lipid and Glucose Study. Acta Diabetol. 2015;52(5):905-915. Source ↗
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This article is for educational purposes only and isn't a substitute for personalized medical advice. Optimal ranges are not diagnostic. Please discuss your results with a qualified healthcare provider. See my full medical disclaimer.
