Common blood tests, explained

When it is ordered. Often ordered for tiredness, suspected infection, bleeding or bruising, before some procedures, and to monitor certain treatments.

HaemoglobinHbg/L or g/dL

What it is: The protein in red cells that carries oxygen.

Why it is measured: Low haemoglobin means anaemia (many causes: iron deficiency, blood loss, chronic disease). High values can follow dehydration or, less often, a bone-marrow condition.

White cell countWBCx10^9/L

What it is: The total number of infection-fighting cells.

Why it is measured: Rises with many infections and inflammation, falls with some viral illnesses and after chemotherapy. The differential below says which type is changed.

Neutrophilsx10^9/L

What it is: The white cells that tackle bacteria first.

Why it is measured: High in bacterial infection and stress; very low (neutropenia) raises infection risk and needs prompt attention.

Lymphocytesx10^9/L

What it is: White cells central to viral defence and immune memory.

Why it is measured: Often raised in viral infections; patterns matter more than single values.

PlateletsPLTx10^9/L

What it is: Tiny cell fragments that help blood clot.

Why it is measured: Low platelets can mean easy bruising or bleeding; high platelets can follow inflammation or, rarely, a marrow disorder.

Mean cell volumeMCVfL

What it is: The average size of your red cells.

Why it is measured: Small cells point toward iron deficiency; large cells toward B12 or folate deficiency or alcohol. A key clue to the type of anaemia.

When it is ordered. Ordered to check kidney function, before and during many medicines, in dehydration or fluid overload, and in most acute illness. Often paired with eGFR.

SodiumNammol/L

What it is: The main salt in the fluid outside your cells.

Why it is measured: Reflects water and salt balance. Abnormal levels usually signal a fluid problem or a medication effect rather than diet.

PotassiumKmmol/L

What it is: A salt vital to nerve and heart-muscle signalling.

Why it is measured: Both high and low potassium affect the heart rhythm, so results outside range are taken seriously. Kidney function and several drugs move it.

Ureammol/L

What it is: A waste product from protein breakdown cleared by the kidneys.

Why it is measured: Rises with reduced kidney function and dehydration; also affected by diet and bleeding in the gut.

Creatininemicromol/L

What it is: A muscle waste product filtered by the kidneys.

Why it is measured: The workhorse marker of kidney function; feeds the eGFR estimate. Muscle mass, age and sex influence it.

Estimated GFReGFRmL/min/1.73m2

What it is: A calculated estimate of how well the kidneys filter.

Why it is measured: Staged into chronic kidney disease categories over time. A single low value can be temporary; the trend matters.

When it is ordered. Ordered for jaundice, abdominal pain, alcohol review, and to monitor medicines that can affect the liver. The pattern can help a clinician narrow the possibilities.

Alanine transaminaseALTU/L

What it is: An enzyme inside liver cells.

Why it is measured: Leaks into blood when liver cells are inflamed or injured (for example fatty liver, hepatitis, some drugs). One of the most useful liver markers.

Aspartate transaminaseASTU/L

What it is: An enzyme found in liver, muscle and other tissues.

Why it is measured: Read with ALT and the clinical context. The AST-to-ALT pattern can add a clue but cannot identify a cause by itself.

Alkaline phosphataseALPU/L

What it is: An enzyme from the liver bile ducts and from bone.

Why it is measured: Rises with bile-duct blockage and with some bone conditions; growing children have naturally higher levels.

Gamma GTGGTU/L

What it is: An enzyme associated with the liver and bile ducts.

Why it is measured: Can help assess whether a raised ALP is from the liver. Alcohol, medicines and several conditions can raise it, so it is not specific.

Bilirubinmicromol/L

What it is: A yellow pigment from the breakdown of old red cells.

Why it is measured: Raised levels can cause jaundice and have several possible causes. Gilbert syndrome is a common, generally benign cause after a clinician confirms the pattern.

Albuming/L

What it is: The main protein the liver makes.

