OGTT

Also seen on reports as: Glucose Tolerance Test, GTT, 75 g OGTT

What is OGTT?

The Oral Glucose Tolerance Test (OGTT) measures how the body handles a fixed amount of sugar over time. The person comes to the lab fasting, has a baseline blood draw, drinks a standard glucose solution (usually 75 grams of glucose dissolved in 250–300 ml of water), and then has blood drawn again at fixed intervals — most commonly at 1 hour and 2 hours after the drink.

It is more rigorous than a fasting or post-meal sugar test because the sugar load is identical for everyone, which removes variability from differently sized meals. This makes it useful when other tests have given borderline or contradictory results.

In pregnancy, the OGTT is the primary test used to diagnose gestational diabetes — a separate set of thresholds applies during pregnancy.

Why doctors order it

OGTT is most often ordered when fasting sugar or HbA1c results are borderline and the doctor wants a clearer picture of how the body handles sugar. It is also used when symptoms (frequent urination, unusual thirst, unexplained weight change) suggest diabetes but the simpler tests have been normal.

In pregnancy, OGTT is part of the standard antenatal package in India and is recommended between weeks 24–28 of pregnancy by most Indian obstetric guidelines (FOGSI / DIPSI). Some women considered higher-risk are tested earlier as well.

OGTT is also occasionally used in research and specialist settings — for example, to evaluate reactive hypoglycaemia, to characterise prediabetes in people with PCOS, or as part of the work-up for unexplained nerve or kidney findings that might be early diabetes complications.

What the result means

Outside pregnancy, the 2-hour value after the 75 g glucose drink is the main number that determines whether the body has impaired sugar handling. The fasting value (drawn at the start) is interpreted using the same cut-offs as a regular FBS test.

In pregnancy, Indian DIPSI (Diabetes in Pregnancy Study Group India) criteria use a non-fasting 75 g 2-hour test with stricter thresholds. Several Indian obstetric centres also follow IADPSG criteria, which use fasting plus 1-hour and 2-hour values. The lab report and the obstetrician will clarify which framework was used.

The OGTT is not a perfect test on its own — values can shift between repeats, and the result is influenced by what the person ate in the days leading up to the test. It is most useful when interpreted by the doctor alongside other tests and the clinical picture.

Common factors that affect the result

OGTT results are sensitive to what the person ate in the days before the test. Several Indian and international guidelines suggest eating a normal carbohydrate intake (not very low and not very high) for at least three days before the test. A very low-carbohydrate diet before OGTT can falsely raise the 2-hour value because the body becomes temporarily less efficient at handling a sudden sugar load.

Acute illness, fever, recent surgery, steroid courses, and unusual stress can all distort OGTT results. So can certain medications — particularly steroids, some psychiatric medications, beta-blockers, and some diuretics. Patients are usually asked to mention all current medications before the test.

The test requires staying seated and not eating, smoking, chewing gum, or doing strenuous activity during the test window. Walking around or leaving the lab between draws can affect the readings.

When to talk to your doctor

• Your fasting value is in the prediabetes range, OR your 2-hour value is 140 mg/dL or higher — either is reason to follow up.
• Your 2-hour value is 200 mg/dL or higher — a repeat OGTT or pairing with HbA1c is typically advised before any diagnosis is made.
• You are pregnant and your OGTT shows any value above the pregnancy thresholds — gestational diabetes is treatable and follow-up matters.
• You experienced symptoms during the test itself — severe nausea, vomiting, fainting, or unusually low sugar afterwards — these should be reported.
• You have PCOS, a strong family history of diabetes, or have had gestational diabetes in a previous pregnancy — the OGTT thresholds for action may be stricter for you.

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