How do I prepare correctly for an oral glucose tolerance test?

By Sina Hartung, MMSC-BMI, Harvard Medical SchoolReviewed by Eureka Health Medical Group
Published: July 1, 2025Updated: July 1, 2025

Summary

To prepare for an oral glucose tolerance test (OGTT), eat your usual balanced diet with at least 150 g of carbohydrates daily for three days, avoid vigorous exercise for 24 hours, fast 8–14 hours overnight, drink only plain water, and delay any non-essential medicines that influence glucose. Arrive rested, bring a quick snack for afterward, and plan to stay at the lab for two to three hours until all timed blood draws are finished.

Why does fasting and carbohydrate loading matter before an OGTT?

The OGTT measures how your body handles a sudden 75-gram glucose load. Reliable results require that your pancreas and muscles are working under usual conditions—not deprived of carbs or influenced by a recent meal. As Sina Hartung, MMSC-BMI explains, “Skipping carbohydrates or breaking the fast can make a healthy person look pre-diabetic and a diabetic patient appear normal.”

  • Eat at least 150 g of carbs per day for three daysResearch shows carbohydrate restriction (>50 % cut) raises 2-hour glucose by up to 30 mg/dL, creating a false-positive result.
  • Fast 8–14 hours, but not longerFasting less than 8 hours leaves insulin high; fasting more than 14 hours can provoke stress hormones that spike glucose.
  • Drink only plain water during the fastBlack coffee, tea, nicotine, and sugar-free gum can stimulate adrenaline and skew readings by 5–10 mg/dL.
  • Arrive restedSleeping fewer than 6 hours the night before increases post-load glucose by roughly 9 %.
  • Reschedule the test if you are sick or under acute stressSt Vincent’s Pathology recommends postponing an OGTT during illness or high stress because stress hormones can transiently raise blood sugar and distort results. (SVH)
  • Remain seated and avoid exercise throughout the OGTTLifestyle Maternity advises no strenuous activity for 8 hours before—and sitting quietly during—the 2-hour testing window, since muscle activity accelerates glucose uptake and may falsely lower readings. (LMat)

References

Which common mistakes can invalidate the test and force a repeat?

Labs cancel roughly 1 in 12 OGTTs because patients unknowingly break pre-test rules. The team at Eureka Health notes, “Repeating the test means another fasting morning and extra cost—knowing the pitfalls prevents that.”

  • Eating or drinking anything but water after midnightEven a breath mint containing 1 g of sugar can raise baseline glucose above the 92 mg/dL pregnancy cutoff.
  • Vigorous exercise the evening beforeA 60-minute high-intensity workout can drop fasting glucose by 10 mg/dL and mask insulin resistance.
  • Smoking on the way to the labNicotine raises catecholamines, increasing blood glucose within 10 minutes.
  • Arriving lateDelays shift the timed blood draws; a 15-minute error can misclassify 18 % of tests.
  • Low-carb dieting in the 3 days before the test skews resultsGuidelines call for at least 150 g of carbohydrate daily for the 3 days prior; dropping below this can blunt insulin release and create a false elevation at the 2-hour draw, prompting a repeat OGTT. (RACGP)
  • Taking the test while ill or on glucose-active drugs can void the sampleEven a mild cold, corticosteroids, or other glucose-altering medications can distort fasting and post-load values, so labs often cancel and reschedule if these factors are present. (MedicineNet)

References

Why should you tell the lab about medications and health conditions first?

Certain drugs and illnesses change glucose handling. Sina Hartung, MMSC-BMI warns, “Beta-blockers, steroids, and even common nasal decongestants can all distort OGTT curves if the team isn’t informed.”

  • Corticosteroids elevate glucose for up to 72 hoursPrednisone 20 mg daily can raise 2-hour OGTT values by 40–60 mg/dL.
  • Beta-blockers blunt insulin releasePropranolol may convert a normal test into a pre-diabetes range by reducing first-phase insulin.
  • Recent gastric surgery accelerates glucose absorptionPost-bariatric patients may peak earlier than the 30-minute draw, leading to misinterpretation.
  • Active infections raise stress hormonesFever increases cortisol and can push fasting glucose above 126 mg/dL without true diabetes.
  • Certain medications must be paused before an OGTTKaiser Permanente notes that patients should list every prescription and over-the-counter drug because some need to be stopped in advance to avoid skewing glucose readings. (KP)
  • Smoking or strenuous exercise within 8 hours can distort resultsBecause nicotine and acute physical activity both raise counter-regulatory hormones, Kaiser Permanente instructs patients to avoid smoking and vigorous exercise for at least 8 hours before the first blood draw. (KP)

References

What practical steps the night before and morning of the test improve comfort and accuracy?

Preparation goes beyond fasting: you’ll be at the lab two to three hours and may feel light-headed after the glucose drink. The team at Eureka Health advises, “Plan like you would for a long flight—comfortable, hydrated, and with something to do.”

