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FBC Blood Test (Full Blood Count): What It Tests, What Results Mean & When to Worry
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed April 2026
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
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An FBC blood test, or full blood count, measures the three main cell types in your blood: red cells and haemoglobin (carry oxygen), white cells (fight infection), and platelets (help clotting). It screens for anaemia, infection and bleeding problems, and is the most ordered blood test in the UK.
The full blood count, or FBC (a CBC in the US), is the most ordered blood test in the UK. GPs use it to screen for anaemia, infection, and bleeding disorders. Yet most people get their results with little explanation. This guide covers every marker, what abnormal results suggest, and when to see your GP.
1. What Is a Full Blood Count?
A full blood count measures the three main cell types in your blood: red blood cells (carry oxygen), white blood cells (fight infection), and platelets (help clotting). It also measures haemoglobin, the protein inside red cells that carries the oxygen.
The NHS uses it as a first-line check for fatigue, bruising, and infections, and before surgery. Despite looking basic, one abnormal value can flag iron or B12 deficiency, thyroid problems, or inflammation.
“Measures the number of blood cells in your blood to check for blood conditions such as iron deficiency anaemia.”
— NHS, Blood tests (Full blood count)
2. What Gets Tested (the Full Panel)
A standard UK FBC reports about 15 measures. The core ones are:
Red cell parameters
- Haemoglobin (Hb): the oxygen-carrying protein, and the key marker for anaemia.
- Red blood cell count (RBC): red cells per litre of blood.
- Haematocrit (Hct / PCV): the share of blood made up of red cells.
- Mean corpuscular volume (MCV): the average red cell size. It classifies anaemia as microcytic (small, often iron deficiency), normocytic, or macrocytic (large, often B12 or folate deficiency).
- Mean corpuscular haemoglobin (MCH): haemoglobin per red cell.
- Red cell distribution width (RDW): the variation in red cell size. A high RDW can flag early deficiency. For every other code on the report, see our blood test abbreviations decoder.
White cell parameters
- White blood cell count (WBC): raised in infection and inflammation; low in viral infection and with some drugs.
- Differential count: splits WBC into five subtypes (see below).
Platelet parameters
- Platelet count (PLT): low raises bleeding risk; high can flag inflammation or iron deficiency.
- Mean platelet volume (MPV): the average platelet size. Larger platelets are younger and more reactive.
3. NHS Reference Ranges
Ranges vary slightly between labs. These values follow standard NHS and British Society for Haematology (BSH) guidelines for adults.
| Marker | Men | Women | Unit |
|---|---|---|---|
| Haemoglobin (Hb) | 130–170 | 120–150 | g/L |
| Red blood cells (RBC) | 4.5–5.5 | 3.8–5.0 | × 10¹²/L |
| Haematocrit (Hct) | 0.40–0.52 | 0.36–0.47 | L/L |
| MCV | 80–100 | 80–100 | fL |
| MCH | 27–32 | 27–32 | pg |
| RDW | 11.5–14.5 | 11.5–14.5 | % |
| White blood cells (WBC) | 4.0–11.0 | 4.0–11.0 | × 10⁹/L |
| Neutrophils | 2.0–7.5 | 2.0–7.5 | × 10⁹/L |
| Lymphocytes | 1.0–4.5 | 1.0–4.5 | × 10⁹/L |
| Monocytes | 0.2–0.8 | 0.2–0.8 | × 10⁹/L |
| Eosinophils | 0.04–0.4 | 0.04–0.4 | × 10⁹/L |
| Basophils | 0.01–0.1 | 0.01–0.1 | × 10⁹/L |
| Platelets (PLT) | 150–400 | 150–400 | × 10⁹/L |
| MPV | 7.5–12.0 | 7.5–12.0 | fL |
Haemoglobin (Hb)
M: 130–170 · F: 120–150 g/L
RBC
M: 4.5–5.5 · F: 3.8–5.0 × 10¹²/L
Haematocrit
M: 0.40–0.52 · F: 0.36–0.47 L/L
MCV
80–100 fL
WBC
4.0–11.0 × 10⁹/L
Neutrophils
2.0–7.5 × 10⁹/L
Lymphocytes
1.0–4.5 × 10⁹/L
Platelets
150–400 × 10⁹/L
Being "within range" is not always optimal. A haemoglobin of 121 g/L in a woman is technically normal but can still cause real fatigue. Symptoms and trends matter more than one number.
