You've had a blood test and the results are sitting in your patient portal, full of numbers and abbreviations you've never seen before. Here is what the most common markers actually mean — and when to take them seriously.

Why blood tests can feel so confusing

Blood tests generate a lot of data. A standard panel might include dozens of individual measurements, each with its own abbreviation, unit, and reference range. Seeing a result flagged as "high" or "low" when you were expecting a clean bill of health can be alarming — even when the finding is minor or clinically insignificant on its own.

The truth is that reference ranges are statistical tools, not hard lines between healthy and unwell. They represent the range within which 95% of the healthy population falls. That means 5% of perfectly healthy people will have at least one result outside the "normal" range at any given time, purely by chance. Context matters enormously — which is why blood results should always be interpreted alongside your symptoms, medical history, and other findings.

With that framing in mind, here is what the most common tests are actually measuring.

Full Blood Count (FBC)

The Full Blood Count is one of the most frequently requested tests. It measures the different types of cells in your blood and gives a snapshot of your general health.

Haemoglobin (Hb). The protein in red blood cells that carries oxygen. A low haemoglobin indicates anaemia — which can cause tiredness, breathlessness, and pallor. High haemoglobin can indicate dehydration or, less commonly, a bone marrow condition.
White blood cells (WBC). The immune system's army. A raised white cell count can indicate infection, inflammation, or — in significant elevations — a blood disorder. A low count can suggest the immune system is suppressed, sometimes as a side effect of medication.
Platelets. The cells responsible for blood clotting. Very low platelets can cause bruising and bleeding; very high platelets may indicate an inflammatory process or, rarely, a bone marrow disorder.
MCV (Mean Corpuscular Volume). The average size of your red blood cells. A low MCV alongside low haemoglobin often points to iron-deficiency anaemia. A high MCV can suggest B12 or folate deficiency.

Kidney function tests (U&Es)

Urea and Electrolytes — commonly abbreviated to U&Es — assess how well your kidneys are filtering waste products from the blood.

Creatinine and eGFR. Creatinine is a waste product cleared by the kidneys. High creatinine suggests reduced kidney function. eGFR (estimated Glomerular Filtration Rate) is a calculation based on creatinine that estimates how efficiently the kidneys are working — a figure above 60 is generally reassuring; below 60, especially if persistent, warrants further investigation.
Sodium and potassium. Electrolytes that regulate fluid balance, nerve function, and heart rhythm. Abnormalities can result from dehydration, dietary factors, kidney problems, or certain medications (particularly blood pressure drugs).
Urea. Another waste product from protein metabolism. Mildly raised urea is common with dehydration or a high-protein diet; significantly raised levels alongside raised creatinine points to kidney impairment.

Liver function tests (LFTs)

Despite the name, liver function tests do not directly measure how well the liver is functioning — they measure markers that rise when liver cells are damaged or when bile flow is obstructed.

ALT (Alanine Aminotransferase). The most sensitive marker of liver cell damage. Raised ALT is associated with fatty liver disease, alcohol use, viral hepatitis, and certain medications. Mild elevations are common and often benign, but persistent or significantly raised ALT needs further assessment.
ALP (Alkaline Phosphatase). Raised in conditions affecting the bile ducts, gallbladder, or bones. A mild isolated elevation can be normal — particularly in teenagers and pregnant women — but significant or persistent rises need investigation.
Bilirubin. A yellow pigment produced when red blood cells are broken down, cleared by the liver. High bilirubin causes jaundice (yellowing of the skin and whites of the eyes). It can reflect liver disease, gallstones obstructing bile flow, or increased breakdown of red blood cells.
Albumin. A protein made by the liver. Low albumin in the context of other abnormal LFTs can indicate chronic liver disease. Low albumin can also occur in malnutrition or chronic inflammation.

Cholesterol and lipid profile

A lipid profile measures the different types of fat circulating in your blood. The key numbers are:

Total cholesterol. A combined figure that includes both "good" and "bad" cholesterol. On its own, it gives a limited picture — what matters more is how the components break down.
LDL cholesterol (Low-Density Lipoprotein). Often called "bad" cholesterol because high levels contribute to the build-up of fatty plaques in arteries, raising the risk of heart attack and stroke.
HDL cholesterol (High-Density Lipoprotein). The "good" cholesterol — it helps remove LDL from the bloodstream. Higher HDL is protective.
Triglycerides. Another form of fat in the blood, influenced heavily by diet (particularly refined carbohydrates and alcohol). High triglycerides alongside low HDL and high LDL significantly increases cardiovascular risk.

Blood glucose and HbA1c

Fasting glucose. Measures blood sugar after not eating for at least 8 hours. A fasting glucose above 7.0 mmol/L on two occasions is diagnostic of diabetes; between 6.1 and 6.9 is classed as impaired fasting glucose (pre-diabetes).
HbA1c. This measures the percentage of haemoglobin that has glucose attached to it — effectively a 3-month average of blood sugar levels. An HbA1c of 48 mmol/mol (6.5%) or above is diagnostic of type 2 diabetes. Between 42 and 47 indicates pre-diabetes. It is more reliable than a one-off fasting glucose and is now the preferred diagnostic test in most settings.

Thyroid function (TFTs)

TSH (Thyroid-Stimulating Hormone). The most sensitive marker of thyroid function. A high TSH suggests the thyroid is underactive (hypothyroidism) — your pituitary gland is working harder to stimulate a sluggish thyroid. A low TSH suggests an overactive thyroid (hyperthyroidism).
Free T4 and Free T3. The actual thyroid hormones circulating in the blood. Measured alongside TSH to confirm the diagnosis and assess severity.

Thyroid problems are very common — particularly in women — and are entirely treatable once identified. Symptoms of an underactive thyroid include fatigue, weight gain, feeling cold, dry skin, and low mood. An overactive thyroid tends to cause weight loss, tremor, palpitations, and anxiety.

When should a result prompt urgent action?

Most blood test abnormalities are mild, stable, and simply require monitoring or a lifestyle adjustment. A small minority require prompt follow-up. Contact your GP promptly if your results include very low haemoglobin (below 70 g/L), significantly impaired kidney function, a very high calcium, or if your GP has specifically flagged a result as urgent.

If you have received a result you do not understand, or one that has been flagged as abnormal without adequate explanation, our private blood testing service can arrange comprehensive panels with full clinical interpretation. Our health screening service goes further still, combining blood tests with a thorough GP review to put your results into full clinical context.

Understanding your blood test results: a plain-English guide