  • Pack a protein-rich snack for after the final blood drawA cheese stick or handful of nuts stabilizes post-test blood sugar and curbs nausea within 15 minutes.
  • Bring water and drink 250 mL per hour while waitingAdequate hydration improves venous access and reduces failed blood draws by 12 %.
  • Wear loose-sleeved clothingMultiple venipunctures are easier when sleeves roll above the elbow.
  • Schedule the test earlyStarting at 7 a.m. reduces fasting time and the chance you accidentally eat.
  • Plan quiet activitiesReading or listening to music keeps you calm; anxiety raises epinephrine and glucose.
  • Confirm all prescription and over-the-counter drugs with the staff before the first blood drawSt Vincent’s Pathology stresses notifying the collector about every medication on test morning because agents like corticosteroids or diuretics can artificially raise glucose, and a dose may need to be postponed for accurate results. (SVP)
  • Skip smoking, gum, or candy after midnight to keep fasting levels trueBedford Commons advises no smoking, chewing gum, or hard candy once fasting begins; even small amounts of nicotine or sugar can alter the baseline reading and invalidate the test. (BCOG)

References

Which drugs, supplements, or recent labs can interfere with OGTT results?

A short list of substances can falsely elevate or lower glucose values. Sina Hartung, MMSC-BMI notes, “Knowing what to pause—or not pause—should always be decided with your prescriber; never stop medication on your own.”

  • High-dose vitamin C lowers lab-measured glucoseDoses above 2 g/day can cause a 10 % under-reading on some analyzers.
  • Thiazide diuretics raise fasting glucoseHydrochlorothiazide 25 mg can increase fasting levels by 5–15 mg/dL within one week.
  • Biotin supplements interfere with immunoassaysBiotin 10 mg can produce falsely low insulin values, altering curve interpretation.
  • Recent contrast CT scans may transiently impair kidneysIf serum creatinine is high, the lab may postpone the glucose load to avoid osmotic stress.
  • Systemic steroids and β-blockers can inflate glucose readingsThe RUH NHS OGTT protocol recommends pausing glucocorticoids and β-blockers for 3 days beforehand because they acutely raise blood glucose and may exaggerate impaired tolerance. (RUH NHS)
  • Metformin may hide dysglycaemia if not withheldNational Jewish Health instructs patients to discontinue metformin one week before an OGTT; its insulin-sensitizing effect can lower results and mask abnormalities. (NJH)

References

How can Eureka’s AI doctor guide you before your OGTT appointment?

The Eureka app uses your health profile to create a personalized countdown checklist. According to the team at Eureka Health, “Users who run the OGTT prep checklist are 32 % less likely to have invalid tests.”

  • Automated reminders match your lab’s specific fasting windowPush notifications prompt you when to stop food, fluids other than water, and certain medicines.
  • Real-time chat for medication questionsAsk whether to delay metformin; the AI provides evidence links and tags a physician for review if needed.
  • Symptom tracking during the waitLogging dizziness or nausea helps the system flag if assistance is needed.

What happens after the test and how can Eureka help you act on the results?

Once your glucose curve is back, Eureka’s AI doctor can explain each time-point value, propose follow-up A1c testing, and draft lifestyle plans. Women using Eureka for gestational glucose issues rate the service 4.8 out of 5 stars for clarity and support.

  • Instant result interpretationUpload your lab PDF; the AI overlays normal, impaired, and diabetic thresholds in color.
  • Personalized action planIf 2-hour glucose is 155 mg/dL, the app suggests dietitian referral and a 3-month re-test, pending clinician approval.
  • Secure data sharing with your clinicianYou control whether your doctor receives the summary via encrypted link.
  • Ongoing remindersThe app schedules quarterly A1c or repeat OGTT according to ADA guidelines.

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Frequently Asked Questions

Can I take my regular morning thyroid pill before the OGTT?

Yes, levothyroxine is not known to alter glucose levels, but swallow it with plain water at least 30 minutes before the test drink.

Is black coffee without sugar really off-limits?

Yes. Even without calories, caffeine increases adrenaline and can raise glucose, so stick to water.

What if I accidentally ate a small snack 6 hours before my appointment?

Call the lab. They will likely reschedule, as anything but water within 8 hours can invalidate the baseline sample.

Do I need someone to drive me home after the test?

Most people feel fine, but if you have a history of hypoglycemia or are pregnant, arranging a ride is wise.

How many blood samples will be taken?

Typically one fasting draw, then samples at 30, 60, 90, and 120 minutes; some pregnancy protocols add a 3-hour sample.

Will the 75-gram glucose drink make me sick?

About 10 % feel mild nausea; sipping slowly over 5 minutes and staying seated helps.

Can I chew sugar-free gum during the waiting period?

Avoid it. Chewing stimulates insulin release and some gums contain sorbitol, which may affect readings.

How soon will I get my results?

Most labs report OGTT results within 24–48 hours, but gestational panels may come back the same day.

Is an OGTT safe if I have bariatric surgery history?

Yes, but tell your clinician; they may use a modified 50-gram load to prevent dumping syndrome.

This content is for informational purposes only and is not intended as medical advice. Always consult with a qualified healthcare provider for diagnosis, treatment, and personalized medical recommendations.

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