This article is from the Interpretation Of Lab Test Profiles, by Ed Uthman firstname.lastname@example.org with numerous contributions by others.
Hyperglycemia can be diagnosed only in relation to time
elapsed after meals and after ruling out spurious influences
(especially drugs, including caffeine, corticosteroids,
estrogens, indomethacin, oral contraceptives, lithium,
phenytoin, furosemide, thiazides, thyroxine, and many more).
Generally, fasting blood glucose >140 mg/dL (7.8mmol/L) and/or
2h postprandial glucose >200 mg/dL (11.1 mmol/L) demonstrated on
several occasions is suggestive of diabetes mellitus; oral
glucose tolerance test is usually not required for diagnosis.
In adults, hypoglycemia can be observed in certain neoplasms
(islet cell tumor, adrenal and gastric carcinoma, fibrosarcoma,
hepatoma), severe liver disease, poisonings (arsenic, CCl4,
chloroform, cinchophen, phosphorous, alcohol, salicylates,
phenformin, and antihistamines), adrenocortical insufficiency,
hypothroidism, and functional disorders (postgastrectomy,
gastroenterostomy, autonomic nervous system disorders). Failure
to promptly separate serum from cells in a blood collection tube
causes falsely depressed glucose levels. If delay in
transporting a blood glucose to the lab is anticipated, the
specimen should be collected in a fluoride-containing tube
(gray-top in the US, yellow in the UK).