Blood glucose test: what it measures, normal fasting levels, and what high or low glucose means

Glucose is the simple sugar that every cell can burn for energy and the one the brain depends on. After a meal, carbohydrate is absorbed as glucose, the pancreas releases insulin, and insulin moves glucose into muscle, fat and liver and switches the liver from making glucose to storing it. Between meals the liver releases glucose to keep the blood level steady. The test measures the concentration of glucose in serum or plasma at the moment of the draw.

Because that concentration changes within minutes of eating, the timing defines the test. A fasting glucose, drawn after at least eight hours without calories, measures the baseline the liver and insulin maintain overnight. A random glucose, drawn any time, is interpreted with a higher threshold and together with symptoms. A two-hour glucose after a 75-gram sugar drink — the oral glucose tolerance test — measures how well the body clears a load, and it is the standard in pregnancy.

The glucose on a basic or comprehensive metabolic panel is a fasting glucose when the panel is drawn fasting, which is why those panels ask for an overnight fast. In the tube, cells keep consuming glucose, so a sample that waits unprocessed reads lower; laboratories use preservative tubes or separate the serum promptly to prevent it.

Screening for prediabetes and diabetes is recommended for all adults from age 35, repeated every three years if normal, and earlier and more often in anyone who is overweight with another risk factor — a parent or sibling with diabetes, high blood pressure, low HDL or high triglycerides, polycystic ovary syndrome, a history of gestational diabetes, physical inactivity, or belonging to a higher-risk ethnic group. People with prediabetes are retested yearly.

A glucose is also the first test for the symptoms of high blood sugar — unusual thirst, frequent urination including at night, blurred vision, unexplained weight loss, slow-healing sores, recurrent yeast or skin infections — and for episodes suggesting low blood sugar: shakiness, sweating, hunger, palpitations, confusion or fainting, especially in people on insulin or sulfonylureas. Pregnant women are screened between 24 and 28 weeks.

Because glucose is on every metabolic panel, most adults have it checked at an annual physical whether or not they are being screened deliberately, and a value in the prediabetes range on a routine panel is one of the most common ways prediabetes is found.

Fasting glucose comes on the basic metabolic panel, the comprehensive metabolic panel and most wellness panels, and can be ordered with an A1c in the same draw, through a doctor or online through TestWell for a draw at a Quest or Labcorp patient service center; a licensed physician reviews the order and results post to your portal within one to three business days against the fasting bands.

Fast for eight to twelve hours — no food, no calorie-containing drinks, no gum or mints — with water allowed and encouraged. Take usual medications unless told otherwise, but note that corticosteroids raise glucose and that some diabetes medications are timed around meals. Schedule a morning draw; a fast that runs much past twelve hours can itself raise glucose slightly through stress hormones.

Avoid intense exercise the morning of the draw (it can raise or lower glucose depending on timing), and postpone the test during an acute illness, which raises glucose for days. A poor night's sleep, heavy caffeine and acute stress push fasting glucose up by a few points. An unexpected result in the prediabetes or diabetes range is confirmed with a repeat fasting glucose or an A1c before a diagnosis is made.

The American Diabetes Association criteria are: normal fasting glucose under 100 milligrams per deciliter (most laboratories print 70 to 99); impaired fasting glucose, or prediabetes, 100 to 125; diabetes 126 or more, confirmed on a second test unless there are clear symptoms with a random glucose of 200 or more. On a two-hour glucose tolerance test, normal is under 140, prediabetes 140 to 199 and diabetes 200 or more. Divide by 18 to convert to millimoles per liter.

The bands are the same for men and women and across adult ages. Fasting glucose drifts up slightly with age and weight, and a result that moves from the 80s into the 90s over a decade is common; a result that crosses 100 is a signal to look at weight, activity and family history and to add an A1c. In pregnancy the thresholds are lower and the tolerance test is used.

A random glucose is read differently: under 140 is normal in most people, and 200 or more with symptoms is diagnostic of diabetes. Values between depend on when the last meal was. A fasting glucose under 70, or any glucose under 55, is hypoglycemia and is significant chiefly in people taking insulin or sulfonylureas, who should treat it, and in people not on diabetes medication, where it prompts a search for a cause.

Persistently high fasting glucose means the balance between the liver's glucose output and insulin's ability to restrain it has shifted — insulin resistance with gradually failing insulin secretion in type 2 diabetes and prediabetes, which account for the great majority of high results, or loss of insulin production in type 1 diabetes, which usually presents quickly with weight loss, thirst and high values. Excess weight, inactivity, family history, age and certain ethnic backgrounds raise the risk of type 2.

Temporary elevations are common and important to recognize: acute illness, infection or surgery (stress hormones), corticosteroid medication, some antipsychotics, diuretics and HIV drugs, a short fast or none at all before the draw, a poor night's sleep, and the overnight rise from the body's own morning cortisol (the dawn phenomenon). Pancreatic disease, Cushing's syndrome, an overactive thyroid, hemochromatosis and pregnancy also raise glucose.

High glucose in the prediabetes range causes no symptoms, which is why it is found by testing. As glucose climbs past about 180 it spills into the urine and produces thirst, frequent urination, blurred vision, fatigue, weight loss and recurrent infections; very high levels with vomiting, deep breathing or confusion are an emergency. A single high value is repeated or paired with an A1c; the diagnosis of diabetes rests on two abnormal tests.

In people on insulin or sulfonylureas, low glucose is a medication effect — too much drug for the food eaten or the exercise done — and it is treated on the spot with fast-acting carbohydrate. Symptoms begin for most people below about 70: shakiness, sweating, hunger, palpitations, anxiety, tingling, then confusion, slurred speech, drowsiness and, below about 40, seizures or loss of consciousness.

In people not on diabetes medication, true hypoglycemia is uncommon and is defined by symptoms with a measured low glucose that resolve when glucose is raised. Causes include alcohol on an empty stomach, reactive hypoglycemia a few hours after a high-carbohydrate meal (especially after bariatric surgery), prolonged fasting or malnutrition, adrenal insufficiency, severe liver or kidney disease, some medications, and rarely an insulin-producing tumor. A fasting glucose in the 60s in a healthy, symptom-free adult is usually normal variation.

A low value can also be an artifact: glucose keeps being consumed by cells in a tube that waits too long before separation, lowering the result by several points an hour. A mildly low result without symptoms is repeated before anything else.

Fasting glucose and A1c answer different questions and are usually read together. Glucose is a snapshot that catches high values on the day and is the only one that detects low glucose; A1c averages three months of glucose and is not affected by a bad night or a stressful morning, but it misreads in anemia, hemoglobin variants, kidney disease and pregnancy. Agreement between the two confirms a diagnosis; disagreement prompts a repeat or a tolerance test.

Fasting insulin, read with fasting glucose, estimates insulin resistance (HOMA-IR) years before glucose rises and is used in metabolic syndrome, PCOS and weight management; C-peptide shows how much insulin the pancreas is still making. The two-hour glucose tolerance test is the most sensitive screen and the standard in pregnancy. Triglycerides, HDL and liver enzymes travel with insulin resistance and are commonly reviewed alongside glucose.