Understanding Your Blood Test Results: A Complete UK Guide

Receiving blood test results can feel overwhelming. A printout covered in numbers, abbreviations, and flags like "H" (high) or "L" (low) is difficult to interpret without medical training. This guide explains what your blood test results actually mean, how to read them, and when you should follow up with your GP.

Why Are Blood Tests Done?

Blood tests are one of the most common and useful diagnostic tools in medicine. Your GP or hospital doctor may request them to:

• Screen for conditions like diabetes, anaemia, or kidney disease before symptoms appear
• Monitor an existing condition and check whether treatment is working
• Investigate symptoms such as fatigue, unexplained weight loss, or persistent pain
• Check how your organs — particularly your liver and kidneys — are functioning
• Assess your cholesterol and cardiovascular risk
• Check vitamin and mineral levels, such as vitamin D, B12, and iron

A single blood draw can yield dozens of individual measurements, making blood tests extraordinarily efficient as a diagnostic tool.

How to Read Your Blood Test Report

Most NHS blood test results are now available online through the NHS App or your GP's patient portal. A typical blood test report includes the following columns:

• Test name: What was measured (e.g., Haemoglobin, Glucose, Creatinine)
• Your result: The actual value recorded from your blood sample
• Units: The unit of measurement (e.g., g/L, mmol/L, µmol/L)
• Reference range: The range considered normal for the general population
• Flag: H (high), L (low), or no flag if within the normal range

It is important to understand that reference ranges vary between laboratories and can differ based on your age, sex, and ethnicity. A result slightly outside the reference range is not always clinically significant — context matters enormously.

The Most Common Blood Test Panels

Full Blood Count (FBC)

The FBC is often the first blood test ordered. It measures the cells in your blood — red blood cells, white blood cells, and platelets. It can detect anaemia, infection, and blood disorders. Key components include haemoglobin, white cell count, and platelet count.

Kidney Function (U&Es — Urea and Electrolytes)

This panel checks how well your kidneys are filtering waste from your blood. It includes sodium, potassium, urea, creatinine, and eGFR (estimated glomerular filtration rate). Low eGFR can indicate chronic kidney disease.

Liver Function Tests (LFTs)

LFTs assess the health of your liver. They measure enzymes and proteins including ALT (alanine transaminase), AST (aspartate transaminase), ALP (alkaline phosphatase), bilirubin, and albumin. Raised liver enzymes can indicate liver inflammation, fatty liver disease, or the effect of medications.

Lipid Profile (Cholesterol)

This measures your total cholesterol, LDL ("bad") cholesterol, HDL ("good") cholesterol, and triglycerides. Your results are used to calculate your cardiovascular risk and guide decisions about lifestyle changes or statin therapy.

Blood Glucose & HbA1c

Fasting glucose measures your blood sugar at a single point in time. HbA1c is a longer-term measure reflecting your average blood sugar over the previous 2–3 months — it is the gold standard test for diagnosing and monitoring diabetes.

Thyroid Function (TFTs)

The thyroid gland controls your metabolism. TFTs typically measure TSH (thyroid stimulating hormone), and sometimes T4 and T3. An underactive thyroid (hypothyroidism) causes fatigue and weight gain; an overactive thyroid (hyperthyroidism) causes weight loss and palpitations.

Iron Studies

Iron studies assess your body's iron stores. They include serum iron, ferritin (stored iron), and transferrin saturation. Low ferritin is the most common cause of iron-deficiency anaemia, which presents as fatigue and breathlessness.

Understanding Reference Ranges

Reference ranges are typically derived from the values seen in 95% of a healthy population. This means that by definition, 5% of healthy people will have a result outside the "normal" range on any given test. A mildly abnormal result does not automatically indicate disease.

Factors that affect whether a borderline result is significant include:

• How far outside the range the result falls
• Whether you have relevant symptoms
• Your age, sex, and medical history
• Trends over time (a result that was normal last year but is now borderline low may be more significant than an isolated borderline result)
• Other results in the same panel that support or refute a diagnosis

What Do "H" and "L" Flags Mean?

High (H) and low (L) flags simply indicate that your result falls outside the laboratory's reference range. They do not automatically mean something is seriously wrong. Your GP or clinician will interpret flags in the context of your overall health, symptoms, and medical history.

However, some abnormal results do require prompt attention. If your GP surgery has flagged a result as requiring urgent follow-up, do not delay contacting them.

When to Contact Your GP

You should contact your GP if:

• You have results flagged as significantly abnormal or "critical"
• Your results have changed noticeably from previous tests
• You have symptoms that accompany an abnormal result
• You are unsure what your results mean
• Your GP surgery has left a message asking you to call

How HealthLens Can Help

HealthLens uses AI to provide a plain-English explanation of each component of your blood test results, helping you understand what each test measures and what your specific values suggest. This is designed to help you prepare for a conversation with your GP — not to replace it.

Our tool can help you understand unfamiliar abbreviations, contextualise values against normal ranges, and identify which results your GP is most likely to want to discuss with you.