How to Prepare for a Fasting Glucose Test: Rules for Taking It
Fasting glucose is one of the simplest and most common tests. It is usually where a check of carbohydrate metabolism and screening for prediabetes and diabetes begins. However, the simplicity is deceptive: the result is affected by the duration of fasting, stress, sleep, recent workouts and even how long the tube sat before processing. The editorial team explains how to take the test correctly.
What 'fasting' actually means
The American Diabetes Association defines the fasting state as no caloric intake for at least 8 hours. In practice, laboratories recommend an interval of 8–14 hours. Less than 8 hours — and you risk getting an elevated level due to the residual effect of dinner; more than 14 hours — the body switches into a prolonged fasting mode, and the result may be atypical.
'No calories' means that not only food is prohibited but also juices, milk, sweet drinks, sports drinks, BCAAs and protein shakes, chewing gum and lozenges. Even a small amount of sugar or amino acids stimulates a hormonal response and affects glucose and insulin levels.
Plain still water is allowed and even desirable: mild dehydration concentrates the blood, and a morning glass of water also helps with the draw itself. The editorial team advises abstaining from coffee and tea, even without sugar: caffeine stimulates the release of adrenaline and can affect glycemia and insulin sensitivity.
Smoking before the test is also undesirable. Nicotine activates the sympathetic nervous system and the release of stress hormones, so cigarettes and electronic nicotine devices should be given up at least an hour before, and preferably from the evening.
The evening and morning before the test
The last dinner should be a normal one, without overeating and a large amount of sweets or alcohol. Alcohol affects glucose ambiguously: it can suppress glucose production in the liver and lower the morning level, and combined with sweet drinks — raise it. Therefore alcohol should be given up at least a day before.
It is important not to change your diet drastically in the days before the test. If a person usually eats enough carbohydrates and switches to keto or fasting before the test, the result stops reflecting the usual state. For the oral glucose tolerance test, the WHO even requires that for three days beforehand a person consume at least 150 g of carbohydrates per day.
Sleep matters. Sleep deprivation and night shifts raise cortisol levels and reduce insulin sensitivity. The result after a sleepless night may be higher than usual, so it is advisable to get a normal night's sleep beforehand.
In the morning, come to the laboratory calmly, without rushing. The optimal time is from 7 to 10 o'clock. Later, due to the long fasting and daily hormonal rhythms, the result changes, and in people prone to diabetes the morning 'dawn phenomenon' can raise glucose already from the early hours.
Training, stress and medications
Physical activity affects glucose in two ways. Regular training improves insulin sensitivity and in the long term lowers fasting glucose. But a very intense workout the day before can temporarily change the picture, and morning cardio right before the test, through the release of adrenaline, can raise the glucose level.
Therefore, the day before the test it is better to limit yourself to light activity, and on the day of the test — not to train before the blood draw. This especially applies to fasted morning workouts, popular among athletes on a cut.
Acute stress, pain, inflammation and infections raise glucose through cortisol, adrenaline and other counter-insular hormones. This state is called stress-induced hyperglycemia. A scheduled test during a cold with a fever or right after an injury is better postponed.
Some medications and drugs also change the glucose level. Below are the main groups that are worth telling your doctor about:
- glucocorticoids (prednisolone, dexamethasone) — raise glucose;
- thiazide diuretics, some beta-blockers, niacin in high doses — may raise it;
- somatotropin and drugs that affect its secretion — worsen insulin sensitivity;
- glucose-lowering drugs — obviously lower it; you must not cancel them on your own.
Laboratory nuances: the tube, the glucometer, timing
Few people know that glucose continues to 'disappear' from the tube after the draw: blood cells use it as fuel. At room temperature, according to guidelines on the laboratory diagnosis of diabetes, the glucose level drops by about 5–7% per hour if the sample is not centrifuged or stabilized.
To prevent this, laboratories use tubes with sodium fluoride (gray caps) or with a citrate buffer, and also separate the plasma as quickly as possible. If you give blood at a point from which samples are transported for a long time, the result may be falsely lowered.
A home glucometer measures glucose in capillary blood, and its accuracy allows deviations that are acceptable for self-monitoring but not for making a diagnosis. A diagnosis of diabetes or prediabetes is made based on a laboratory test of venous plasma.
Also pay attention to the units of measurement: in Ukraine mmol/L is used, in the USA — mg/dL. To convert a value to mg/dL it is divided by approximately 18. When comparing your results with foreign sources, it is easy to make a mistake precisely here.
How to read the result
| Fasting plasma glucose | Assessment (ADA) | What to do |
|---|---|---|
| 3.9–5.5 mmol/L | Normal | Monitor routinely |
| 5.6–6.9 mmol/L | Impaired fasting glycemia (prediabetes) | See a doctor, assess HbA1c |
| ≥ 7.0 mmol/L | Possible diabetes | Repeat the test, examination by an endocrinologist |
| < 3.9 mmol/L | Low level | Determine the cause together with a doctor |
One elevated result does not equal a diagnosis. Under the ADA criteria, a diagnosis of diabetes requires confirmation: a repeat fasting glucose test, HbA1c or an oral glucose tolerance test, if there are no obvious symptoms of hyperglycemia.
Low fasting results in a lean athlete on a caloric deficit are usually normal, but a sharp drop with symptoms of weakness, trembling and sweating requires examination.
Fasting glucose reflects only one moment. To assess long-term glycemic control it is supplemented with a glycated hemoglobin test, and to detect insulin resistance a doctor may prescribe fasting insulin with calculation of the HOMA-IR index.
Editorial conclusions
For a fasting glucose test you need 8–14 hours without calories; only plain water is allowed, while coffee, smoking and sports drinks are undesirable.
The result is distorted by stress, sleep deprivation, illness, intense training and some medications, and in the laboratory — by a delay in processing the tube.
A single deviation always requires a repeat test and a doctor's consultation, and a glucometer does not replace a laboratory test.
We also recommend reading our articles on preparing for the glycated hemoglobin test (HbA1c), on cortisol and on triglycerides.
References
- American Diabetes Association Professional Practice Committee. 2. Diagnosis and classification of diabetes: Standards of Care in Diabetes—2024. Diabetes Care. 2024;47(Suppl 1):S20–S42.
- Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Clin Chem. 2011;57(6):e1–e47.
- World Health Organization. Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia: report of a WHO/IDF consultation. Geneva: WHO; 2006.
- Rifai N, Horvath AR, Wittwer CT (eds). Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. 6th ed. Elsevier; 2018.
- Matthews DR, Hosker JP, Rudenski AS, et al. Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia. 1985;28(7):412–419.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