4. Red Blood Cells Explained
Red blood cells are made in the bone marrow and live about 120 days. They carry oxygen from your lungs to every tissue via haemoglobin.
Low haemoglobin / low RBC (anaemia)
Anaemia affects about 1.8 billion people worldwide (WHO), and iron deficiency anaemia affects about 3% of UK men and 8% of premenopausal women. The MCV shows what kind you have:
- Low MCV (<80 fL), microcytic: usually iron deficiency, also thalassaemia trait or chronic disease.
- Normal MCV (80–100 fL), normocytic: chronic disease, kidney disease, early iron deficiency, or blood loss.
- High MCV (>100 fL), macrocytic: B12 or folate deficiency, alcohol, or liver disease.
High haemoglobin (polycythaemia)
A mildly raised Hb on one test is most often dehydration. Other causes are lung disease, smoking, or high altitude. Rarely, it is polycythaemia vera. If your Hb stays high, your GP should investigate.
Why RDW matters
RDW measures variation in red cell size. A high RDW can flag mixed deficiencies (iron plus B12), early anaemia, or chronic inflammation. Research in The Lancet Haematology links a raised RDW to higher all-cause mortality.
5. White Blood Cells Explained
White blood cells (leucocytes) are your immune front line. The "differential" splits them into five subtypes:
- Neutrophils (50–70%). Raised by bacterial infection, inflammation, or stress. Low with some viruses or drugs.
- Lymphocytes (20–40%). Raised by viral infection or, in older adults, leukaemia. Low with HIV or severe illness.
- Monocytes (2–8%). Persistently high points to chronic inflammation.
- Eosinophils (1–4%). Raised by allergies, parasites, or drug reactions.
- Basophils (<1%). The rarest, involved in severe allergic reactions.
6. Platelets Explained
Platelets are small cell fragments that help blood clot.
- Low (thrombocytopenia, <150). Viral infection, autoimmune ITP, or liver disease. Below 50 needs urgent review; below 10 is an emergency.
- High (thrombocytosis, >400). Usually reactive, from iron deficiency, infection, or inflammation. It settles once the cause is treated.
- MPV. A high MPV with a low count means the marrow is making new platelets.
7. GP Full Blood Count vs Helvy
Your GP orders an FBC on the NHS for free. So when does a private test make sense?
| NHS GP | Helvy | |
|---|---|---|
| Full blood count | Yes | Not in Helvy panels — a GP test |
| Cost | Free | From £99 |
| Wait for appointment | 1–3 weeks | Order today, test at home |
| Results turnaround | 1–7 days | Usually 5 working days from sample receipt |
| Explanation of results | Brief GP comment | Full biomarker report |
| Wider wellness markers | Only what GP requests | Thyroid, cholesterol, vitamin D, B12 |
| Retest tracking | Request new appointment | Dashboard with trends |
| Validated methods | GP visit if needed | ISO 15189 at UKAS-accredited UK labs |
Full blood count
NHS: yes
Helvy: not in panels (a GP test)
Wait time
NHS: 1–3 weeks
Helvy: order today
Wider markers
NHS: only what GP requests
Helvy: thyroid, cholesterol, vitamins
Results
NHS: brief GP comment
Helvy: full biomarker report
The value of private testing is context. An FBC tells you a lot, but pairing it with iron studies, B12, folate, and thyroid gives the full picture.
8. Interpreting Your Results — Common Patterns
GPs read patterns, not single numbers:
- Low Hb + low MCV + low ferritin. Classic iron deficiency anaemia. Look for heavy periods, diet, or gut blood loss.
- Low Hb + high MCV. Usually B12 or folate deficiency, alcohol, or an underactive thyroid.
- Raised WBC + neutrophils. Usually bacterial infection or inflammation.
- Raised WBC + lymphocytes. Often viral. In adults over 50, a lasting rise needs a check for chronic lymphocytic leukaemia (CLL).
- Low Hb + low WBC + low platelets (pancytopenia). This needs urgent investigation and referral.
