Random Blood Sugar

The random blood sugar is a plasma glucose measured at any time, without regard to the last meal. Glucose is measured by glucose oxidase or hexokinase methods on venous plasma in the laboratory, or by capillary meter at the bedside. It is the first test in a patient with hyperglycaemic or hypoglycaemic symptoms, an acutely unwell patient, or a patient with altered consciousness or seizures. It also supports diagnosis of diabetes when symptoms are present.

A random value does not define metabolic control and cannot exclude diabetes when normal. It is not used on its own to screen asymptomatic people, since values vary with the timing and content of meals.

Indications. Polyuria, polydipsia, weight loss, blurred vision, recurrent infections, confusion, collapse, seizures, coma, suspected stroke, sepsis, acute illness in people with diabetes, hypoglycaemia symptoms (sweating, tremor, hunger, confusion), neonatal and paediatric illness, and monitoring during steroids or parenteral nutrition.

Hyperglycaemia and diabetes.

• A random plasma glucose of 11.1 mmol/L (200 mg/dL) or more with classic symptoms (polyuria, polydipsia, weight loss) diagnoses diabetes.
• Without symptoms, a raised random value is confirmed by a fasting glucose, glucose tolerance test or HbA1c.
• The usual renal threshold for glycosuria is about 10 mmol/L (180 mg/dL).

Diabetic emergencies.

• Diabetic ketoacidosis: glucose over 11 mmol/L (200 mg/dL), or known diabetes, with blood ketones (beta hydroxybutyrate) of 3 mmol/L or more and a bicarbonate under 15 mmol/L or a venous pH under 7.3. Glucose can be near normal with SGLT2 inhibitor therapy, pregnancy, fasting or alcohol.
• Hyperosmolar hyperglycaemic state: glucose over about 30 mmol/L (540 mg/dL), effective osmolality over 320 mOsm/kg, no marked ketosis, and a pH over 7.3.

Hypoglycaemia.

• In diabetes, a glucose under 3.9 mmol/L (70 mg/dL) is an alert value, and under 3.0 mmol/L (54 mg/dL) is clinically significant hypoglycaemia. Severe hypoglycaemia is an event needing help from another person.
• In people without diabetes, hypoglycaemia is shown by Whipple's triad: symptoms, a low plasma glucose (about 3.0 mmol/L or under, 55 mg/dL), and relief of symptoms when glucose is raised.

Hospital hyperglycaemia. A glucose over 7.8 mmol/L (140 mg/dL) on admission without known diabetes needs follow up testing. Persistent values over 10 mmol/L (180 mg/dL) are usually treated.

Raised.

• Diabetes of any type, including type 1 (young, rapid onset), type 2, gestational and monogenic diabetes.
• Stress response to acute illness: infection, myocardial infarction, stroke, trauma, burns, surgery.
• Drugs: glucocorticoids, thiazide diuretics, beta blockers, olanzapine and clozapine, tacrolimus, ciclosporin, sympathomimetics, protease inhibitors, immune checkpoint inhibitors (autoimmune diabetes), asparaginase.
• Endocrine and pancreatic disease: Cushing syndrome, acromegaly, phaeochromocytoma, hyperthyroidism, pancreatitis, pancreatic cancer, haemochromatosis, cystic fibrosis.
• Parenteral nutrition, dextrose infusions and sampling from the infusion arm.
• Recent meal (post-prandial peak 1 to 2 hours after eating).

Lowered.

• Drugs: insulin, sulphonylureas and meglitinides (the commonest cause), also quinine, fluoroquinolones, pentamidine, salicylates, tramadol and alcohol.
• Critical illness and sepsis, liver failure, adrenal insufficiency, hypopituitarism, renal failure, malnutrition and prolonged fasting, severe malaria.
• Insulinoma, non-islet cell tumour hypoglycaemia (IGF-2), post-bariatric hypoglycaemia, reactive hypoglycaemia, autoimmune insulin syndrome, factitious use of insulin or sulphonylureas.
• Children and infants: hyperinsulinism, ketotic hypoglycaemia, glycogen storage disease, fatty acid oxidation disorders, hereditary fructose intolerance, growth hormone or cortisol deficiency.

Critical values and findings needing immediate action. Laboratory triggers vary.

• Glucose under 2.5 mmol/L (45 mg/dL), or under 3.0 mmol/L with confusion, seizures or reduced consciousness.
• Glucose over 25 mmol/L (450 mg/dL), or over 11 mmol/L with ketones and acidosis, or over 30 mmol/L with high osmolality.
• Any unconscious, seizing or confused patient needs a glucose check at once, since hypoglycaemia mimics stroke and intoxication.
• A hypoglycaemic patient with unknown cause needs a critical sample taken at the time of hypoglycaemia, before treatment where this does not delay it.

Interferences and pitfalls.

• In unspun blood at room temperature, glucose falls by about 5 to 7% (0.4 to 0.6 mmol/L) an hour from glycolysis, faster with a high leucocyte, red cell or platelet count and in neonates. Samples go into fluoride oxalate tubes and are centrifuged within about 30 minutes, since fluoride takes 1 to 2 hours to act.
• Plasma glucose is about 10 to 15% higher than whole blood glucose, and venous values are lower than capillary values after meals.
• Capillary meters are accurate to about 15% and are not used to diagnose diabetes. Interference arises from maltose, icodextrin and galactose (falsely high on some meters), high or low haematocrit, poor peripheral perfusion, hypoxia and high dose vitamin C or paracetamol on some meters.
• Neonatal bedside readings are unreliable at low values, and a low result is confirmed by the laboratory without delaying treatment.
• Sampling from an arm with a dextrose infusion gives a spuriously high result.
• Euglycaemic diabetic ketoacidosis occurs with SGLT2 inhibitors, so a normal glucose does not exclude ketoacidosis.

Special groups.

• Neonates: an operational threshold of about 2.6 mmol/L (47 mg/dL) is used to act in the first days, and persistent or severe hypoglycaemia risks brain injury.
• Children: diabetes thresholds match those of adults, and ketoacidosis is the common first presentation of type 1 diabetes.
• Pregnancy: a random glucose does not diagnose gestational diabetes, which needs a fasting value or glucose tolerance test. Corticosteroids for foetal lung maturation raise glucose.
• Older adults: hypoglycaemia presents atypically with confusion or falls, and the renal threshold for glucose rises with age.
• Chronic kidney disease: insulin clearance falls and hypoglycaemia is more frequent.

Follow up tests. Fasting plasma glucose, oral glucose tolerance test, HbA1c, blood ketones, venous blood gas, serum osmolality, urea and electrolytes, islet autoantibodies (GAD65, IA-2, ZnT8), C-peptide, and at the time of hypoglycaemia insulin, C-peptide, proinsulin, beta hydroxybutyrate, cortisol, growth hormone and a sulphonylurea screen, urine ketones, lipase, thyroid function tests.