Full Blood Count (FBC): How to Read Hb, White Cells and Platelets

You open a patient's blood results and see Hb 82, WCC 14.6, neutrophils 12.1 and platelets 410. Four numbers are abnormal. But which one matters most, what might the pattern mean, and what should you check next?

That is the skill this guide is designed to build. A full blood count (FBC) is not a list of numbers to memorise. It is a set of related measurements that can give you clues about anaemia, infection and inflammation, bleeding risk, bone-marrow function and haematological disease.

By the end, you should be able to open an FBC, work through it in a sensible order, recognise common patterns and explain what you would check at the bedside.

First: what is actually in an FBC?

An FBC usually gives you three main groups of information:

• Red cells: haemoglobin (Hb), red blood cell count, haematocrit and red-cell indices such as MCV and MCH.
• White cells: total white cell count (WCC) and the differential, including neutrophils, lymphocytes, monocytes, eosinophils and basophils.
• Platelets: the platelet count, which forms part of your assessment of haemostasis and bleeding risk.

The laboratory may also add analyser flags, blood-film comments or recommendations for further review.

Do not memorise one universal 'normal' range. Reference intervals vary between laboratories and patient groups. Age, pregnancy and other physiological factors can affect interpretation. Use the reference interval displayed beside the patient's result.

You have opened the FBC. Where do you start?

Before trying to diagnose anything, scan the panel and ask five questions:

1. What is abnormal?
2. Is anything substantially different from the previous result?
3. Do several abnormalities form a recognisable pattern?
4. Does that pattern fit what is happening clinically?
5. Is there anything that needs prompt escalation?

A useful habit is: result → trend → patient → action.

1. Haemoglobin: start by asking whether the patient is anaemic

Haemoglobin is the oxygen-carrying protein inside red blood cells. When the haemoglobin concentration is below the appropriate reference interval, the patient is anaemic.

But anaemia is a finding, not a diagnosis. An Hb of 82 g/L tells you that anaemia is present; it does not tell you why.

Possible causes include blood loss, iron deficiency, vitamin B12 or folate deficiency, chronic inflammation, chronic kidney disease, haemolysis and bone-marrow disorders.

Do not look at Hb without looking backwards

Suppose today's Hb is 82 g/L.

If the patient's Hb has been around 80–85 g/L for several weeks, that is different from an Hb that was 126 g/L yesterday and is 82 g/L today. The second pattern should immediately make you ask about acute blood loss, haemodilution, the clinical condition and whether the result is plausible.

At the bedside, look for symptoms and signs such as fatigue, breathlessness, dizziness, pallor, tachycardia, chest discomfort or evidence of bleeding. The patient's symptoms, observations, comorbidities and rate of change all matter.

2. MCV: use red-cell size to organise your thinking

Once you have identified anaemia, look at the mean cell volume (MCV). MCV describes the average size of the circulating red blood cells and helps you organise the differential diagnosis.

Low MCV: microcytic pattern

A low MCV means the red cells are, on average, smaller than expected. Common considerations include iron deficiency and thalassaemia, although other causes exist.

Do not diagnose iron deficiency from MCV alone. Iron deficiency can exist with a normal MCV, particularly earlier in its development or when more than one process is affecting the red cells. Ferritin and other investigations are interpreted alongside the history and clinical context.

Normal MCV: normocytic pattern

A normal MCV does not mean the anaemia is unimportant. Normocytic anaemia may occur with acute blood loss, chronic inflammation, kidney disease, haemolysis, bone-marrow disease or mixed deficiencies.

High MCV: macrocytic pattern

A high MCV means the red cells are larger on average. Causes include vitamin B12 or folate deficiency, alcohol-related changes, liver disease, hypothyroidism, some medicines and bone-marrow disorders.

The useful question is not simply “Is the MCV high or low?” It is “Does the MCV help explain why this patient's Hb is low, and what other investigations would help?”

3. Other red-cell indices: useful, but do not get lost in them

Haematocrit (Hct) describes the proportion of blood volume occupied by red cells. Mean cell haemoglobin (MCH) reflects the average amount of haemoglobin within a red cell. Red-cell distribution width (RDW) describes variation in red-cell size.

These can add detail to the pattern, but on placement you do not need to solve every index before recognising an important Hb abnormality. Start with Hb + MCV + trend + patient, then add the other indices when they help answer a specific question.

4. White cell count: do not stop at 'the WCC is high'

The total WCC combines several different white-cell populations. A raised WCC is therefore only the beginning of the interpretation.

If the WCC is abnormal, look at the differential. Ask which cell type is driving the change.

Neutrophils

Neutrophils are important in the innate immune response and often rise with bacterial infection, but neutrophilia is not synonymous with bacterial infection. It may also occur with inflammation, physiological stress, smoking, tissue injury and corticosteroid treatment.

