Ogtt test

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Here is a thorough overview of the OGTT (Oral Glucose Tolerance Test) based on authoritative medical textbooks:

Oral Glucose Tolerance Test (OGTT)

What Is It?

The OGTT is a serial measurement of plasma glucose before and after a specific amount of oral glucose. It provides a standardized method to evaluate how efficiently the body clears a glucose load. Although more sensitive than fasting plasma glucose (FPG), it has poor reproducibility and is affected by many non-diabetic factors, which limits its routine clinical use.

Pre-Test Conditions (MANDATORY)

Before performing an OGTT, all of the following conditions must be met:

Discontinue (where possible) medications known to affect glucose tolerance
3 days of unrestricted diet with at least 150 g of carbohydrate/day
Fast for 10-16 hours prior to the test
Ambulatory outpatients only - bed rest impairs glucose tolerance
Patient must remain seated and not smoke during the test
Test should begin between 7:00 AM and 9:00 AM
Not performed on hospitalized, acutely ill, or inactive patients

- Tietz Textbook of Laboratory Medicine, 7th Ed., p. 1732

Glucose Dose

Population	Dose
Non-pregnant adults	75 g anhydrous glucose in 300 mL water, ingested over 5 minutes
Children	1.75 g/kg body weight, up to a maximum of 75 g
Gestational diabetes (2-step)	100 g over 3 hours

Diagnostic Thresholds (Non-Pregnant Adults - 75 g OGTT)

Using the 2-hour plasma glucose value:

Category	2-Hour Plasma Glucose
Normal	< 140 mg/dL (< 7.8 mmol/L)
Prediabetes / IGT	140-199 mg/dL (7.8-11.0 mmol/L)
Diabetes Mellitus	≥ 200 mg/dL (≥ 11.1 mmol/L)

- Quick Compendium of Clinical Pathology, 5th Ed.

Abnormal diagnostic tests should be repeated before a diagnosis is established, unless two different diagnostic methods are both abnormal.

Four Routes to Diagnose Diabetes

Method	Prediabetes	Diabetes
FPG (≥8h fast)	100-125 mg/dL	≥ 126 mg/dL
OGTT 2h glucose	140-199 mg/dL	≥ 200 mg/dL
HbA1c	5.7%-6.4%	≥ 6.5%
Random glucose + symptoms	-	≥ 200 mg/dL

- Quick Compendium of Clinical Pathology, 5th Ed.

Factors Affecting OGTT Results (Non-Diabetic Causes)

Patient Preparation:

Duration of fast
Prior carbohydrate intake
Medications (thiazides, oral contraceptives, corticosteroids)
Trauma, intercurrent illness, age, activity, weight

During the Test:

Posture, anxiety, caffeine, smoking, activity, time of day
Sample preservation

Administration:

Form of glucose (anhydrous vs. monohydrate)
Quantity ingested, volume of solution, rate of ingestion

- Tietz Textbook of Laboratory Medicine, 7th Ed.

Clinical Indications for OGTT

OGTT is rarely used routinely in the US for diagnosing diabetes (FPG and HbA1c are preferred), but it IS indicated for:

Gestational Diabetes Mellitus (GDM) - primary indication
Postpartum screening of women who had GDM (screening for type 2 DM)
Impaired Glucose Tolerance (IGT) diagnosis - these patients have increased cardiovascular risk but may not have IFG by ADA criteria
Unexplained nephropathy, neuropathy, or retinopathy when random glucose is < 140 mg/dL
Epidemiologic/population studies

OGTT for Gestational Diabetes (GDM)

Two approaches are used at 24-28 weeks gestation:

1-Step Approach (75 g, IADPSG/ADA criteria)

After an overnight fast of ≥8 hours - diagnose GDM if any 1 threshold is met or exceeded:

Timepoint	Threshold
Fasting	≥ 92 mg/dL (5.1 mmol/L)
1 hour	≥ 180 mg/dL (10.0 mmol/L)
2 hours	≥ 153 mg/dL (8.5 mmol/L)

2-Step Approach (50 g screen → 100 g confirmatory, ACOG criteria)

Step 1: 50 g glucose to a non-fasting patient; measure at 1 hour. If glucose ≥ 135-140 mg/dL (depending on cut-off desired), proceed to Step 2.
Step 2: 100 g glucose after overnight fast; diagnose GDM if any 2 of these values (Carpenter & Coustan criteria) are met or exceeded:

Timepoint	C&C Criteria
Fasting	95 mg/dL
1 hour	180 mg/dL
2 hours	155 mg/dL
3 hours	140 mg/dL

- Quick Compendium of Clinical Pathology, 5th Ed.; Creasy & Resnik's Maternal-Fetal Medicine

Limitations of OGTT

Poor reproducibility - approximately 20% of OGTTs fall into a non-diagnostic category
Requires strict patient preparation (fasting, diet, activity)
Time-consuming (2-3 hours in clinic)
Can only be performed in ambulatory outpatients
Affected by many non-diabetic factors (see table above)
Some organizations recommend repeating the test on 2 separate occasions unless results are grossly abnormal

Summary

The OGTT remains the gold standard for diagnosing impaired glucose tolerance and gestational diabetes, though FPG and HbA1c are more practical for routine diabetes screening. Its key cut-off for diabetes in non-pregnant adults is a 2-hour glucose ≥ 200 mg/dL after a 75 g glucose load.

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