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A single blood test can report dozens of numbers — sodium, potassium, creatinine, ALT, HbA1c, cholesterol — each with its own unit and “normal range”. Behind every one is a piece of chemistry: a molecule or ion that tells a story about how well a particular organ is working, and an analytical technique that measures its concentration accurately in a tiny sample. This guide explains what blood is made of, what the common tests measure, and how the laboratory actually measures them.
(This article explains the chemistry. It isn’t medical advice — reference ranges differ between laboratories, and results should always be interpreted by a health professional.)
What’s in blood?
An adult has roughly 5 litres of blood. Spin a tube of it in a centrifuge and it separates into layers:
- Plasma (about 55 %): a straw-coloured liquid, over 90 % water, carrying dissolved ions, glucose, proteins, hormones, waste and gases.
- Buffy coat (under 1 %): white blood cells and platelets.
- Red blood cells (about 45 %): packed with haemoglobin. The percentage by volume is called the haematocrit.
If blood is allowed to clot before spinning, the liquid left is serum — plasma without the clotting proteins. Many chemistry tests are run on serum or plasma.
Units: mmol/L and friends
Most countries report blood chemistry in SI units, usually millimoles per litre (mmol/L, which is the same as mmol dm⁻³). The United States often uses mass units such as mg/dL. Converting between them is a direct application of molar mass (see what is molar mass?).
Worked example: glucose. Glucose, C₆H₁₂O₆, has M = 180.16 g mol⁻¹. A fasting glucose of 5.0 mmol/L in mg/dL:
5.0 mmol/L × 180.16 mg/mmol = 900.8 mg/L = 90 mg/dL (since 1 dL = 0.1 L).
So the rough conversion factor for glucose is 18. For cholesterol (M ≈ 386.7 g mol⁻¹), it’s about 38.7. You can check molar masses with the molar mass calculator and practise conversions with the molarity calculator.
The electrolyte panel
| Test | Typical adult range | Tells you about |
|---|---|---|
| Sodium (Na⁺) | 135–145 mmol/L | Water balance |
| Potassium (K⁺) | 3.5–5.0 mmol/L | Kidney function, heart rhythm risk |
| Chloride (Cl⁻) | 98–106 mmol/L | Acid–base and fluid balance |
| Hydrogencarbonate (HCO₃⁻) | 22–29 mmol/L | Acid–base status |
For the chemistry behind each ion, see electrolytes in the body.
How it’s measured: most modern analysers use ion-selective electrodes (ISEs). An ISE has a membrane that lets through, or binds, only one type of ion. The potential difference across the membrane depends on the logarithm of that ion’s activity, following the Nernst equation — the same principle as a pH electrode (see how a pH meter works). A potassium electrode, for example, uses valinomycin, a ring-shaped molecule that binds K⁺ far more strongly than Na⁺.
A useful bit of chemistry doctors calculate is the anion gap:
anion gap = [Na⁺] − ([Cl⁻] + [HCO₃⁻])
Blood is electrically neutral, so the “gap” represents negative ions not measured routinely — mainly proteins. A high gap suggests extra acids in the blood, such as ketones in diabetic ketoacidosis (see ketosis) or lactate.
Glucose and HbA1c
- Fasting glucose (typically about 4–6 mmol/L) is measured by enzyme methods. The enzyme glucose oxidase oxidises glucose, producing hydrogen peroxide, which is then detected by a colour-forming reaction or an electrode. Home glucose meters use a similar enzyme on a test strip and measure the electric current produced. Enzymes are ideal because they’re highly specific: glucose oxidase ignores almost every other sugar.
- HbA1c measures the percentage of haemoglobin that has had glucose attached to it (glycated) through a slow, non-enzymatic reaction between glucose and an amine group on the protein. Since red blood cells live about 120 days, HbA1c reflects average glucose over the previous two to three months. It’s reported in mmol/mol (IFCC units) or as a percentage.
