Metabolic Glossary, Batch 1 – Metabolic Media

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The short answer: This glossary defines ten metabolic lab and monitoring terms, from A1c to CGM time in range. Only some have published thresholds: the ADA’s 2026 Standards set cut points for A1c, fasting glucose, and OGTT glucose, and a 2019 international consensus set CGM time-in-range targets for adults with diabetes. HOMA-IR, fasting insulin, and the TyG index have no agreed range.

A glossary is a reference desk: it says what a term means, who published any threshold, and what the number can’t tell you. This page defines terms. It doesn’t interpret anyone’s results, and a clinician who knows a person’s history does that. Guideline figures are stated as of the ADA’s 2026 Standards, and the page was written on Sept 30, 2026. A second batch of terms is planned.

Each entry follows the same pattern: a plain definition, what the test measures, the threshold (or the words “No agreed range”), and its limits.

Glucose tests

A1c (glycated hemoglobin, HbA1c)

Definition: A1c is the percentage of hemoglobin, the oxygen-carrying protein in red blood cells, that has glucose attached to it.

What it measures: It reflects average glucose exposure over roughly the past 2–3 months, not a single moment (ADA Standards of Care 2026, Table 2.3).

Threshold: As of the ADA’s 2026 Standards, the prediabetes range is 5.7–6.4% (39–47 mmol/mol), and the diabetes range is 6.5% or higher (48 mmol/mol or higher) when measured with an NGSP-certified method. For how these ranges sit alongside the other two tests, see Prediabetes by the Numbers.

Limits: Hemoglobin variants and conditions that change red blood cell turnover can alter the relationship between A1c and glucose. In those cases the ADA says plasma glucose criteria should be used (ADA 2026, recommendation 2.4).

Fasting plasma glucose (FPG)

Definition: Fasting plasma glucose is the glucose concentration in blood plasma after a period with no calorie intake.

What it measures: One moment, after fasting for at least 8 hours (ADA 2026, Table 2.1).

Threshold: The ADA’s 2026 prediabetes range is 100–125 mg/dL (5.6–6.9 mmol/L), called impaired fasting glucose. The diabetes range is 126 mg/dL or higher (7.0 mmol/L or higher). The WHO and several other groups set the lower limit at 110 mg/dL (6.1 mmol/L); the ADA lowered its own to 100 mg/dL in 2003 (ADA 2026 text).

Limits: The ADA lists high within-person variability and poor preanalytic stability as drawbacks compared with A1c (Table 2.3).

2-hour OGTT glucose (oral glucose tolerance test)

Definition: An oral glucose tolerance test measures plasma glucose 2 hours after a laboratory-administered glucose drink.

What it measures: How glucose levels respond over 2 hours to a 75-gram glucose load, using the WHO protocol the ADA specifies. This is a description of a lab procedure, not preparation instructions.

Threshold: The ADA’s 2026 prediabetes range for the 2-hour value is 140–199 mg/dL (7.8–11.0 mmol/L), called impaired glucose tolerance. The diabetes range is 200 mg/dL or higher (11.1 mmol/L or higher).

Limits: It is a timed, multi-step lab test, so it captures a different aspect of glucose handling than A1c or fasting glucose.

One caveat across all three tests: the ADA states that risk is continuous and extends below the lower limit of each range. A cut point is a convention, not a line where risk switches on.

Insulin and insulin-based indexes

Fasting insulin

Definition: Fasting insulin is the concentration of insulin in blood after an overnight fast.

What it measures: The amount of circulating insulin at one moment. It doesn’t directly show how well cells respond to insulin.

Threshold: No agreed range. It isn’t part of the ADA 2026 diagnostic criteria, which use glucose and A1c.

Limits: Insulin assays differ. In a 2007 report from an ADA workgroup comparing 12 commercial insulin methods, the between-method variation ranged from 12% to 66%, with a median of 24% (Marcovina et al., Clinical Chemistry). A result from one lab’s method may not match another’s.

HOMA-IR (homeostatic model assessment of insulin resistance)

Definition: HOMA-IR is a calculation that uses one fasting glucose value and one fasting insulin value to estimate insulin resistance. For how insulin resistance works, see Insulin Resistance, Explained.

What it measures: A model-based estimate, not a direct measurement. In the 1985 paper that introduced it, the estimate correlated with the euglycemic clamp (Rs = 0.88), but its coefficient of variation was 31%, which the authors said limits its use (Matthews et al., Diabetologia 1985). A 2004 review by the same group describes cohort and epidemiological studies as appropriate uses (Wallace et al., Diabetes Care 2004).

