How to Interpret a CBC Report
The complete blood count (CBC) is the most commonly ordered blood test. It measures the three main types of cells in blood โ red cells, white cells, and platelets โ along with several indices that describe those cells' characteristics. Reading a CBC well means understanding not just whether individual values are in range, but what patterns of results suggest and what follow-up tests they point to.
What a CBC includes โ and what each index measures
A standard CBC includes three main cell type counts and several derived indices. The WBC differential โ which breaks down white cells by type โ may be included automatically or ordered separately. Here is every marker you are likely to see on a CBC report, with typical reference ranges:
| Abbreviation | Full name | What it measures | Typical range | Unit |
|---|---|---|---|---|
| RBC | Red blood cell count | Number of red blood cells per unit volume | Men: 4.5โ5.9 | Women: 4.1โ5.1 | ร 10ยนยฒ/L (or 10โถ/ฮผL) |
| Hgb / Hb | Haemoglobin | Concentration of the oxygen-carrying protein in red blood cells | Men: 13.5โ17.5 | Women: 12.0โ15.5 | g/dL |
| Hct / PCV | Haematocrit / Packed cell volume | Proportion of blood volume occupied by red blood cells | Men: 41โ53% | Women: 36โ46% | % |
| MCV | Mean corpuscular volume | Average volume (size) of a single red blood cell | 80โ100 | fL (femtolitres) |
| MCH | Mean corpuscular haemoglobin | Average amount of haemoglobin in a single red blood cell | 27โ33 | pg |
| MCHC | Mean corpuscular haemoglobin concentration | Average concentration of haemoglobin within a given volume of red blood cells | 315โ360 | g/L (or 31.5โ36.0 g/dL) |
| RDW | Red cell distribution width | Variation in red blood cell size (anisocytosis) | 11.5โ14.5 | % |
| WBC | White blood cell count | Total number of white blood cells per unit volume | 4.0โ11.0 | ร 10โน/L (or 10ยณ/ฮผL) |
| Neutrophils | Neutrophil count (part of WBC differential) | Most abundant WBC; primary responder to bacterial infection | 1.8โ7.5 | ร 10โน/L |
| Lymphocytes | Lymphocyte count (part of WBC differential) | Key role in viral immunity and antibody production | 1.0โ4.0 | ร 10โน/L |
| Monocytes | Monocyte count | Phagocytic WBCs involved in chronic inflammation | 0.2โ0.8 | ร 10โน/L |
| Eosinophils | Eosinophil count | WBCs involved in allergic responses and parasitic infections | 0.0โ0.5 | ร 10โน/L |
| Basophils | Basophil count | Rare WBCs involved in inflammatory reactions | 0.0โ0.1 | ร 10โน/L |
| PLT | Platelet count | Small cell fragments that initiate blood clot formation | 150โ400 | ร 10โน/L (or 10ยณ/ฮผL) |
Reference ranges are approximate and vary between laboratories. Always use the range printed on your own lab report.
Reading the red cell section: RBC, Hgb, Hct, MCV, MCH, MCHC, RDW
The red cell indices work together. In practice, clinicians focus on a few key patterns rather than looking at each value in isolation:
- Is there anaemia? Anaemia is defined by a haemoglobin below the reference interval for age and sex. Mild anaemia is Hgb 100โ129 g/L (10โ12.9 g/dL) in adults; moderate is 80โ99 g/L; severe is below 80 g/L. These thresholds differ slightly in pregnancy and in children.
- What type? MCV classifies anaemia as microcytic (<80 fL), normocytic (80โ100 fL), or macrocytic (>100 fL). This narrows the differential significantly before any additional tests are ordered.
- What does RDW add? RDW measures variation in red cell size. A high RDW in a microcytic anaemia favours iron deficiency over thalassaemia trait (which typically has a normal RDW and a very low MCV). A high RDW in a macrocytic anaemia alongside a low MCH pattern suggests a mixed deficiency (e.g., iron + B12).
- MCHC as a quality indicator. MCHC above the reference interval (hyperchromia) is uncommon but seen in hereditary spherocytosis and with haemolysis. Very high MCHC can indicate a sampling or analytical problem.
Classifying anaemia by MCV โ a practical framework
When haemoglobin is low, MCV guides the next steps:
| Anaemia type | MCV | Additional clues | Common causes |
|---|---|---|---|
| Microcytic anaemia (low MCV) | < 80 fL | Low MCH, low MCHC; often elevated RDW in iron deficiency | Iron deficiency anaemia (most common), thalassaemia trait, sideroblastic anaemia |
| Normocytic anaemia (normal MCV) | 80โ100 fL | MCV normal; check reticulocyte count, creatinine, TSH | Anaemia of chronic disease, acute blood loss, haemolytic anaemia, renal anaemia, hypothyroidism |
| Macrocytic anaemia (high MCV) | > 100 fL | High MCH; check B12, folate, reticulocyte count, LFTs, alcohol history | B12 or folate deficiency, alcohol excess, liver disease, hypothyroidism, myelodysplastic syndrome, hydroxyurea |
In practice, multiple causes of anaemia can coexist. Someone with inflammatory bowel disease may have both iron deficiency (microcytic tendency) and B12/folate deficiency (macrocytic tendency) โ the MCV may appear deceptively normal while RDW and the MCH pattern reveal a mixed picture.
Reading the white cell section: WBC and the differential
A high WBC (leukocytosis) or low WBC (leukopenia) should be interpreted together with the WBC differential โ which shows which type of white cell is driving the change.
