High fasting glucose: what it means, what prediabetes is and what to record at home

You had a routine blood test and fasting glucose came out at 104, 108, 112 mg/dL: a few points above the 100 that the lab marks as the limit. You searched for “high fasting glucose” and then for “symptoms of high blood sugar”, and found none in yourself. That is to be expected: a slightly high glucose cannot be felt. The classic symptoms, intense thirst, urinating all the time, losing weight without trying, appear when glucose is clearly high, not at 108.^[9] This text explains what that number means, what can push it up on a single day, which categories guidelines use and what is worth recording at home so that the next appointment starts with data and not with a scare.

First, the urgent part. If you are very thirsty and urinating often together with unexplained weight loss, or if your meter reads above 240 mg/dL and there is also nausea, vomiting, a very dry mouth, shortness of breath or fruity-smelling breath, stop reading: seek medical care now.^[7]

High glucose can almost never be felt

The search for “symptoms of high blood sugar” has an uncomfortable answer: in the band where most people fall, between 100 and 125 mg/dL, there are no symptoms. The NIDDK, the US diabetes institute, says it plainly: many people with type 2 diabetes have no symptoms, or they are so mild that people do not notice them, and when they appear they develop slowly, over several years.^[9] The same goes for prediabetes: most people feel nothing.^[4] The symptoms that do exist, urinating more, intense thirst, hunger even after eating, blurred vision, fatigue, sores that are slow to heal, belong to clearly high glucose levels.^[9] If you feel well and your result is 110, what you have is a number that asks for context.

What “fasting” means for this test

It seems obvious, but it is the most common cause of a first high result that does not repeat later. For glucose, fasting means not having had anything to eat or drink, except water, for at least 8 hours before the draw.^[1] The Mexican standard defines it the same way: abstaining from caloric intake for 8 hours.^[6] Water yes; coffee with milk, juice, sugary gum or a biscuit “so as not to go on an empty stomach”, no. The draw is usually done in the morning, after the overnight fast.^[2]

If you had dinner at eleven and the draw was at six, or you had a light breakfast on the way to the lab, the result is not a fasting glucose. Note it as such, with the time of your last meal, and repeat under proper conditions before drawing conclusions.

What pushes up a single result

Blood glucose is not a fixed value: it responds to what happened in the last hours and days. Several everyday situations can push a result above 100 without any underlying change:

• A short night. A single night of too little sleep makes the body use insulin less efficiently, and the next morning’s glucose reflects it.^[8]
• Illness or stress. The stress of an infection, even a cold or flu, raises glucose; so does the emotional stress of a bad week.^[7]
• The dawn phenomenon. Between 4 and 5 in the morning the body releases a surge of hormones that prepares waking, and in some people that raises glucose in the early hours.^[7] A draw at 7 in the morning falls right after.
• Too little water. With less water in the body, glucose is more concentrated in the blood.^[8]
• Medicines. Glucocorticoids (steroids such as prednisone or dexamethasone, used for inflammation, allergies or asthma) affect the way insulin works; so can some antipsychotics, antiseizure drugs, diuretics and beta-blockers.^[9]
• An incomplete fast, the point above: a sugared coffee at six in the morning is enough.

None of this erases a result. What it does is add a note in the margin: “I slept four hours”, “I had the flu”, “I took prednisone that week”. With that note, the next result can really be compared. The lab compares against the table; you can compare against yourself, as we explain in this text.

The categories guidelines use

The American Diabetes Association (ADA) defines three bands for each of the three tests used to assess glucose: fasting plasma glucose, glycated haemoglobin (HbA1c) and glucose 2 hours into a tolerance test with 75 g of glucose.^[1]

The Mexican standard NOM-015-SSA2-2010 uses the same bands under other names: “abnormal fasting glucose” for 100 to 125 mg/dL, “glucose intolerance” for 140 to 199 mg/dL at 2 hours after the load, and calls prediabetes having one of the two or both.^[6] For diabetes, its criteria match: classic symptoms plus a random glucose of 200 mg/dL, a fasting glucose of 126 mg/dL, or a glucose of 200 mg/dL at 2 hours after the load, always confirmed on a different day when there are no unequivocal symptoms.^[6]

Why two results are needed

Something almost nobody takes into account when reading their sheet: none of these criteria is used only once. The ADA states that each test, as a rule, needs to be repeated on a second day to diagnose diabetes, unless glucose is very high or there are clear symptoms along with a positive result.^[1] The NIDDK puts it the same way: any test used to diagnose diabetes requires confirmation with a second measurement on a different day.^[3] A result of 128 on a Monday with the flu is not a diagnosis: it is the first half of one, or nothing, depending on what the repeat says.