Threshold: No agreed range. There is no consensus cutoff, and results vary with the insulin assay and the population studied.

Limits: Because it is built on a fasting insulin value, it inherits that measurement’s variability.

C-peptide

Definition: C-peptide is a fragment released in equal amounts with insulin when the pancreas makes insulin.

What it measures: It stays in the blood longer than insulin and isn’t affected by insulin taken as medicine, so it reflects the body’s own insulin production. It can be measured in blood or urine (MedlinePlus, National Library of Medicine, accessed Sept 30, 2026).

Threshold: No single agreed range for a general reader.

Limits: According to MedlinePlus, it is not used to diagnose diabetes.

Triglyceride-glucose (TyG) index

Definition: The TyG index is a number computed from fasting triglycerides and fasting glucose, proposed as a low-cost stand-in for insulin resistance when insulin isn’t measured.

What it measures: An indirect estimate. A 2008 study of 748 apparently healthy adults in Mexico compared it with HOMA-IR (Simental-Mendía et al., Metabolic Syndrome and Related Disorders). A 2010 study of 99 participants compared it with the euglycemic-hyperinsulinemic clamp and reported a correlation of r = −0.681 (Guerrero-Romero et al., Journal of Clinical Endocrinology & Metabolism).

Threshold: No agreed cutoff. The two papers report different “best” values, and cutoffs differ by population, so none is given here.

Limits: It is a research surrogate, and no guideline body among this page’s sources adopts it.

CGM metrics and A1c conversions

Time in range (TIR, CGM)

Definition: Time in range is the share of continuous glucose monitor (CGM) readings, or hours per day, that fall between 70 and 180 mg/dL (3.9–10.0 mmol/L).

What it measures: Glucose patterns across the day and night, which A1c averages out.

Threshold: The 2019 international consensus (Battelino et al., Diabetes Care) set targets for adults with type 1 or type 2 diabetes: more than 70% of readings in range (over 16 hours 48 minutes a day), less than 4% below 70 mg/dL, and less than 25% above 180 mg/dL. It lists a lower target of more than 50% in range for older or high-risk adults, paired with a stricter limit of less than 1% below 70 mg/dL, and a different range (63–140 mg/dL) for pregnancy. ADA 2026 (Table 6.2) carries the same metrics and asks for at least 14 days of wear. These targets are for people with diabetes; they aren’t thresholds for people without it. Targets are individualized.

Limits: As of the ADA’s 2026 Standards, there is insufficient evidence to use CGM to screen for or diagnose prediabetes or diabetes. Regulatory status changes, so this is a dated statement: as of June 2026, the FDA cleared the first over-the-counter CGM for adults 18 and older in March 2024 and the first for children 2 and older on June 12, 2026, both for people who do not use insulin. The FDA says users should consult their health care provider before making any medication adjustments based on the device’s output (FDA press announcements). This page doesn’t cover clearances after June 2026.

eAG (estimated average glucose)

Definition: eAG converts an A1c percentage into an estimated average glucose in mg/dL or mmol/L.

What it measures: A calculated estimate from a regression equation, eAG (mg/dL) = 28.7 × A1c − 46.7, developed in the A1c-Derived Average Glucose study of 507 people: 268 with type 1 diabetes, 159 with type 2, and 80 without diabetes (Nathan et al., Diabetes Care 2008). As one worked example, an A1c of 7.0% gives 154 mg/dL by that equation.

Threshold: None. It is a unit conversion, not a cut point.

Limits: It is a population-level relationship, not an individual’s measured average, and it can differ from a person’s actual glucose readings.

GMI (glucose management indicator)

Definition: GMI is an estimate of A1c calculated from a CGM’s average glucose.

What it measures: Average sensor glucose, expressed on the A1c scale. The term replaced “eA1C” after the FDA determined the earlier name needed to change (Bergenstal et al., Diabetes Care 2018).

Threshold: None of its own. ADA 2026 (Table 6.2) describes GMI as a calculated value approximating A1c that isn’t always equivalent to it.

Limits: GMI and a lab A1c can differ for the same person, because they measure different things over different periods.

What don’t glossary terms tell you?

A number is a snapshot. Each result comes with a method, a lab, and a date, and different tests capture different things: an average over weeks, one fasting moment, a 2-hour response, or continuous readings.
Thresholds are conventions. Named bodies set them in a given year, and they aren’t medical verdicts. The ADA states that risk is continuous.
Indexes are estimates. HOMA-IR, TyG, eAG, and GMI are calculated from other measurements, and their own authors describe precision limits.
Interpretation belongs to a clinician. This page doesn’t interpret anyone’s results.

What questions can you bring to your care team?

