Carbohydrates and glucose metabolism

Glucose is the single most tested analyte in clinical chemistry, and for good reason: a wrong result changes how a patient with diabetes is treated. The board loves this topic because it lets one question test physiology, a diagnostic cutoff, a laboratory method, and a specimen error all at once. Master the flow from hormonal control to the tube in your hand and you convert several near-certain items into points.

How the body keeps glucose in range

Blood glucose is the balance between what enters the blood and what leaves it. After a meal, glucose rises and the pancreatic beta cells release insulin, the only hormone that lowers blood glucose. Insulin moves glucose into cells, drives glycogenesis (storing glucose as glycogen), and blocks the pathways that would raise glucose.

When you fast, glucose falls and the counter-regulatory hormones take over. Glucagon from the pancreatic alpha cells is the main one; it triggers glycogenolysis (breaking glycogen back to glucose) and gluconeogenesis (making new glucose from amino acids and glycerol). Epinephrine, cortisol, and growth hormone also raise glucose. Because four hormones push glucose up and only one pushes it down, the body is built to defend against low sugar first.

Two vocabulary pairs are worth locking in:

• Glycogenesis builds glycogen; glycogenolysis breaks it down.
• Gluconeogenesis makes glucose from non-carbohydrate sources; glycolysis burns glucose for energy.

Diagnosing diabetes mellitus

Diabetes is diagnosed by demonstrating chronic hyperglycemia through any of four standard approaches. Learn the principle of each, not just the number.

• Fasting plasma glucose (FBS) after at least eight hours without food. The diagnostic threshold is a fasting value at or above 7.0 mmol/L (126 mg/dL).
• Two-hour plasma glucose during a 75 g oral glucose tolerance test (OGTT) at or above 11.1 mmol/L (200 mg/dL).
• Glycated hemoglobin (HbA1c) at or above 6.5%. HbA1c forms by the slow, non-enzymatic attachment of glucose to hemoglobin, so it mirrors average glucose over the red-cell lifespan.
• A random plasma glucose at or above 11.1 mmol/L (200 mg/dL) in a patient with classic symptoms (polyuria, polydipsia, weight loss).

Values between normal and diabetic define the at-risk states: impaired fasting glucose and impaired glucose tolerance (prediabetes). Remember that HbA1c is unreliable when the red-cell lifespan is abnormal: it reads falsely low in hemolytic anemia or recent blood loss and can mislead in hemoglobinopathies.

Worked example: reading an OGTT

A 44-year-old has a fasting glucose of 6.4 mmol/L and a two-hour OGTT value of 12.0 mmol/L. The fasting value sits below the 7.0 mmol/L diabetic cutoff, but the two-hour value of 12.0 mmol/L is above 11.1 mmol/L. A single value in the diabetic range on a validated test supports the diagnosis, so this OGTT is diagnostic of diabetes on the two-hour reading. The lesson: check every timepoint against its own cutoff, not just the fasting one.

Hypoglycemia and severe hyperglycemia

Hypoglycemia is a low blood glucose, commonly flagged below about 3.9 mmol/L (70 mg/dL), and it is a true emergency because the brain runs almost entirely on glucose. Adrenergic warning signs (sweating, tremor, palpitations) come first; neuroglycopenic signs (confusion, seizures, coma) follow as glucose keeps dropping.

At the other extreme, uncontrolled hyperglycemia produces two classic crises. Diabetic ketoacidosis (DKA), typical of type 1 diabetes, combines high glucose, ketones, and a metabolic acidosis. Hyperosmolar hyperglycemic state, more typical of type 2 diabetes, shows very high glucose and severe dehydration with little or no ketone production.

Measuring glucose in the laboratory

Two enzymatic methods dominate, and the board expects you to tell them apart.

• Hexokinase is the reference (comparison) method. It is highly specific and less prone to interference, which is why method-comparison studies use it as the standard.
• Glucose oxidase is common on analyzers and reagent strips. It is specific for glucose, but strong reducing substances can interfere with some end-point systems.

Specimen handling: protecting the true value

This is where careless technique destroys a good sample. Red cells and leukocytes keep consuming glucose after collection through glycolysis, so glucose in an unpreserved tube falls at roughly 5 to 7 percent per hour at room temperature, and faster if the white-cell count is high. Two defenses:

1. Separate the plasma or serum from the cells promptly (ideally within an hour) so the cells can no longer eat the glucose.
2. Use a glucose-preservative tube. The classic additive, sodium fluoride, blocks glycolysis by inhibiting the enzyme enolase. It is packaged in the gray-top tube.

A common trap: fluoride stops glycolysis but works slowly, so it does not fully prevent the early drop in the first hour. Prompt separation still matters.

Ketones

When cells cannot use glucose, they burn fat and produce three ketone bodies: beta-hydroxybutyrate, acetoacetate, and acetone. In active ketoacidosis, beta-hydroxybutyrate predominates. This matters because the routine nitroprusside reaction (used on many urine strips and tablets) detects acetoacetate and acetone but not beta-hydroxybutyrate. Early in DKA, a nitroprusside test can therefore underestimate the true ketone load, so a direct beta-hydroxybutyrate measurement is preferred when available.

Common traps

• Choosing insulin as a hormone that raises glucose. Insulin is the only one that lowers it.
• Forgetting that glucose in an unpreserved tube falls, not rises, before analysis.
• Assuming sodium fluoride gives instant, complete protection; it acts slowly.
• Treating a nitroprusside ketone result as complete when it misses beta-hydroxybutyrate.
• Reading HbA1c at face value in a patient with hemolysis or recent blood loss, where it reads falsely low.
• Confusing glycogenolysis (breaking glycogen) with gluconeogenesis (making new glucose).

Recap

Insulin lowers glucose; glucagon, epinephrine, cortisol, and growth hormone raise it. Diagnose diabetes by a fasting glucose at or above 7.0 mmol/L, a two-hour OGTT at or above 11.1 mmol/L, an HbA1c at or above 6.5%, or a symptomatic random glucose at or above 11.1 mmol/L. Hexokinase is the reference method and glucose oxidase the workhorse. Protect the specimen with prompt separation and a sodium fluoride (gray-top) tube, and remember that the nitroprusside ketone test misses beta-hydroxybutyrate, the dominant ketone in early DKA.