Standard Operating Procedure

Subject Glucose Tolerance Test Index Number Lab-4075 Section Laboratory Subsection Chemistry - Automated Category Departmental Contact Amy VanLin Last Revised 10/15/2018

References Required document for Laboratory Accreditation by the College of American Pathologists (CAP), Centers for Medicare and Medicaid Services (CMS), and/or COLA.

Applicable To Employees of the Gundersen Health Systems laboratories, Gundersen Tri-County Hospital laboratories, Gundersen St. Joseph’s Health Services laboratories, Gundersen Palmer Lutheran Hospital laboratories and Gundersen Boscobel Area Hospital laboratories.

Detail Diabetes screening and diagnostic tests are simple procedures which give accurate laboratory results when performed correctly. They can be misleading if the patient is not properly prepared.

The concentration of plasma glucose is regulated within narrow limits by a fine balance between glucose production and utilization. The major hormones involved in this regulation are insulin, glucagon, glucocorticoids, epinephrine, and growth hormone. Insulin promotes glucose utilization and fat deposition while the other hormones stimulate glucose production and lipolysis. Within 30 minutes following the ingestion of a carbohydrate-containing meal, plasma glucose increases, and insulin is released from the beta cells of the islets of Langerhans. The insulin increases cellular glucose uptake and utilization and promotes the storage of glucose as liver glycogen. The subsequent secretion of the other hormones prevents an excessive fall of glucose and hypoglycemia does not occur.

The most common cause of hyperglycemia is genetic diabetes mellitus. Other etiologies include obesity, Cushing’s syndrome, acromegaly, hyperthyroidism, pheochromocytoma, hypokalemia, gastrectomy, starvation, chronic uremia, chronic liver disease, and drug-induced hyperglycemia.

The laboratory system uses ADA and WHO guidelines for routine diagnosis of Type 1 or Type 2 diabetes mellitus in non-pregnant patients. The ADA does not recommend the routine use of the oral glucose tolerance test (OGTT) for diagnosing either Type 1 or Type 2 diabetes, but does recommend the use of a fasting (at least 8 hours) glucose measurement.

WHO recommends the 2-hour OGTT only when the fasting glucose is between 110-126 mg/dL. Patient preparation for an OGTT must include 3 days of unrestricted diet and is conducted after an overnight fast (8-14 hours). The WHO recommended OGTT requires only two blood samples, a fasting glucose value and a two-hour post glucose (75 g) value.

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Standard Operating Procedure

A diagnosis of diabetes mellitus is made in a non-pregnant adult if the fasting glucose is equal to or greater than 126 mg/dL. A normal glucose is less than 140 mg/dL two hours after the glucose challenge. Impaired glucose tolerance is diagnosed when the two-hour value is between 140-199 mg/dL. A value at two hours equal to or greater than 200 mg/dL, if confirmed, is diagnostic of diabetes mellitus.

The use of a 3-hour or longer OGTT in routine practice for non-pregnant patients is not recommended by the ADA or WHO and is not available in our laboratory system.

SPECIMEN: Collection and Handling: Universal Precautions apply.

Plasma (preferred specimen) or serum (serum specimen must be separated immediately after clotting). The only acceptable anticoagulant is heparin. The stability of glucose in specimens is affected by storage temperature, bacterial contamination, and glycolysis. Plasma or serum samples should be separated from the cells or clot within an hour of being drawn. Specimens that cannot be separated from the cells within 30 minutes should be placed on ice or refrigerated.

When blood is drawn and stands uncentrifuged at room temperature, the average decrease in serum glucose is ~7% in 1 hour. This decrease is the result of glycolysis. The rate of in vitro glycolysis is higher in the presence of leukocytosis or in patients with increased hematocrits.

Stability (if separated from cells): 24 hours at 2-8oC in primary green top 3 days at 2-8oC if specimen poured off gel. Additional blood samples may be collected at the request of the ordering physician, however; no interpretation is available except for a standard GTT.

The patient must consume the appropriate dosage of Dexicola within a 5-minute period. If the patient vomits at any time between consuming the Dexicola and collection of the final specimen, the GTT will be discontinued. If possible, the provider can interpret based on the specimens already collected.

Blood specimens will be obtained from the patient at the following intervals: 3 Hour GTT for gestational diabetes: Fasting, 1, 2, 3 hours 2 Hour GTT: Fasting, 2 hours 1 Hour Post-Dexicola: 1 hour

Implementation PROCEDURE NOTES: N/A

CALCULATIONS: N/A

INTERPRETATION:

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Standard Operating Procedure

A. Non-pregnant adults: Oral glucose tolerance is not necessary if the patient has a fasting plasma glucose level of 126 mg/dL or greater. If the oral glucose is given, the diagnosis of diabetes mellitus is made if the 2-hour sample is 200 mg/dL or greater. B. Pregnant women 1. One hour dexicola screening test: Patient is given 50 grams dexicola. Collect the blood specimen one hour after ingestion. A glucose result of 135 mg/dL or greater should be followed up by a 3-hour glucose tolerance test. 2. Three hour glucose tolerance test: Following an oral glucose load of 100 grams, the diagnosis of gestational diabetes may be made if two plasma glucose values equal or exceed the following: Fasting 95 mg /dL 1 Hour 180 mg/dL 2 Hours 155 mg/dL 3 Hours 140 mg/dL C. Children: An oral glucose load of 1.75 g/kg ideal body weight up to 75 grams is used. The diagnosis of diabetes mellitus is made if the 2-hour sample is 200 mg/dL or greater.

Reporting Critical Results: Please refer to Lab-0130 Critical Call Values, Lab Reporting Protocol.

REVIEW AND CHANGES: This document and all attached forms should be reviewed optimally on an annual basis with 2 years as the maximum review date. Review will be done by the Technical Leader, Supervisor, Manager, Medical Director or designated person. Changes require retyping document or form and review by the Medical Director.

REFERENCES: 1. Clinical Chemistry 48:3 pages 436-472 (2002). 2. ACOG Practice Bulletin #137 (August 2013). 3. Gundersen Health System Provider Guidelines, Gestational Diabetes Mellitus (GDM), Pregnancy (January 2016).

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