Why it is measured: A low level can reflect long-standing liver disease, poor nutrition or inflammation. It also affects how other results are read.

When it is ordered. Ordered in cardiovascular risk checks, family history of high cholesterol, and to monitor cholesterol treatment. Often part of a wider risk score with blood pressure and other factors.

Total cholesterolTCmmol/L

What it is: All the cholesterol carried in your blood.

Why it is measured: A broad marker; the breakdown below is more informative than the total alone.

LDL cholesterolLDLmmol/L

What it is: The cholesterol that can build up in artery walls.

Why it is measured: The main treatment target: lowering it reduces heart-attack and stroke risk in people at risk.

HDL cholesterolHDLmmol/L

What it is: The cholesterol that helps clear fat from the blood.

Why it is measured: Higher is generally more favourable; read as part of the whole profile, not alone.

TriglyceridesTGmmol/L

What it is: The most common type of fat in the body.

Why it is measured: Rises with recent food, alcohol and untreated diabetes; very high levels carry their own risks. Fasting may be requested.

Non-HDL cholesterolmmol/L

What it is: Total cholesterol minus the HDL part.

Why it is measured: A modern combined risk marker that does not need fasting, now used in many guidelines.

When it is ordered. Ordered for tiredness, weight change, palpitations, low mood, and to monitor thyroid treatment. TSH is usually the first-line test.

Thyroid stimulating hormoneTSHmIU/L

What it is: The pituitary hormone that signals the thyroid to work.

Why it is measured: Commonly used as a first-line thyroid test. The direction of change can suggest reduced or increased thyroid activity, but it is interpreted with FT4, symptoms and context.

Free T4FT4pmol/L

What it is: The main thyroid hormone circulating in blood.

Why it is measured: Added when TSH is abnormal to confirm and grade the problem.

Free T3FT3pmol/L

What it is: The more active thyroid hormone.

Why it is measured: Requested in specific situations, such as some overactive-thyroid pictures.

When it is ordered. Ordered to diagnose and monitor type 2 diabetes and prediabetes, and in routine risk checks. HbA1c gives a longer view than a single glucose reading.

HbA1cmmol/mol or %

What it is: The share of haemoglobin coated with sugar, reflecting the last 2 to 3 months.

Why it is measured: Used to diagnose and monitor diabetes because it averages out day-to-day swings. Reported in mmol/mol (IFCC) and sometimes percent (NGSP).

Glucosemmol/L

What it is: The sugar level in the blood at the moment of the sample.

Why it is measured: A fasting or random reading; changes quickly with food, so timing matters. Used with HbA1c and symptoms.

When it is ordered. Ordered when the FBC suggests anaemia, in tiredness, and in pregnancy. Separates iron deficiency from B12 or folate deficiency.

Ferritinmicrograms/L

What it is: A protein that stores iron and is used to assess iron stores.

Why it is measured: A low result supports iron deficiency. Inflammation and some illnesses can raise ferritin, so a result is read with other findings.

Vitamin B12ng/L or pmol/L

What it is: A vitamin needed to make healthy red cells and nerves.

Why it is measured: Deficiency causes a large-cell anaemia and nerve symptoms; common in some diets and with certain medicines.

Folatemicrograms/L

What it is: A B vitamin essential for cell production.

Why it is measured: Deficiency also causes large-cell anaemia; important before and during pregnancy.

When it is ordered. Ordered in suspected infection or inflammatory disease, and to monitor known conditions. They flag activity but not the cause.

C-reactive proteinCRPmg/L

What it is: A protein the liver makes quickly when inflammation rises.

Why it is measured: Goes up within hours of infection or injury and falls as things settle, so it is useful for tracking. It does not locate the problem.

Erythrocyte sedimentation rateESRmm/hr

What it is: How fast red cells settle in a tube, an indirect inflammation measure.

Why it is measured: A slower, longer-term marker than CRP; used in certain chronic inflammatory conditions.