Fasting Plasma Glucose


The fasting plasma glucose (or fastingblood sugar) test is used to measure plasma glucose levels after a fast of at least 8 hours. This test is commonly used to screen fordiabetes mellitus and prediabetes, in which absence of deficiency of insulin allows persistently high glucose levels.

  • To screen for diabetes mellitus and prediabetes.
  • To monitor drug or diet therapy in the patient with diabetes mellitus.
  • To monitor for hyperglycemia and hypoglycemia.
  1. Explain to the patient that this test detects disorders of glucose metabolism and aids in the diagnosis of diabetes.
  2. Tell the patient that the test requires a blood sample. Explain who will perform the venipuncture and when.
  3. Explain to the patient that he may experience slight discomfort from the tourniquet and needle puncture.
  4. Instruct the patient to fast for 12 to 14 hours before the test.
  5. Notify the laboratory and physician of medications the patient is taking that may affect test results; it may be necessary to restrict them.
  6. Alert the patient to the symptoms of hypoglycemia such as weakness, restlessness, nervousness, hunger, and sweating and tell him to report such symptoms immediately.
  1. Perform a venipucture and collect the sample in a 5-ml clot-activator tube.
  2. Send the sample to the laboratory immediately.
  3. Note on the laboratory results when the patient last ate, when the sample was collected, and when the patient received the last pretest dose of insulin or oral antidiabetic drug (if applicable).
Nursing Interventions
  1. Apply direct pressure to the venipucture site until bleeding stops.
  2. Provide a balanced meal or a snack.
  3. Instruct the patient that he may resume his usual medications that were stopped before the test.
Normal Results
  • Results vary according to the laboratory procedure.
  • After at least an 8-hour fast, 70 to 100 mg of true glucose per deciliter of blood (SI, 3.9 to 5.6 mmol/L).
Abnormal Results

Elevated Levels

  • Diabetes mellitus (fasting plasma glucose levels of 126 mg/dL {SI, 7 mmol/L} or more obtained on two or more occasions).
  • Impaired fasting glucose or impaired glucose tolerance (levels ranging from 110-125 mg/dL).
  • Pancreatitis, recent acute illness (such as myocardial infarction), Cushing’s syndrome, acromegaly, and pheochromocytoma.
  • Acute stress
  • Hyperthyroidism
  • Pancreatic cancer
  • Hyperlipoproteinemia
Interfering Factors
  1. Recent illness, infection,, or pregnancy (possible increase).
  2. Glycolisis resulting from failure to refrigerate the sample or to send it to the laboratory immediately (possible false negative).
  3. Acetaminophen, if using the glucose oxidase or hexokinase method (possible false positive).
  4. Arginine, benzodiazepines, chlorthalidone, corticosteroids, and dextrothyroxine may cause an increase.
  5. Ethacrynic acid may cause hyperglycemia; large doses in patients with uremia can cause hypoglycemia.
  6. Alcohol, beta-adrenergic blockers, insulin, monoamine oxidase inhibitors, and oral antidiabetic agents (possible decrease).
  7. Stenous exercise (decrease)
  8. Drug interactions: numerous medications may alter blood glucose levels.
  • Hematoma at the venipuncture site.