Glucose Tolerance Tests

Glucose Tolerance Tests is a topic covered in the Davis's Lab & Diagnostic Tests.

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Synonym/Acronym:
Standard oral tolerance test, standard gestational screen, standard gestational tolerance test, GTT.

Rationale
To evaluate blood glucose levels to assist in diagnosing diabetes.

Patient Preparation
There are no activity or medication restrictions unless by medical direction; additionally, there are no fluid restrictions prior to the gestational screen (unless by medical direction). Instruct the patient to fast for at least 8 hr before the standard oral and standard gestational GTTs and not to consume any caffeinated products or chew any type of gum before specimen collection for the test; these factors are known to elevate glucose levels.

Normal Findings
Method: Spectrophotometry.

Standard Oral Glucose Tolerance (Up to 75-g Glucose Load)Conventional UnitsSI Units (Conventional Units × 0.0555)
Nondiabetic, fasting sampleLess than 100 mg/dLLess than 5.6 mmol/L
Nondiabetic, 2-hr sampleLess than 140 mg/dLLess than 7.8 mmol/L
Prediabetes, fasting sample100–125 mg/dL5.6–6.9 mmol/L
Prediabetes, 2-hr sample140–199 mg/dL7.8–11 mmol/L
Plasma glucose values are reported to be 10% to 20% higher than serum values. A diagnosis of diabetes is made when the fasting glucose is equal to or greater than 126 mg/dL (7 mmol/L) or the 2-hr sample is equal to or greater than 200 mg/dL (11.1 mmol/L).
Tolerance Tests for Gestational Diabetes
One-Step Approach (75-g Glucose Load)Conventional and SI Units (SI = Conventional Units × 0.0555)
Fasting sampleLess than 93 mg/dL (SI: Less than 5.1 mmol/L)
 1-hr sampleLess than 181 mg/dL (SI: Less than 10 mmol/L)
 2-hr sampleLess than 154 mg/dL (SI: Less than 8.5 mmol/L)
Plasma glucose values are reported to be 10% to 20% higher than serum values. A diagnosis of gestational diabetes is made when any of the three thresholds are met or exceeded.
Tolerance Tests for Gestational Diabetes
Two-Step ApproachConventional and SI Units (SI = Conventional Units × 0.0555)
Step 1Standard Gestational Screen (50-g Glucose Load)
1-hr sample performed while the patient is not fastingLess than 141 mg/dL; if results exceed 140 mg/dL, the 100-g GTT should be performed (American College of Obstetricians and Gynecologists [ACOG])Less than 7.8 mmol/L; if results exceed 7.7 mmol/L, the 75-g GTT should be performed
Step 2Gestational GTT (100-g Glucose Load)Carpenter and CoustanNational Diabetes Data Group
Fasting sampleLess than 95 mg/dL (SI: Less than 5.3 mmol/L)Less than 105 mg/dL (SI: Less than 5.8 mmol/L)
 1-hr sampleLess than 180 mg/dL (SI: Less than 10 mmol/L)Less than 190 mg/dL (SI: Less than 10.5 mmol/L)
 2-hr sampleLess than 155 mg/dL (SI: Less than 8.6 mmol/L)Less than 165 mg/dL (SI: Less than 9.2 mmol/L)
 3-hr sampleLess than 140 mg/dL (SI: Less than 7.8 mmol/L)Less than 145 mg/dL (SI: Less than 8 mmol/L)
Plasma glucose values are reported to be 10% to 20% higher than serum values.GTT = glucose tolerance test.A diagnosis of gestational diabetes is made when two or more of the four thresholds are met or exceeded.

Critical Findings and Potential Interventions

Glucose


Adults & Children
  • Less than 40 mg/dL (SI: Less than 2.22 mmol/L)
  • Greater than 400 mg/dL (SI: Greater than 22.2 mmol/L)

Newborns
  • Less than 32 mg/dL (SI: Less than 1.8 mmol/L)
  • Greater than 328 mg/dL (SI: Greater than 18.2 mmol/L)

Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.

Consideration may be given to verification of critical findings before action is taken. Policies vary among facilities and may include requesting immediate recollection and retesting by the laboratory or retesting using a rapid point-of-care testing instrument at the bedside, if available.

Symptoms of decreased glucose levels include headache, confusion, polyphagia, irritability, nervousness, restlessness, diaphoresis, and weakness. Possible interventions include oral or IV administration of glucose, IV or intramuscular injection of glucagon, and continuous glucose monitoring.

Symptoms of elevated glucose levels include abdominal pain, fatigue, muscle cramps, nausea, vomiting, polyuria, polyphagia, and polydipsia. Possible interventions include fluid replacement in addition to subcutaneous or IV injection of insulin with continuous glucose monitoring.

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