What HbA1c adds and when it misleads

Glycated haemoglobin measures what proportion of the haemoglobin in your red blood cells has glucose attached. Since those cells live for a few months, the result reflects your average glucose over the past three months, not that morning’s.^[3] That is why it complements fasting glucose well: a short night or a flu moves the glucose of the day, but barely moves HbA1c. The Mexican standard describes it the same way, as the average of the last 12 weeks.^[6]

But HbA1c has blind spots. Anything that changes the life span of red blood cells changes the result: recent blood loss, a transfusion or dialysis alter it; iron deficiency, for example iron-deficiency anaemia, can falsely raise it; kidney failure and liver disease also distort it.^[3] And there are haemoglobin variants, more common in people of African, Mediterranean or Southeast Asian descent, or with relatives with sickle cell anaemia or thalassaemia, that interfere with some measurement methods.^[3] If your HbA1c and your glucose readings tell different stories, HbA1c may not be a reliable test for you, and your doctor will go by glucose.^[2]

Prediabetes: a stage that can be slowed down

If two results on different days fall between 100 and 125 mg/dL, or HbA1c is between 5.7 and 6.4 %, the word that appears is prediabetes. It is very common: in the United States more than 2 in 5 adults have it, and you can have it for years with no clear symptoms.^[5] The factors that go with it are well known: excess weight, being 45 or older, a parent or sibling with type 2 diabetes, being physically active fewer than three times a week, having had gestational diabetes or a baby weighing more than 4 kilos, and polycystic ovary syndrome.^[5] The NIDDK adds a large waist, sleep apnoea, smoking and some medicines to the list, glucocorticoids among them.^[4]

What makes this stage different is that it can be slowed down. In the study that gave rise to the prevention programs, people at high risk who lost between 5 and 7 % of their starting weight reduced their chance of developing diabetes.^[4] The US national prevention program, based on that study, works with those two numbers: losing 5 to 7 % of body weight and accumulating at least 150 minutes a week of brisk walking or similar activity, and reports that this way the risk of type 2 diabetes can fall by 58 %, and by 71 % in people over 60.^[5]

Two honest caveats. Those percentages are averages from a structured program, not an individual promise. And “slowing down” does not erase the risk forever: the NIDDK recommends that anyone with prediabetes be checked for type 2 diabetes every year.^[2] What is in your hands now is to record well, so that you and your doctor can see whether the number moves and in which direction. If you also get sleepy after meals, this text on glucose and insulin completes the picture; if your triglycerides also came out high, this other one explains why they usually go together.

Warning signs: when you do not log, you consult

A glucose of 105 or 118 is not an emergency. What follows is, with or without a lab result:

• Intense thirst and very frequent urination, especially if they come together with unexplained weight loss or blurred vision.^[9]
• Meter reading above 240 mg/dL at home, with symptoms: the ADA asks to check urine for ketones from that value and not to exercise if they are present.^[7]
• Nausea and vomiting, a very dry mouth, shortness of breath or fruity-smelling breath: these are signs of ketoacidosis, which is life-threatening and needs immediate treatment.^[7]
• Glucose of 200 mg/dL or more at any time of day together with the classic symptoms: that picture meets the criterion for diabetes and does not wait for the routine repeat.^[1]

Home glucose log: two weeks

If you have or can get a glucose meter, a short, orderly log gives your doctor something no isolated result gives: how your glucose behaves on a normal day. There is no need to prick your finger ten times a day. Two moments are enough, for one or two weeks:

1. Fasting glucose, on waking, before eating or drinking anything but water. Note the time and how many hours have passed since your last meal.
2. Glucose 2 hours after starting to eat, at one or two meals a day. Two hours is the time point the tolerance test uses, so your log speaks the same language as the lab.^[1]
3. What you ate at that meal, in one line: “two quesadillas and a soda” is enough. Without the meal, the 2-hour number cannot be interpreted.
4. How you slept, whether you were ill, whether you took any new medicine and whether it was a very stressful day.^[8]
5. Weight and waist once a week, same time, fasting.

In the end you will have between 14 and 28 values with context. What matters is not that one comes out at 112, but whether the morning ones stay consistently above 100, how much they rise after meals and what happened on the highest days.

What to bring to the appointment

• All your fasting glucose and HbA1c results with dates, not just the latest, and the note on the conditions of each one: hours of fasting, sleep, illness, medicines.
• The one- or two-week home log, with the meals.
• Current weight, waist and blood pressure.
• Family history: diabetes in parents or siblings, gestational diabetes, polycystic ovary syndrome.
• The list of medicines, including steroid pills or injections in recent months.

With that, your doctor can decide whether to repeat fasting glucose, whether to order HbA1c or a tolerance test, and how to follow up. The decision is theirs; the data can be yours.