Understanding your blood test results

How to read a blood test report

Most blood test reports give you four things per test: the test name, your result, the reference range, and a flag (H, L, or asterisk) if the result is outside that range.

Two things to understand before diving in:

1. Reference ranges are statistical. They’re typically derived from the middle 95% of healthy people. By definition, 1 in 20 healthy people will fall outside a single reference range purely by chance. If you do 10 tests, the chance at least one is “abnormal” for a healthy person is over 40%.
2. Context matters more than the number. Trend over time, symptoms, age, sex, medications, and other results are all part of interpretation.

Here are the common tests, what they measure, and how to think about results.

Full blood count (FBC)

The FBC counts the cells in your blood — red cells, white cells, and platelets. Key components:

Haemoglobin (Hb)

The protein in red blood cells that carries oxygen. Low = anaemia (could be iron deficiency, B12, blood loss, chronic disease). High = polycythaemia (could be dehydration, lung disease, rarely a blood disorder).

Typical ranges (UK): men 13.0–17.0 g/dL; women 12.0–15.0 g/dL.

White cell count (WCC)

Immune cells. High = infection, inflammation, sometimes blood disorders. Low = some viral infections, medications, occasionally serious conditions.

The differential (breakdown into neutrophils, lymphocytes, etc) often matters more than the total.

Platelets

Cells involved in clotting. Low = bleeding risk; multiple causes. High = often inflammation, sometimes more concerning.

MCV (mean cell volume)

Size of red blood cells. Low = often iron deficiency. High = often B12/folate deficiency, alcohol, thyroid problems.

Iron studies

Ferritin

The key marker of iron stores. Low ferritin = iron deficiency (whether or not anaemia is present yet). This is the most useful single iron test.

Below 30 mcg/L generally indicates iron deficiency. Some labs flag below 15 or 20 — but functional iron deficiency can occur up to 50–70 in symptomatic patients (particularly women with heavy periods).

Ferritin can also rise as an inflammation marker (acute phase reactant), so a normal or high ferritin doesn’t exclude iron deficiency if there’s inflammation present.

Transferrin saturation

The percentage of iron-carrying protein actually carrying iron. Low = iron deficiency. Very high = iron overload (haemochromatosis).

Thyroid function

TSH (thyroid-stimulating hormone)

The pituitary’s message to the thyroid: “make more hormone.” High TSH = underactive thyroid (the gland isn’t responding). Low TSH = overactive thyroid.

Reference range typically 0.4–4.0 mIU/L. Many endocrinologists treat above 4 or sometimes above 2.5 in specific contexts (pregnancy, fertility).

Free T4 (thyroxine)

The main thyroid hormone. Low T4 + high TSH = clear hypothyroidism. Normal T4 + slightly raised TSH = subclinical hypothyroidism (debated whether to treat).

Thyroid antibodies (TPO, Tg)

Auto-antibodies against the thyroid. Present in autoimmune thyroid disease. Predictor of future thyroid problems even if function is currently normal.

Vitamin D

Measured as 25-hydroxyvitamin D (the storage form). UK ranges:

• Below 25 nmol/L — deficient. Treatment dose needed.
• 25–50 nmol/L — insufficient. Supplement.
• 50–75 nmol/L — adequate.
• 75–125 nmol/L — sufficient.
• Above 125 nmol/L — high. Reduce supplementation.

See our vitamin D article for the bigger picture.

B12 and folate

Both involved in red cell production and nerve function. Deficiency can cause anaemia, fatigue, neurological symptoms.

B12: most labs flag below 200–300 pg/mL but some people are symptomatic in the “low-normal” range (200–400). Active B12 testing (holotranscobalamin) is sometimes more accurate.

Folate: depends on the lab. Less commonly deficient than B12.

Liver function (LFTs)

Several tests grouped together:

• ALT, AST — enzymes released when liver cells are damaged. Raised in fatty liver, alcohol, viral hepatitis, medications.
• ALP (alkaline phosphatase) — from liver, bone, and other sources. Raised in bile duct problems, bone disease, sometimes other causes.
• GGT — often elevated in alcohol use; also bile duct disease.
• Bilirubin — the breakdown product giving bruises their yellow colour. Raised in jaundice, Gilbert’s syndrome (a harmless variant), liver disease.
• Albumin — main protein made by liver. Low in chronic liver disease, poor nutrition, kidney loss.

Mildly raised liver enzymes are common (often fatty liver, alcohol, medications); significantly raised levels warrant proper investigation.

Kidney function

Creatinine and eGFR

Creatinine is a waste product cleared by the kidneys. Higher creatinine = worse kidney function. eGFR (estimated glomerular filtration rate) translates this to a percentage of normal kidney function.

Above 90 = normal. 60–89 = mildly reduced (often age-related). 30–59 = moderately reduced. Below 30 = severe and needs specialist input.

Urea

Another kidney waste product. Affected by hydration, diet, and kidney function.

Electrolytes (sodium, potassium)

Vital for cellular function. Abnormalities can be from medications, kidney disease, hormonal causes.

Cholesterol and lipids

Total cholesterol

The headline number. Less useful on its own than the breakdown.

LDL cholesterol (“bad”)

The cholesterol associated with cardiovascular risk. Target depends on overall risk — for most adults under 3.0 mmol/L is reasonable; lower for high-risk patients.

HDL cholesterol (“good”)

Protective. Higher is better. Above 1.0 in men, above 1.2 in women.

Triglycerides

Another fat in the blood. Affected by recent meals (so often done fasting), alcohol, diabetes. High = increased cardiovascular and pancreatic risk.

Non-HDL cholesterol

Total minus HDL. Increasingly used as the main cardiovascular risk marker.

HbA1c and glucose

HbA1c

Reflects average blood glucose over about 3 months. Used to diagnose and monitor diabetes.

• Below 42 mmol/mol — normal
• 42–47 mmol/mol — pre-diabetes / increased risk
• 48 mmol/mol or above — diabetes

Fasting glucose

Less commonly used now that HbA1c is widely available.

CRP and ESR

Both markers of inflammation in the body.

CRP (C-reactive protein) rises within hours of inflammation and falls quickly when it settles. Below 5 mg/L normal; significantly raised in infection, inflammation, autoimmune disease.

ESR (erythrocyte sedimentation rate) is slower to rise and fall. Influenced by age, sex, anaemia. Less specific.

When “abnormal” doesn’t mean “wrong”

Many flagged results don’t need any action:

• Mild deviations in healthy people — statistical artefact
• Gilbert’s syndrome — harmless variant causing mildly raised bilirubin
• Mild ALT/AST elevation after exercise or alcohol
• Mildly raised TSH in someone without thyroid symptoms
• Slightly low haemoglobin in someone who’s otherwise well

Others do warrant action:

• Significantly abnormal values, even if you feel well
• Results that match your symptoms
• Trends moving in the wrong direction over multiple tests
• Combinations that point to a specific diagnosis (e.g. low Hb + low ferritin + high MCV)

The skill of your doctor is in telling these apart, putting them in context, and not chasing every flagged result with another test. If your results haven’t been explained to you properly, that’s a reasonable thing to ask for.