Kidney function
The kidneys filter waste from the blood (see the kidneys as chemical filters). Two waste products show how well they’re working:
- Urea, CO(NH₂)₂, is made in the liver from the nitrogen of broken-down amino acids. It rises with kidney problems, but also with dehydration or a high-protein meal.
- Creatinine comes from the steady breakdown of creatine phosphate in muscles (see what happens chemically when you exercise). Because muscles produce it at a fairly constant rate, blood creatinine is a good marker of kidney filtration. A classic method, the Jaffe reaction (from 1886), reacts creatinine with picric acid in alkaline solution to form an orange-red complex measured by its absorbance; many labs now use more specific enzyme methods.
From creatinine, age and sex, labs calculate an eGFR — an estimate of how much blood the kidneys filter per minute.
Liver function
“Liver function tests” are a mix of enzymes and other molecules:
- ALT and AST (alanine and aspartate aminotransferases): enzymes normally inside liver cells. When cells are damaged, they leak into the blood. Their “concentration” is actually measured as activity — how fast they catalyse a reaction — in units per litre (U/L).
- ALP (alkaline phosphatase): raised in blocked bile ducts and in bone growth (it’s normally higher in growing teenagers).
- Bilirubin: a yellow-orange product of the breakdown of haem from old red blood cells. The liver makes it water-soluble so it can be excreted in bile. When it builds up, skin and eyes turn yellow (jaundice).
- Albumin: the most abundant plasma protein, made by the liver. It carries fatty acids, hormones and drugs, and holds water in the blood by osmosis.
Enzyme activity is measured by following a reaction with a spectrophotometer: for example, ALT activity is linked to a reaction that converts NADH to NAD⁺, and the fall in absorbance at 340 nm (where NADH absorbs but NAD⁺ doesn’t) shows how fast the reaction is going (see enzyme kinetics).
Lipids
A lipid profile measures fats carried in the blood (see cholesterol chemistry):
- Total cholesterol
- HDL cholesterol (“good”): lipoproteins that carry cholesterol back to the liver
- LDL cholesterol (“bad”): carries cholesterol to tissues; high levels are linked to fatty deposits in arteries
- Triglycerides
Cholesterol is insoluble in water, so it travels in lipoproteins — particles with a shell of phospholipids and proteins around a fatty core. Labs measure cholesterol with a chain of enzymes that ends with a coloured product, and LDL is often calculated from the other values rather than measured directly.
Other common tests
- Calcium (bound partly to albumin, so often “corrected” for albumin level)
- Iron studies: iron, ferritin (the iron-storage protein) and transferrin (see iron in the body)
- Thyroid hormones and vitamin D, usually by immunoassays — tests using antibodies that bind a specific molecule (see the immune system as chemistry)
- CRP (C-reactive protein), a marker of inflammation
Getting it right: pre-analytical chemistry
Many “wrong” results come from what happens before the sample reaches the analyser:
- Haemolysis: if red cells burst (from a tight tourniquet or rough handling), they release potassium, falsely raising the K⁺ result.
- Delay: red cells keep using glucose in the tube, so glucose falls if the sample waits. Grey-topped tubes contain sodium fluoride, which blocks glycolysis to stop this — an enzyme inhibitor used on purpose (see enzyme inhibition).
- Wrong tube: tubes containing the anticoagulant potassium EDTA must not be used for potassium or calcium tests, because EDTA adds potassium and grabs calcium.
- Fasting: triglycerides rise after a meal.
Key takeaways
- Blood is plasma (mostly water with dissolved substances) plus cells; many tests use serum or plasma.
- Results are mostly given in mmol/L; converting to mg/dL uses the molar mass.
- Electrolytes are measured with ion-selective electrodes; glucose and many other molecules with specific enzymes.
- Creatinine and urea reflect kidney function; ALT, AST, ALP, bilirubin and albumin reflect liver function.
- Sample handling — haemolysis, delay, the right tube — matters as much as the analysis.
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