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Blood Test Tracker

Glucose, OGTT 3-Hour

Blood glucose three hours after a 100 g sugar drink. It is the last value of the 3-hour glucose tolerance test used in pregnancy.

AI-verified against cited sources. Awaiting expert reviewAlso called 3h-PG, OGTT-3hLOINC 1530-5

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What it is

In pregnancy, one common way to check for gestational diabetes has two steps. First comes a 1-hour screening test with a 50 g sugar drink. If that is high, you do a longer fasting test with a 100 g drink and blood draws before the drink and at 1, 2 and 3 hours. This result is the last sample. A 75 g test can also include a 3-hour sample, but the standard 75 g test used outside pregnancy stops at 2 hours and no widely used cutoffs exist for a 3-hour value.

Why it matters

In the two-step approach the American Diabetes Association (ADA) diagnoses gestational diabetes when at least two of the four values meet or exceed their thresholds. The 3-hour value is one of the four. Gestational diabetes can affect the health of both parent and baby, so catching it lets the care team plan monitoring and next steps. A single high value is not the whole picture, and your clinician reads all four together.

If it’s low

By three hours after the sugar drink, glucose usually settles back toward where it started, and in some people it can dip below it. A low number on its own is not part of the gestational diabetes criteria. A dip with shakiness, sweating or confusion is worth telling your clinician about. A sample that sat too long before testing can also read falsely low.

Common causes

  • A drop in glucose after the sugar load (sometimes called reactive hypoglycemia)
  • A sample that sat for hours before the blood cells were separated
  • A very long fast before the test, or heavy alcohol use
  • Liver or kidney disease
  • Underactive adrenal glands or certain pituitary conditions
  • Rarely, a pancreas tumor that makes too much insulin

What can help

  • If you had symptoms during the test, tell the person doing it and your clinician
  • Ask whether more testing is needed to understand a low result
  • Ask whether the sample handling or timing could explain it

If it’s high

On the 100 g test in pregnancy, a 3-hour result of 140 mg/dL (7.8 mmol/L) or higher meets one of the four Carpenter-Coustan thresholds the ADA lists. Some labs use the older NDDG limit of 145 mg/dL (8.1 mmol/L). The ADA diagnoses gestational diabetes when at least two of the four values meet their thresholds, and the American College of Obstetricians and Gynecologists notes that one elevated value can be used. A high number can also have everyday explanations, such as not fasting or illness.

Common causes

  • Gestational diabetes
  • Prediabetes or diabetes that was not known before pregnancy
  • Not fasting, or not following the test preparation
  • Illness, infection or other stress on the body
  • Some medicines, such as corticosteroids

What can help

  • Ask your clinician how all four values fit together and what the next steps are
  • Ask which glucose goals apply to you during pregnancy
  • Regular physical activity and balanced meals are the main lifestyle levers clinicians discuss for glucose that runs high

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

The usual range depends on sex. Choose women or men to see the one that applies.

What can affect your result

  • Which glucose load was used: the 3-hour thresholds are set for the 100 g drink
  • Fasting for at least 8 hours beforehand
  • How far along the pregnancy is
  • Illness, stress, surgery or injury
  • Medicines, including corticosteroids
  • Sample handling: unspun blood loses glucose as cells use it
  • Not fasting for at least 8 hours beforehand, or eating or drinking anything besides water during the 3-hour test, can change results.
  • The 100 g load is the one the 3-hour pregnancy thresholds were set for. A 75 g or 50 g drink gives results that are not interchangeable.
  • Illness, infection, surgery and other physical stress raise glucose, as can some medicines (for example corticosteroids).
  • Vomiting the drink can make the result unreliable.
  • A sample that sits before the blood cells are separated can read falsely low, because the cells use up glucose in the tube.

Morning, after an overnight fast of at least 8 hours (ADA). Sample taken exactly 3 hours after the drink. Reflects test day only. For a 2 to 3 month average, look at A1c.

When to talk to a clinician

If your result meets a threshold and you haven't heard from your clinician, call to ask about next steps. Contact the clinician who ordered the test today if the lab flagged the result as critical. Call 911 or your local emergency number for confusion, fainting or seizures, or for very high glucose with vomiting, belly pain, fast breathing or extreme drowsiness.

Questions you could ask

  • Was this a 100 g test, and was I fasting properly?
  • Which criteria does my lab use, and how do all four values fit together?
  • What are the next steps, and which glucose goals apply to me?
  • Could the sample handling or timing explain a low result?
  • Do I need any follow-up tests?

Questions people ask

What if only one of my four values is high?

The ADA lists the Carpenter-Coustan criteria as needing at least two of the four values (fasting, 1, 2 and 3 hours) at or above their thresholds, while the American College of Obstetricians and Gynecologists notes that one elevated value can be used. Ask your clinician which approach they follow and what it means for you.

Why is there a 3-hour value on the 100 g test but not the 75 g test?

The two tests come from different research. The 75 g test is based on the HAPO study and ends at 2 hours. The 100 g test goes back to older work and keeps the 3-hour sample. They use different thresholds, so results are not interchangeable.

Does my lab use 140 or 145 mg/dL for the 3-hour value?

It depends on the criteria. Carpenter-Coustan uses 140 mg/dL (7.8 mmol/L), and the older NDDG uses 145 mg/dL (8.1 mmol/L). The ADA suggests the lower Carpenter-Coustan limits when the two-step approach is used. Your report or clinician can tell you which your lab applies.

Show the science and sources

Units

  • 1 mg/dL = 1 mg/dL
  • 1 mg/dL = 0.05551 mmol/L
  • 1 mg/dL = 0.01 g/L

Open the OGTT 3-Hour Glucose unit converter

For clinicians and researchers

Venous plasma or serum glucose at 180 minutes of a fasting 100 g OGTT. ADA 2026 (Section 2), two-step strategy: step 1 is a nonfasting 50 g glucose load test with a 1-hour result of 130, 135 or 140 mg/dL (ACOG allows any of these) triggering step 2; step 2 is the fasting 100 g OGTT, with gestational diabetes diagnosed when at least two of fasting 95, 1-hour 180, 2-hour 155 and 3-hour 140 mg/dL (5.3, 10.0, 8.6, 7.8 mmol/L) are met or exceeded (Carpenter-Coustan). ACOG notes one elevated value can be used, and accepts either Carpenter-Coustan or NDDG thresholds. NDDG (1979) venous plasma thresholds are 105, 190, 165 and 145 mg/dL. Both sets are numerical conversions of the original O'Sullivan and Mahan (1964) whole-blood values, and the ADA says the lower Carpenter-Coustan thresholds appear advantageous when the two-step approach is used. The 75 g one-step strategy ends at 2 hours and has no 3-hour criterion. No standard reference interval exists for a 3-hour sample outside pregnancy.

LOINC 1530-5 (primary), 14764-5, 1533-9, 32320-4. Specimen: serum-or-plasma.

Sources

  1. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026, American Diabetes Association (Diabetes Care) (2026)
  2. Review of the Screening Guidelines for Gestational Diabetes Mellitus: How to Choose Wisely, Indian Journal of Community Medicine (2023)
  3. Glucose (PubChem compound 5793), PubChem, National Library of Medicine
  4. Critical Values List (CORP-APPEND-0104A, Rev. 46), ARUP Laboratories (2026)
  5. Critical (Panic) Values, with adult reference intervals (document DX_TL_811850, 01/19/2024), Labcorp (2024)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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