Nursing care
Glucose Tolerance Test: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
The glucose tolerance test requires a fasting glucose, then a 100-gram oral glucose load, with blood drawn again at one, two, and three hours. Two or more values at or above the threshold at any of these four points confirms a diagnosis of gestational diabetes.
Why this value is ordered
The glucose tolerance test is ordered to diagnose gestational diabetes, usually between 24 and 28 weeks of pregnancy, or earlier in women with risk factors such as obesity, a prior macrosomic infant, or a family history of diabetes. Pregnancy hormones, particularly human placental lactogen, increase insulin resistance as pregnancy advances, and this test detects whether the pancreas can compensate.
It is typically ordered after an abnormal screening result, most often a one-hour 50-gram glucose challenge test that did not require fasting. The full three-hour test with fasting and a 100-gram load is the confirmatory step, not the initial screen, and this sequencing is a common point of confusion on exam questions.
Interpreting the number in context
The patient fasts overnight, has a fasting glucose drawn, then drinks a 100-gram glucose solution. Blood is drawn again at one hour, two hours, and three hours after the load, giving four total values to compare against threshold cutoffs that vary slightly by laboratory and by which diagnostic criteria the institution uses.
A single abnormal value at any one of the four draws is not diagnostic on its own. The test is read as a set: two or more values meeting or exceeding the threshold, at any of the four time points, establishes the diagnosis of gestational diabetes. One elevated value with the rest normal usually prompts either a repeat test or closer dietary monitoring rather than an immediate diagnosis.
Critical values and what to do
A markedly elevated fasting glucose before the load is even given, particularly one that meets criteria for overt diabetes rather than gestational diabetes, should prompt notification of the provider before proceeding with the test, since giving a large glucose load to a severely hyperglycaemic patient is not the priority action.
During the test, watch for symptomatic hypoglycaemia in the hours after the load as insulin response peaks, and for vasovagal symptoms or vomiting after the concentrated glucose drink, which can invalidate the result and require rescheduling. If the patient vomits within roughly 30 minutes of drinking the solution, the test generally needs to be repeated on another day rather than continued.
Related tests read alongside it
The one-hour 50-gram glucose challenge test is the screening step that precedes this test in the two-step diagnostic pathway used in much of the United States; an abnormal screen leads to the three-hour confirmatory test described here. Some institutions instead use a one-step approach with a 75-gram load and only two post-load draws, so confirm which protocol your unit and lab follow before counselling a patient on what to expect.
Haemoglobin A1c is not used to diagnose gestational diabetes because the physiological rise in red cell turnover during pregnancy makes it unreliable for this purpose, though it remains useful for identifying pre-existing diabetes at the first prenatal visit. Once gestational diabetes is diagnosed, self-monitored fingerstick glucose becomes the ongoing test read alongside fetal growth ultrasounds to guide management.
Nursing implications
Confirm the patient has fasted for the required period, usually 8 to 14 hours, and has avoided smoking, since both affect the fasting result. Explain that she must remain in the department or lab for the full three hours without eating, drinking anything other than water, or engaging in vigorous activity, since exercise alters glucose uptake and can skew the timed draws.
Space the venous draws accurately at one, two, and three hours from the start of the glucose drink, not from when the fasting sample was taken, and document the exact draw times since even small deviations affect interpretation. If any single value returns abnormal, prepare the patient for the likelihood of either a repeat test or referral for dietary and glucose monitoring education while the full results are confirmed.
What patients ask about it
Patients often ask whether they can eat or drink during the three-hour wait, and the answer is water only, with no food, gum, or other beverages until the final draw is complete. They also ask why the drink tastes so unpleasant and whether nausea is normal; mild nausea from the concentrated glucose load is common, but vomiting may mean the test needs to be repeated.
A frequent worry is what a diagnosis of gestational diabetes means for the baby, and the honest answer is that with monitoring and, when needed, treatment, most women with gestational diabetes have healthy pregnancies, though it does raise the risk of a larger baby and later type 2 diabetes for the mother. Reassure without minimising: this is a manageable diagnosis, not a crisis, but it does change the surveillance plan for the rest of the pregnancy.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our maternity and newborn practice questions are the closest set to what this page covers.
Common questions
How many abnormal values confirm gestational diabetes on the glucose tolerance test?
Two or more abnormal values out of the four draws, fasting, one hour, two hours, or three hours, confirm the diagnosis. A single abnormal value alone is not diagnostic and usually leads to a repeat test or closer monitoring.
What is the difference between the glucose challenge test and the glucose tolerance test?
The glucose challenge test is a one-hour, 50-gram screening test that does not require fasting. The glucose tolerance test is the three-hour confirmatory test that follows an abnormal screen, requiring fasting, a 100-gram load, and draws at one, two, and three hours.
Can a patient eat during the glucose tolerance test?
No. The patient must remain fasting except for water throughout the full three hours between the glucose drink and the final blood draw. Eating, chewing gum, or vigorous activity during that window can alter the result.
What happens if a patient vomits during the glucose tolerance test?
Vomiting shortly after drinking the glucose solution, generally within about 30 minutes, usually invalidates the test because the full dose was not absorbed. The test typically needs to be rescheduled rather than continued.