What is the difference between the GCT and the OGTT?

The 1-hour 50g glucose challenge test (GCT) is a screening test — a positive result (typically ≥130 or ≥140 mg/dL depending on the lab) means you need further testing. The 3-hour 100g OGTT is the diagnostic test; gestational diabetes is confirmed when 2 or more of the 4 values meet or exceed the threshold.

Which criteria should I use — Carpenter-Coustan or NDDG?

The American Diabetes Association (ADA) endorses the Carpenter-Coustan thresholds, which are slightly lower and therefore catch more cases. The National Diabetes Data Group (NDDG) thresholds are older and slightly higher. Your care provider or lab report will state which set was used.

What GCT cutoff is considered positive?

Most centers use ≥140 mg/dL as the cutoff for a positive 1-hour GCT, though some use ≥130 mg/dL to increase sensitivity. This tool lets you enter your provider's specific cutoff so you can see where your value falls.

Can I have gestational diabetes if only one OGTT value is high?

By the Carpenter-Coustan and NDDG criteria, a diagnosis of gestational diabetes requires 2 or more elevated values on the 3-hour OGTT. One elevated value is not diagnostic, though many providers will still recommend dietary changes.

Are these values in mg/dL or mmol/L?

This tool uses mg/dL, which is the standard unit in the United States. If your lab reports values in mmol/L, multiply by 18 to convert (e.g., 7.8 mmol/L × 18 = 140 mg/dL), or use a unit converter first.

Does this tool diagnose gestational diabetes?

No. This tool provides an educational interpretation based on published thresholds. Only a licensed clinician can make a clinical diagnosis. Always discuss your results with your care provider.

GTT Glucose Screening Interpreter

Name: GTT Glucose Screening Interpreter
Availability: InStock
Author: Nham Vu

Enter your 1-hour GCT or 3-hour OGTT glucose values to get an immediate interpretation using Carpenter-Coustan or NDDG diagnostic criteria.

Test Settings

Test Type

Positive Cutoff (mg/dL)

Or type your provider's custom cutoff

1-Hour Glucose (mg/dL)

For educational use only. Not a substitute for clinical evaluation. Always discuss results with your healthcare provider.

Enter your values and press Interpret

Results and threshold comparison will appear here.

Summary

Enter your 1-hour GCT or 3-hour OGTT glucose values to get an immediate interpretation using Carpenter-Coustan or NDDG diagnostic criteria.

How it works

Choose whether you are interpreting a 1-hour GCT or a 3-hour OGTT.
Select your diagnostic criteria: Carpenter-Coustan (ADA preferred) or NDDG (older standard).
Enter glucose values in mg/dL for the time points that apply to your test.
The tool compares each value against the threshold for your chosen criteria.
A summary shows which values are elevated and whether the result is positive or negative.
For the 3-hour OGTT, gestational diabetes is diagnosed when 2 or more values meet or exceed the thresholds.

Use cases

Understand a 1-hour glucose challenge result before your follow-up appointment.
Determine how many values on a 3-hour OGTT were above threshold.
Compare Carpenter-Coustan and NDDG thresholds side by side for your values.
Prepare informed questions for your OB or midwife after receiving lab results.
Cross-check results when switching care providers using different criteria sets.
Educate patients on which specific time-point values triggered a positive screen.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu