Oral Glucose Tolerance Test Results in Diabetes
In a patient with diabetes, the oral glucose tolerance test (OGTT) will show a 2-hour plasma glucose level ≥200 mg/dL (11.1 mmol/L) after a 75-gram glucose load. 1
Diagnostic Threshold for Diabetes
The OGTT diagnostic criteria for diabetes mellitus are clearly defined:
- 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) after ingestion of 75 grams of anhydrous glucose dissolved in water 1
- The test must be performed following World Health Organization standards with an 8-hour fast prior to glucose administration 1
- Blood is drawn at baseline (fasting) and 2 hours after the glucose load 1
Confirmation Requirements
A single OGTT result showing 2-hour glucose ≥200 mg/dL requires confirmation with repeat testing on a separate day, unless the patient presents with unequivocal hyperglycemia or classic diabetic symptoms. 1
- If two separate OGTT results both show 2-hour glucose ≥200 mg/dL, diabetes is confirmed 1
- Alternatively, if the OGTT and a different test (such as fasting plasma glucose ≥126 mg/dL or A1C ≥6.5%) are both abnormal, diabetes is confirmed even from the same sample 1
- A single abnormal OGTT combined with classic symptoms (polyuria, polydipsia, weight loss) and random glucose ≥200 mg/dL confirms diabetes without additional testing 1
Clinical Context and Test Characteristics
The OGTT is considered a reference standard diagnostic test for diabetes, though it has important limitations 1:
- Poor reproducibility is a significant concern—studies show only 50% reproducibility of abnormal results on repeat testing 2
- The test requires strict patient compliance with 8-hour fasting and 2-hour waiting period 1
- Day-to-day variance in glucose values can affect results 1
Alternative Diagnostic Methods
When OGTT cannot be completed or is impractical, alternative diagnostic criteria include 1:
- Fasting plasma glucose ≥126 mg/dL (7.0 mmol/L) on two separate occasions
- A1C ≥6.5% using NGSP-certified laboratory method on two occasions
- Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with classic hyperglycemic symptoms
Important Clinical Pitfalls
The OGTT should not be used in conditions with increased red blood cell turnover (sickle cell disease, pregnancy second/third trimesters, glucose-6-phosphate dehydrogenase deficiency, hemodialysis, recent blood loss/transfusion, erythropoietin therapy), as these conditions invalidate glucose measurements—only plasma glucose criteria should be used in these patients 1
If a patient develops significant nausea or vomiting during the OGTT, the test must be stopped immediately and rescheduled, as partial glucose ingestion invalidates the results since diagnostic thresholds are calibrated specifically to the complete 75-gram load 3
The test cannot be interpreted with incomplete glucose ingestion, and forcing completion during gastrointestinal distress compromises both patient safety and test validity 3