How to Read Blood Test Results
Feb 6, 2026
How to Read Blood Test Results
Understanding your blood test results empowers you to take control of your health—knowing what each marker means, what normal ranges are, and when to be concerned helps you have informed conversations with your doctor and make better health decisions. This guide explains how to interpret the key markers you'll find on most blood test reports.
At Blood Test London, every result comes with a doctor always reviewing every result and interpretation—but understanding the basics yourself is valuable.
Get Blood Tests with a doctor always reviewing every result
Understanding Your Report Structure
Common Elements
Element | What It Means |
|---|---|
Test name | What was measured |
Your result | The number or value |
Unit | How it's measured (mmol/L, g/L, etc.) |
Reference range | "Normal" range for the lab |
Flag | H (high), L (low), or blank (normal) |
Important Notes on Reference Ranges
Ranges vary between labs (different equipment, methods)
"Normal" doesn't always mean "optimal"
Your individual normal may differ from population average
Context matters—age, sex, health status affect interpretation
Key Blood Markers Explained
Full Blood Count (FBC)
Marker | What It Shows | Typical Range |
|---|---|---|
Haemoglobin (Hb) | Oxygen-carrying capacity | 120-170 g/L |
Red blood cells (RBC) | Number of red cells | 4.0-5.5 ×10¹²/L |
White blood cells (WBC) | Immune cells | 4.0-11.0 ×10⁹/L |
Platelets | Clotting cells | 150-400 ×10⁹/L |
MCV | Red cell size | 80-100 fL |
What abnormalities suggest:
Low Hb → Anaemia (investigate cause)
High WBC → Infection, inflammation, or other conditions
Low platelets → Bleeding risk, investigate cause
High MCV → B12/folate deficiency, alcohol excess
Metabolic Panel
Marker | What It Shows | Typical Range |
|---|---|---|
Glucose (fasting) | Blood sugar | 3.9-5.5 mmol/L |
HbA1c | 3-month average sugar | Below 42 mmol/mol |
Sodium | Electrolyte balance | 136-145 mmol/L |
Potassium | Electrolyte balance | 3.5-5.0 mmol/L |
Creatinine | Kidney function | 60-110 μmol/L |
eGFR | Estimated kidney function | >90 mL/min |
How to Interpret Flags
"H" (High) Flag
Doesn't always mean something is wrong:
Slightly high: Often not significant
Moderately high: Investigate or retest
Very high: Needs attention
"L" (Low) Flag
Similarly graduated:
Slightly low: May be normal for you
Moderately low: Often worth investigating
Very low: Needs attention
No Flag (Within Range)
Usually reassuring
But "normal" has a wide range
Optimal health may be in the narrower middle
Understanding Common Panels
Liver Function Tests (LFTs)
Marker | What It Shows | Concern If |
|---|---|---|
ALT | Liver cell damage | Elevated |
AST | Liver/muscle damage | Elevated |
ALP | Bile ducts/bone | Elevated |
GGT | Alcohol, bile ducts | Elevated |
Bilirubin | Jaundice marker | Elevated |
Albumin | Liver production | Low |
Pattern recognition:
ALT > AST → Fatty liver, hepatitis
AST > ALT → Alcohol damage
High ALP + GGT → Bile duct issue
Lipid Profile (Cholesterol)
Marker | Optimal | Risk |
|---|---|---|
Total cholesterol | <5.0 mmol/L | >6.0 |
LDL ("bad") | <3.0 mmol/L | >4.0 |
HDL ("good") | >1.0 (men), >1.2 (women) | Below these |
Triglycerides | <1.7 mmol/L | >2.3 |
Thyroid Function
Marker | Normal | Indicates If Abnormal |
|---|---|---|
TSH | 0.4-4.0 mIU/L | High = underactive; Low = overactive |
Free T4 | 12-22 pmol/L | Thyroid hormone level |
Free T3 | 3.1-6.8 pmol/L | Active thyroid hormone |
Get Your Results Explained by a Doctor
When to Be Concerned
Seek Urgent Advice If:
Very high or very low values (far outside range)
Multiple abnormalities together
Results that have changed dramatically from previous tests
Results that match symptoms you're experiencing
Usually Not Urgent:
Slightly out of range values
Single isolated abnormality
Stable results that have been monitored
Results flagged but close to normal
Questions to Ask Your Doctor
What does this result mean for my health?
Do I need any follow-up tests?
Is this likely related to my symptoms?
What can I do to improve this marker?
When should I retest?
Is this something that needs treatment?
Why Doctor Review Matters
Blood tests don't exist in isolation. Interpretation requires:
Your medical history
Your symptoms
Your medications
Previous results (trends)
Clinical judgment
At Blood Test London, a doctor always reviews every result who considers all these factors—not just flags on a report.
Book Blood Tests with a doctor always reviewing every result
All our tests include:
Results in 2-3 days
Doctor review of every result
Written interpretation
GP consultation available if needed
Phone: 020 3951 3429
FAQ
Why do reference ranges differ between labs?
Different equipment, methods, and populations used to establish ranges. Always compare results to the same lab's range.
My result is flagged but close to normal—should I worry?
Usually not. Minor deviations are often not significant. Context and trends matter more than single values.
Can I interpret results myself?
You can understand the basics, but professional interpretation considers factors you may not know are relevant.
What if my GP and private lab have different ranges?
Both are valid. Compare your result to the range provided with that specific test.
Blood Test London by The Wellness. Blood tests with A doctor always reviews every result, results in 2-3 days.