If you’re curious about your own results, these questions can help:

Which test and method was used, and was it repeated?
Which units does the report use?
Which of these measures does my clinician find most useful, and why?

For more, see more Read explainers and more Labs & Metrics pieces as they publish.

Frequently asked questions

What is the difference between A1c and fasting glucose?

A1c reflects average glucose over roughly the past 2–3 months. Fasting plasma glucose is a single measurement after at least 8 hours without calories (ADA Standards of Care 2026, Tables 2.1 and 2.3). Both have ADA 2026 thresholds: an A1c of 5.7–6.4% and a fasting glucose of 100–125 mg/dL fall in the prediabetes range. The ADA notes that risk is continuous.

Is there a normal HOMA-IR range?

There is no consensus cutoff. HOMA-IR was built as a research model, its authors reported a 31% coefficient of variation, and results vary with the insulin assay and population (Matthews et al., 1985; Wallace et al., 2004; Marcovina et al., 2007).

What is time in range on a CGM?

It is the share of readings between 70 and 180 mg/dL. The 2019 international consensus target for adults with type 1 or type 2 diabetes is more than 70% of readings, which is over 16 hours 48 minutes a day (Battelino et al., Diabetes Care 2019). The target is for people with diabetes.

What is the difference between eAG and GMI?

eAG converts a lab A1c into an estimated average glucose (Nathan et al., 2008). GMI estimates A1c from a CGM’s average glucose (Bergenstal et al., 2018). Each is an estimate and can differ from the measured value it approximates.

What does C-peptide measure?

It is released in equal amounts with insulin and reflects the body’s own insulin production. It isn’t affected by insulin taken as medicine (MedlinePlus, National Library of Medicine). MedlinePlus says it is not used to diagnose diabetes.

Sources

American Diabetes Association Professional Practice Committee. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2026. Diabetes Care 2026;49(Suppl 1):S27. (PMC)
Battelino T, Danne T, Bergenstal RM, et al. Clinical Targets for Continuous Glucose Monitoring Data Interpretation: Recommendations From the International Consensus on Time in Range. Diabetes Care 2019;42(8):1593–1603. (PubMed)
American Diabetes Association. 6. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes—2026. Diabetes Care 2026;49(Suppl 1):S132. (PMC)
Matthews DR, Hosker JP, Rudenski AS, Naylor BA, Treacher DF, Turner RC. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 1985;28(7):412–419.
Wallace TM, Levy JC, Matthews DR. Use and Abuse of HOMA Modeling. Diabetes Care 2004;27(6):1487–1495.
Marcovina S, Bowsher RR, Miller WG, et al.; Insulin Standardization Workgroup. Standardization of insulin immunoassays: report of the American Diabetes Association Workgroup. Clin Chem 2007;53(4):711–716.
Nathan DM, Kuenen J, Borg R, Zheng H, Schoenfeld D, Heine RJ; A1c-Derived Average Glucose (ADAG) Study Group. Translating the A1C assay into estimated average glucose values. Diabetes Care 2008;31(8):1473–1478.
Bergenstal RM, Beck RW, Close KL, et al. Glucose Management Indicator (GMI): A New Term for Estimating A1C From Continuous Glucose Monitoring. Diabetes Care 2018;41(11):2275–2280. (PubMed)
Simental-Mendía LE, Rodríguez-Morán M, Guerrero-Romero F. The product of fasting glucose and triglycerides as surrogate for identifying insulin resistance in apparently healthy subjects. Metab Syndr Relat Disord 2008;6(4):299–304.
Guerrero-Romero F, et al. The product of triglycerides and glucose, a simple measure of insulin sensitivity. Comparison with the euglycemic-hyperinsulinemic clamp. J Clin Endocrinol Metab 2010;95(7):3347–3351.
MedlinePlus, National Library of Medicine. C-Peptide Test. Accessed Sept 30, 2026.
U.S. Food and Drug Administration. FDA Clears First Over-the-Counter Continuous Glucose Monitor. Press announcement, Mar 5, 2024.
U.S. Food and Drug Administration. FDA Clears First Over-the-Counter Continuous Glucose Monitor for Children. Press announcement, June 12, 2026.

This article is for general education. It isn’t a substitute for personal medical advice. If you have questions about prediabetes, blood sugar, or your lab results, talk with a licensed clinician who knows your health history.

Medical disclaimer: Content on Metabolic Media is for educational purposes only and is not medical advice, diagnosis, or treatment. We are not a medical provider, pharmacy, or telehealth clinic. Always consult a licensed clinician. If you have a medical emergency, call 911 (US) or your local emergency number.