So if a patient has WCC 15.2 × 10⁹/L and neutrophils 13.4 × 10⁹/L, do not write “infection” purely from the blood result. Check the patient: temperature, NEWS2 observations, symptoms, examination findings, recent surgery, medicines, inflammatory markers, cultures and the wider clinical assessment.

Where does CRP fit?

You will often see C-reactive protein (CRP) reviewed alongside the FBC, although CRP is not part of the full blood count. CRP is an acute-phase protein produced by the liver and its concentration can rise in response to inflammation.

A raised CRP can support the clinical picture when infection or another inflammatory process is suspected, but CRP does not diagnose infection on its own. It can rise with bacterial and viral infection, inflammatory disease, tissue injury, surgery and other causes of inflammation.

Think of WCC, neutrophils and CRP as clues that need a patient attached to them. For example:

WCC 15.2 | neutrophils 13.4 | CRP 186 mg/L

These results suggest a significant inflammatory response, but they still do not tell you where the inflammation is coming from or prove that the cause is bacterial. Now assess the patient: observations and NEWS2, symptoms, examination findings, likely source, cultures or other investigations, treatment already given and the direction of the trend.

CRP is particularly useful as a trend. A falling value may support improvement in the right clinical context, while a rising value may prompt reassessment, but neither should override what is happening clinically.

Low neutrophils: a different kind of concern

A reduced neutrophil count is neutropenia. Infection risk increases as neutropenia becomes more severe, particularly in patients receiving anticancer treatment or with haematological disease.

An unwell patient who is known or suspected to be neutropenic requires prompt assessment and escalation according to their clinical pathway. Do not wait for the patient to look dramatically unwell before raising concern.

Lymphocytes

Lymphocyte abnormalities may occur with viral infections, immune conditions, medicines and haematological disorders. As with neutrophils, a single result does not diagnose the cause.

Monocytes, eosinophils and basophils

These are not 'extra numbers' to ignore. Monocytes participate in immune and inflammatory responses. Eosinophilia can occur with allergic disease, parasitic infection and some drug reactions. Basophil abnormalities are less common but can be relevant in inflammatory and haematological conditions.

Persistent, marked or unexplained abnormalities need clinical interpretation and may require further investigation.

5. Platelets: think haemostasis, bleeding and the trend

Platelets contribute to primary haemostasis by adhering and aggregating at sites of vascular injury. The platelet count therefore matters when assessing bleeding risk, but the number alone does not tell you whether a patient will bleed.

Low platelets: thrombocytopenia

Thrombocytopenia can occur because platelet production is reduced, platelets are being destroyed or consumed, or because of sequestration and other processes. Causes include infection, medicines, immune thrombocytopenia, liver disease, disseminated intravascular coagulation and bone-marrow disorders.

If the platelet count is low, ask:

• Is the patient bleeding or bruising?
• How low is the count, and is it falling?
• What was the previous platelet count?
• Are they taking anticoagulants or antiplatelet medicines?
• Are they septic, post-operative or otherwise acutely unwell?
• Is there a laboratory comment about platelet clumping?

Platelet transfusion decisions depend on the cause, bleeding, procedures and clinical context; they are not made from one count in isolation. UK guidance therefore uses different thresholds for different clinical situations.

High platelets: thrombocytosis

Thrombocytosis may be reactive, for example following infection, inflammation, bleeding or iron deficiency. Persistent thrombocytosis can also occur with myeloproliferative and other haematological disorders and requires appropriate investigation.

6. Read the pattern, not five separate abnormalities

This is where FBC interpretation becomes useful.

Pattern A: low Hb + low MCV

Hb 88 g/L | MCV 72 fL

This is a microcytic anaemia pattern. Iron deficiency is an important possibility, but it is not the only one. Look for ferritin and other iron studies, bleeding history, dietary history and the clinical context rather than diagnosing from two numbers.

Pattern B: raised WCC + neutrophilia

WCC 16.1 × 10⁹/L | neutrophils 14.2 × 10⁹/L

This is a neutrophilic leukocytosis pattern. Infection is one possibility, but now ask whether the patient actually has evidence of infection or another reason for neutrophilia. If CRP has also been measured, use it as another piece of the inflammatory picture rather than as proof of infection.

Pattern C: falling Hb + falling platelets in an acutely unwell patient

A falling Hb and platelet count together deserve more attention than either result considered in isolation. Depending on the clinical context, bleeding, haemodilution, consumption, sepsis, medicines and haematological processes may need consideration.

Your role is not to independently diagnose a complex coagulation disorder from the FBC. Your role is to recognise the concerning trend, assess the patient and escalate clearly.

7. Could the result be wrong?

Before building a clinical theory around an unexpected result, check whether there is a reason the sample may be unreliable.

FBC samples are commonly collected into an EDTA tube, typically purple-topped in UK adult blood-collection systems, although local tube systems and policies must always be followed.

Problems can include:

• a clotted sample
• platelet clumping, which can produce a falsely low automated platelet count
• dilution if blood is collected inappropriately from or near an intravenous infusion
• incorrect collection or handling
• laboratory analyser flags requiring blood-film review.

