Page 579 - Concise Pathology for Exam Preparation ( PDFDrive )

564 SECTION II Diseases of Organ Systems

• In the fasting state, glucose levels in arterial and venous blood are similar. However,
postprandially arterial and capillary bloods have glucose levels about 20 mg/dL higher
than venous blood. This is because extraction of glucose by tissues in presence of insu-
lin gets elevated in response to nutrient absorption in gastrointestinal tract.
• Quantitative determinations of glucose are based on a variety of chemical and enzymatic
methods. The older chemical methods are less specific. Newer enzymatic methods (glucose
oxidase, hexokinase) for glucose analysis are highly specific.
• In whole blood, clotted and kept at room temperature, glucose disappears at a rate of
approximately 7% per hour owing to ongoing glycolytic activity of leukocytes and red
cells. It is thus preferable to collect blood in tubes containing fluoride—a strong in-
hibitor of glycolysis as well as citrate (acidity) to immediately inhibit glycolysis (grey-
stoppered vacutainer tubes).
Oral Glucose Tolerance Test (OGTT)
This test is intended to measure capability and timely response of the insulin-secreting
cells to integrated signals provided by GI hormones and rising blood glucose levels.

Patient Preparation
Put the patient for 3 days or more on a normal diet including at least 150 g of carbohy-
drates per day. In morning after an overnight fast, 75 or 100 g of an aqueous solution of
glucose is given.
Steps
1. The patient, who should have been taking an unrestricted carbohydrate diet for at least
3 days or more prior to the test, fasts overnight (at least 8 h).
2. The patient should rest for at least half an hour before starting the test. A sample of
blood is drawn to estimate the glucose level.
3. A glucose load of 75 g dissolved in 300 mL of water is given orally.
4. Blood samples are withdrawn at half-hourly intervals for 2 h (½ h, 1 h, 1½ h and 2 h)
and glucose levels are estimated.
Interpretation
1. Normal (in nonpregnant adult)
(a) Fasting value: ,95 mg/dL
(b) Value at 1 h: ,180 mg/dL
(c) Value at 2 h: ,155 mg/dL
(d) Value at 3 h: ,140 mg/dL
2. Indicative of impaired glucose tolerance (IGT)
(a) Fasting value: 110–126 mg/dL
(b) At least one of the values at 30, 60 or 90 min .200 mg/dL and value at 120 min
between 140 and 200 mg/dL
3. Indicative of diabetes mellitus
(a) If the fasting glucose determination revealed diabetic values (.126 mg/dL), the
OGTT should not be performed
4. If the fasting glucose fell into the IGT range (110–126 mg/dL) and an OGTT is per-
formed, the results are indicative of diabetes mellitus if two or more of venous plasma
concentrations are reached or exceeded.
5. The criteria for diagnosis of diabetes during pregnancy (gestational diabetes) are stricter
than outlined above for nonpregnant adults. This is because even mild diabetes during
pregnancy becomes a significant risk factor for fetal morbidity and mortality. Thus, the
OGTT is performed with 100 g of glucose, and it indicates gestational diabetes when
two or more of the following values (in mg/dL) are reached or exceeded: fasting 110,
1 h 190, 2 h 165, 3 h 145.

2-h Postprandial Plasma Glucose
This determination has no standardized role for diagnostic purposes. It is, however, often
valuable when attempting to optimize patients’ treatment. Normally, 120-min values are
below 140 mg/dL.

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