9. Who Should Get an FBC
NICE recommends an FBC for a wide range of symptoms. Consider testing if you:
- Feel persistently tired, weak, or short of breath
- Bruise easily or bleed for a long time after minor cuts
- Have heavy periods (the top cause of iron deficiency)
- Eat vegetarian or vegan (higher B12, folate, and iron risk)
- Get recurrent infections, sore throats, or mouth ulcers
- Are pregnant or planning pregnancy
- Take drugs that affect blood counts
- Have a family history of blood disorders
- Want a proactive baseline, especially over 40
10. Conditions an FBC Can Help Detect
An FBC rarely gives a diagnosis alone, but it flags clues for dozens of conditions:
Iron deficiency anaemia
Low Hb, low MCV, low MCH
B12 / folate deficiency
Low Hb, high MCV
Thalassaemia trait
Low MCV, normal or high RBC
Chronic kidney disease
Low Hb, normocytic
Hypothyroidism
Low Hb, macrocytic or normocytic
Bacterial infection
Raised WBC + neutrophils
Viral infection
Raised lymphocytes
Allergic conditions
Raised eosinophils
Autoimmune disease
Low WBC, low lymphocytes
Liver disease
High MCV, low platelets
Blood cancers
Abnormal WBC differential
Chronic inflammation
Raised WBC, raised platelets
An abnormal FBC is a starting point, not an endpoint. Further tests usually confirm the diagnosis.
11. How to Prepare for the Test
The FBC needs no fasting. But if your test also includes cholesterol or triglycerides, take a fasting morning sample.
- Hydrate well beforehand; dehydration can falsely raise haemoglobin
- Water is fine even when fasting for other markers
- Tell the doctor about iron, B12, or anticoagulant use, which affect results
- Avoid intense exercise for 24 hours before (it can raise WBC)
12. Beyond the FBC — Additional Tests
An FBC gives a broad overview. Specific concerns warrant targeted follow-up:
- Iron studies (ferritin) if MCV is low.
- B12 and folate if MCV is high or you eat plant-based.
- Reticulocyte count or blood film to show marrow response and abnormal cells.
- Thyroid and liver tests if MCV is raised.
- CRP or ESR if WBC is raised without obvious infection.
13. When to See Your GP
Book a GP appointment if any of the following apply:
- Haemoglobin below range, especially below 100 g/L
- WBC well outside range (<3.0 or >15.0) with no cause
- Platelets below 100, which raises bleeding risk
- Pancytopenia (low Hb, WBC, and platelets)
- A lasting rise in WBC or lymphocytes over 50 (rule out CLL)
- Unexplained weight loss with any abnormal result
The NICE NG12 guidelines advise an urgent FBC for unexplained fatigue, recurrent infections, bruising, or swollen glands. An abnormal FBC can trigger the two-week-wait cancer referral.
14. Frequently Asked Questions
What is an FBC blood test?
An FBC blood test is a full blood count. It counts and sizes the red cells, white cells and platelets in a small blood sample. The NHS uses it as a first-line check for tiredness, bruising and infection, and before surgery. It is a screening test, so it shows the pattern rather than the cause, and a qualified clinician reads it alongside your symptoms.
What is a normal haemoglobin level in the UK?
The NHS range is 130–170 g/L for men and 120–150 g/L for women. In pregnancy it drops as blood volume rises, with a lower threshold of 110 g/L in the first trimester (NICE NG201).
Do I need to fast before a full blood count?
No. Food does not affect the FBC. If your test also includes cholesterol, triglycerides, or fasting glucose, fast overnight. Water is always fine.
Can stress or exercise affect my FBC results?
Yes. Intense exercise can raise white cells for 24–48 hours, and dehydration raises haemoglobin and haematocrit. For the most accurate baseline, test on a rest day and stay hydrated.
How often should I get a full blood count?
There is no set NHS interval for healthy adults. With symptoms or risk factors, annual testing is reasonable. For a baseline, an FBC every 1–2 years gives useful trend data.
What does it mean if my MCV is high but haemoglobin is normal?
A raised MCV without anaemia can mean early B12 or folate deficiency, regular alcohol, an underactive thyroid, or liver disease. Check B12, folate, and thyroid function before it becomes anaemia.
Is an FBC the same as a CBC?
Yes. FBC is the UK term; CBC is the US term. They measure the same things. US labs report haemoglobin in g/dL, not g/L, so multiply g/dL by 10 to convert.
RELATED GUIDES
Stop guessing. Start knowing.
Helvy panels do not include a full blood count, which stays a GP test. But if an FBC has not explained your symptoms, our General Energy & Wellness panel checks thyroid (TSH, free T4), cholesterol, vitamin D, and B12. Results usually within 5 working days.