Fasting Glucose 100–125 mg/dl (5.6–6.9 mmol/l): What It Means and What Next

The 'normal' range for fasting glucose is not the result of a single, universal measurement but a decision by experts who set the threshold based on the risk of later diabetes and its complications. The American Diabetes Association (ADA) holds that impaired fasting glucose begins at 100 mg/dl (5.6 mmol/l) and extends to 125 mg/dl (6.9 mmol/l). The World Health Organization (WHO) uses a higher lower bound: 110 mg/dl (6.1 mmol/l), with the same upper bound of 125 mg/dl. Both organizations diagnose diabetes at a fasting glucose of ≥126 mg/dl (7.0 mmol/l), confirmed accordingly by a second measurement.

In practice, the 100 mg/dl threshold in line with the ADA is the one most often seen in laboratories and diabetes guidelines, which is why a printout showing 104 mg/dl is often annotated as above the normal range. For the patient, this difference has a concrete consequence: the same result, for example 105 mg/dl, is classified as IFG by the ADA but still falls within normal by the WHO. This does not mean that one organization is right and the other wrong — it reflects a compromise between sensitivity (catching more people at risk) and specificity (avoiding labelling people at low risk).

This result captures the sense of the two thresholds well. People with a glucose of 100–109 mg/dl, who are still 'normal' by the WHO but already have IFG by the ADA, form a group at lower risk than people with a result of 110–125 mg/dl — and it is precisely this difference that averages out into the lower rate for the ADA definition. In other words: the higher the value within the 100–125 range, the greater the probability of progression, and a result of 124 mg/dl is a different clinical situation from 101 mg/dl even though both fall within the same range.

Fasting glucose seems like a stable number, but even in a healthy person its value changes from day to day. It is influenced by, among other things, the length and quality of sleep, stress, the previous evening's dinner, the previous day's physical activity, infection, medications, and the early-morning surge of hormones (cortisol and growth hormone) that increases glucose production in the liver towards morning — a phenomenon we describe separately in our article on the dawn phenomenon. On top of this comes the analytical error of the measurement itself. The result is that a value of 104 mg/dl one week can be 96 mg/dl the next, without any real change in metabolism.

What does this mean for the 100–125 range? Selvin's study concerned the 126 mg/dl threshold, but the variability it describes applies across the whole range of values. With a coefficient of variation of around 5.7%, a result of 105 mg/dl could, on repeat testing, come out at roughly 93 to 117 mg/dl (this is a simplified estimate based on a normal distribution and the average coefficient of variation, not a result from a specific study). Such a spread is enough for a person to land in the IFG category on one measurement and in the normal range on the next, and for a person with a result of 120 mg/dl not to fall automatically to normal at the next draw.

Blood cells — primarily red and white blood cells — continue to consume glucose after the draw if the sample sits at room temperature and is not centrifuged or does not contain a glycolysis inhibitor. This process, called in vitro glycolysis, causes the glucose concentration in the tube to fall over time, which can give a falsely low result. That is why blood for glucose measurement is usually collected into tubes containing sodium fluoride (a glycolysis inhibitor), and the sample should be centrifuged quickly. A falsely low result is an error in the opposite direction to the one that interests us here (it can mask IFG rather than create it), but it illustrates that the value on the printout is the product of a measurement process, not a direct readout from the body.

The reverse side of the issue, described in the analytical literature, is more interesting. Tubes containing citrate, which inhibit glycolysis very quickly, yield significantly higher glucose results than tubes containing fluoride only, on which many of the studies that set the current diagnostic thresholds were based. According to the review by Pasqualetti et al. (2017), this can lead to a different clinical classification of the same people. In practice, this means the same body, tested in different laboratories using different collection systems, can get results differing by a few mg/dl — and at a value of 99 or 101 mg/dl, those few units decide whether the printout is marked 'above normal'.