If a result looks completely inconsistent with the patient or with previous results, check the laboratory comments and escalate it. Do not simply decide that an abnormal result must be an error.

A repeatable FBC routine for placement

Use the same sequence each time until it becomes automatic:

1. Scan the whole FBC. What is outside the laboratory reference interval?
2. Check Hb. Is the patient anaemic, and has it changed?
3. If Hb is low, check MCV. Microcytic, normocytic or macrocytic?
4. Check WCC and differential. Which white-cell population is driving the abnormality?
5. If inflammatory markers are available, put them in context. CRP can add information, but it does not diagnose infection by itself.
6. Check platelets. High, low, stable or falling?
7. Compare with previous results. Acute change or established pattern?
8. Read laboratory comments. Any clot, platelet clumping, film comment or analyser flag?
9. Go back to the patient. Observations, symptoms, bleeding, infection risk, medicines and current illness.
10. Escalate the pattern. Give exact values and trends rather than saying only “the FBC is abnormal”.

Patient scenarios: practise the interpretation

Scenario 1: the Hb has fallen

Mrs Reid, 74, is one day after surgery. She feels light-headed when sitting out of bed and her heart rate has increased.

Yesterday: Hb 118 g/L | platelets 242 × 10⁹/L
Today: Hb 86 g/L | platelets 225 × 10⁹/L

What matters? The Hb has fallen by 32 g/L alongside new symptoms and tachycardia. Do not just document “anaemia”. Assess the patient, check observations and evidence of bleeding, review fluid balance and escalate the change promptly.

A useful escalation sounds like: “Her haemoglobin has fallen from 118 to 86 g/L since yesterday. She is newly light-headed and her heart rate has increased. I am concerned about the fall in Hb and possible post-operative blood loss.”

Scenario 2: raised WCC and CRP still need a patient

Mr Lewis has WCC 15.8 × 10⁹/L, neutrophils 13.6 × 10⁹/L and CRP 124 mg/L. He received corticosteroids earlier and has no fever or new localising infective symptoms.

Can you call this bacterial infection? No. The blood results show an inflammatory pattern, but they do not establish the cause. Neutrophilia may be influenced by corticosteroids and physiological stress, while CRP can rise in many inflammatory states. Assess the patient, review observations, symptoms, likely source, other investigations and previous results before interpreting the pattern.

Scenario 3: low platelets

Ms Patel's platelet count has fallen from 168 to 58 × 10⁹/L over several days. She has new bruising.

What should catch your attention? The combination of a substantial downward trend and clinical evidence of bleeding tendency. Check medicines, the wider FBC, laboratory comments and the patient's clinical condition, and escalate promptly. The cause cannot be determined from the platelet count alone.

Scenario 4: the FBC is 'normal' but the patient is deteriorating

A patient has an FBC within the laboratory reference intervals but is hypotensive, tachycardic and increasingly confused.

Does the FBC reassure you? Not enough to ignore the patient. Blood results are one part of assessment and may lag behind acute clinical change. Escalate the deterioration using the patient's observations and clinical findings.

How to hand over an abnormal FBC

A vague handover such as “their bloods are bad” is difficult to act on. Give the information that shows why you are concerned.

For example:

“The Hb has fallen from 108 to 76 g/L since yesterday. The patient is newly tachycardic at 112 bpm and has passed melaena. I am concerned about active bleeding.”

That communicates the result + trend + patient + concern.

The bit to remember

When you open an FBC, think:

Hb → MCV → WCC differential → platelets → previous results → patient.

If CRP is available, use it as an additional inflammatory marker, not as a diagnosis.

Do not try to diagnose every abnormality from the laboratory screen. Your first job is to recognise the pattern, notice important changes, connect them to the patient and escalate appropriately.

For the wider approach to reference intervals, trends and sample problems, continue with Blood Test Results Explained.

References

National Health Service (NHS) (n.d.) Blood tests. Available at: NHS blood tests 

National Institute for Health and Care Excellence (NICE) (n.d.) Anaemia: iron deficiency. Clinical Knowledge Summary. Available at: NICE CKS: iron-deficiency anaemia 

British Society for Haematology (2024) Identification and management of preoperative anaemia in adults: A British Society for Haematology Guideline update. Available at: British Society for Haematology 

British Society for Haematology (2016, reviewed 2022) Guidelines for the use of platelet transfusions. Available at: British Society for Haematology platelet guideline 

Gloucestershire Hospitals NHS Foundation Trust (n.d.) Full Blood Count (FBC). Available at: Full Blood Count (FBC) 

Gloucestershire Hospitals NHS Foundation Trust (n.d.) C-reactive protein (CRP). Available at: C-reactive protein (CRP) 

Nursing and Midwifery Council (NMC) (2018, updated 2025) The Code: Professional standards of practice and behaviour for nurses, midwives and nursing associates. Available at: NMC Code