High WBC (leukocytosis)
The most common causes are bacterial infection (usually neutrophilia), physical or emotional stress, corticosteroid medication (neutrophilia with relative lymphopenia), and smoking. In a healthy adult, mild leukocytosis in the absence of symptoms is often benign and transient.
A very high WBC (>30 ร 10โน/L), or leukocytosis combined with anaemia, thrombocytopenia, or abnormal cell morphology on the blood film, may prompt urgent haematology review to rule out haematological malignancy.
Low WBC (leukopenia)
Mild leukopenia is common in viral infections (particularly lymphopenia in COVID-19, HIV, and influenza) and in certain ethnic groups where lower neutrophil counts are a normal variant (benign ethnic neutropenia). More significant leukopenia can result from vitamin B12 or folate deficiency, some medications, autoimmune conditions, and bone marrow disorders.
Neutrophilia (high neutrophils)
Bacterial infection is the classic cause. Other common causes include inflammation, surgery, trauma, corticosteroids, and cigarette smoking. A very high neutrophil count with immature cells ("left shift" โ seen on blood film review) suggests a significant acute infection or bone marrow response.
Lymphocytosis (high lymphocytes)
Viral infections are the most common cause (EBV infectious mononucleosis, CMV, whooping cough). In older adults, a persistent lymphocytosis warrants consideration of chronic lymphocytic leukaemia (CLL), which is common and often indolent.
Eosinophilia (high eosinophils)
Most commonly caused by allergic conditions (asthma, hay fever, eczema, drug reactions) or parasitic infection. In the UK and Australia, parasites are a less common cause than allergy; in travellers or those from endemic regions, parasitic causes should be considered. Marked eosinophilia (>1.5 ร 10โน/L) may warrant further investigation.
Reading the platelet count
Platelets are small cell fragments produced in the bone marrow that initiate clotting when a vessel is damaged.
- Low platelets (thrombocytopenia, <150 ร 10โน/L): Mild thrombocytopenia (100โ150) is common and often benign. Values below 50 ร 10โน/L carry a risk of spontaneous bleeding; below 20 ร 10โน/L requires urgent assessment. Causes include immune thrombocytopenic purpura (ITP), viral infections, B12/folate deficiency, liver disease (spleen sequestration), heparin, and bone marrow disorders.
- High platelets (thrombocytosis, >400 ร 10โน/L): More commonly reactive โ infection, inflammation, iron deficiency, and post-splenectomy are the most frequent causes. Essential thrombocythaemia (ET), a primary bone marrow disorder, should be considered when platelets are markedly elevated (>600โ700 ร 10โน/L) without an obvious reactive cause.
A falsely low platelet count ("pseudothrombocytopenia") can occur due to platelet clumping in EDTA anticoagulant tubes. If the automated count is unexpectedly low and the clinical picture does not fit, the sample may be repeated in a citrate tube or the blood film reviewed.
Reading the CBC as a whole โ patterns that matter
The most informative insights from a CBC come from recognising patterns across multiple indices rather than fixating on a single abnormal value:
Pancytopenia (all three cell lines low)
Low RBC/Hgb + low WBC + low platelets simultaneously suggests a problem at the level of bone marrow production or excessive peripheral cell destruction. Causes include aplastic anaemia, B12/folate deficiency, myelodysplastic syndrome, bone marrow infiltration, hypersplenism, and haematological malignancy. Pancytopenia always warrants further investigation.
Anaemia with high platelets
A common combination seen in iron deficiency (iron deficiency stimulates platelet production as a side effect of thrombopoietin signalling). The platelet count typically normalises with iron treatment.
Macrocytosis without anaemia
High MCV with normal Hgb is common and not necessarily sinister. Alcohol excess, liver disease, hypothyroidism, and certain medications (methotrexate, hydroxyurea, azathioprine) are common causes. B12 and folate deficiency initially causes macrocytosis before dropping the haemoglobin.
WBC differential with blasts or immature cells
If the automated analyser flags abnormal cells or blasts, a manual blood film review by a haematologist is standard. The presence of circulating blast cells is an urgent finding.
Frequently asked questions
What is a CBC blood test?
A CBC (complete blood count) is a panel of tests that measures the different cells circulating in your blood: red blood cells (RBC), white blood cells (WBC), and platelets. It also measures indices that describe the characteristics of those cells, such as haemoglobin, haematocrit, MCV, MCH, MCHC, and RDW.
What does a high WBC mean?
An elevated white blood cell count (leukocytosis) can result from bacterial infection, viral infection, inflammation, stress, corticosteroid use, smoking, or โ less commonly โ haematological conditions. A differential WBC count (showing which type of white cell is elevated) helps narrow down the cause.
What causes a low MCV?
A low MCV (microcytosis) means red blood cells are smaller than usual. The most common causes are iron deficiency anaemia and thalassaemia trait. MCV should be interpreted alongside haemoglobin, MCH, ferritin, and clinical context.
Is a high RDW serious?
An elevated RDW on its own is not a diagnosis โ it indicates variability in red cell size (anisocytosis). Combined with a low MCV, it strongly suggests iron deficiency. Combined with a high MCV, it points toward B12 or folate deficiency. A high RDW with normal MCV and Hgb may be seen early in mixed deficiency or haemolytic conditions.
Do I need to fast before a CBC?
No. A CBC does not require fasting. However, some tests ordered at the same time (glucose, lipids) may require fasting โ confirm with your clinician what preparation is needed for the full blood draw.
Trusted reference sources
- MedlinePlus: Complete blood count (CBC)
- NHS: Types of blood tests
- Mayo Clinic: Complete blood count (CBC)
- Labcorp: CBC with differential reference
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