Function

• Glucose testing is typically used to rule out diabetes and pre-diabetes. Patients should be tested regularly if diabetes runs in the family, they are overweight, or over the age of 40, says LabTestsOnline.org.
  
  

Procedure

• Most glucose tests are performed after an eight to 10-hour fast, reports LabTestsOnline.org. If you are diabetic, your doctor may schedule a test immediately after a meal as well.
  
  

Hyperglycemia

• Hyperglycemia---extremely high blood sugar---usually indicates diabetes. Moderately high levels may indicate pre-diabetes. Elevated blood sugar can also stem from stress, chronic renal failure, pancreatic cancer, extreme food intake, or drugs such as lithium, estrogens, and antidepressants, says LabTestsOnline.org.
  
  

Hypoglycemia

• According to LabTestsOnline.org, low glucose levels---hypoglycemia---could be from adrenal insufficiency, extensive liver disease, hypothyroidism or insulin overdose. Excessive use of alcohol, acetaminophen or anabolic steroids can also skew test results.
  
  

Considerations

• Obstetricians typically recommend women 24 to 28 weeks pregnant have a one-hour glucose-challenge test, to identify gestational diabetes. Left untreated, it can result in high birth weight and/or low glucose levels for the baby.