According to the ADA Standards of Care (2025), diagnosing diabetes in the absence of unequivocal hyperglycemia requires two abnormal results — either from the same sample (for example fasting glucose and HbA1c) or from two separate tests. For the 100–125 mg/dl range, which lies below the diabetes threshold, a sensible second step is to repeat the fasting glucose and measure HbA1c, because this allows you to judge whether the elevated glycemia is a persistent trait of metabolism or an episode. HbA1c reflects average glycemia over the past 2–3 months and is therefore less sensitive to one-off fluctuations, but it has its own limitations (iron deficiency, anemia, hemoglobin variants) — we write about the discrepancies between these two tests in a separate article on HbA1c versus fasting glucose.

The third tool is the oral glucose tolerance test (OGTT), in which glucose is measured fasting and two hours after drinking 75 g of glucose. It detects impaired glucose tolerance (IGT, 2-hour result of 140–199 mg/dl), which is not visible in fasting glucose alone. Data from the US population (Tucker's study of 7,412 adults from NHANES) show how poorly classifications from different tests agree: for a diagnosis of diabetes by OGTT, agreement with HbA1c was only 34% and with fasting glucose 44%. This does not mean one test is 'bad' — each measures a different aspect of glucose homeostasis. It means that in a person with IFG, combining several tests gives a fuller picture than any one of them.

The scale of risk is more nuanced than headlines about an 'epidemic of prediabetes' suggest. A review published in The Lancet (Tabák et al., 2012) estimates that each year 5–10% of people with prediabetes progress to diabetes, while the same proportion returns to normal glycemia. This means prediabetes is not a one-way path — it is dynamic, and with the same abnormality some people can improve and some can worsen. In Morris's meta-analysis (described above), the pooled incidence for ADA-defined IFG was about 3.5 cases per 100 person-years, and for WHO-defined IFG about 4.7 per 100 person-years. When IFG coexisted with impaired glucose tolerance, the risk was clearly higher: about 7 cases per 100 person-years.

What moves an individual up or down this scale? Based on epidemiological reviews, several factors can be named: a glucose value in the upper part of the range (close to 125 mg/dl), coexisting impaired glucose tolerance on an OGTT, abdominal obesity, a family history of diabetes, higher triglycerides and lower HDL, hypertension, low physical activity, and age. Tabák's review also stresses that prediabetes is associated simultaneously with insulin resistance and impaired beta-cell function — changes that begin before glucose starts to rise — and that in observational studies it is associated with early forms of nephropathy, chronic kidney disease, small-fibre neuropathy and retinopathy, as well as with an increased risk of macrovascular disease. That is why a result of 100–125 mg/dl is worth treating as a prompt for a comprehensive cardiovascular risk assessment, not just a 'sugar number'.

Not every result of 100–125 mg/dl means the beginning of metabolic disorders. A temporary rise in glucose can result from a recent infection or inflammation, intense physical or mental stress, short sleep, alcohol the previous evening, medications (including glucocorticoids, some antipsychotics and thiazide diuretics), and from the natural morning rise in glucose tied to the hormonal rhythm. In some people the culprit is a late, large dinner — we describe this mechanism in more detail in a separate text. On the other hand, low physical activity in preceding days, excess visceral fat and polycystic ovary syndrome are conditions in which elevated fasting glucose has an insulin-resistant basis and is often persistent.

It is also worth remembering the opposite error: the belief that a result of 98 mg/dl on a single measurement rules out a problem. Normal fasting glucose with elevated insulin, post-meal glucose intolerance or a high HbA1c is possible, and people with early insulin resistance often have normal glucose values for years thanks to increased insulin secretion — we discuss this mechanism in our knowledge base entry on high insulin despite normal glucose. Fasting glucose is an important but only one of several windows on carbohydrate metabolism.

A fasting glucose of 100–125 mg/dl is one of the most common surprises on a blood test printout — and one of the easiest to misread in both directions. An overly sharp reaction leads to drastic diets and unnecessary anxiety, an overly dismissive one to missing the window in which lifestyle changes have the greatest effect. The sensible path is dull but effective: confirm the result, supplement the diagnostics with HbA1c (and, if in doubt, an OGTT), assess the full metabolic risk profile, and only then plan actions tailored to what the tests actually showed.