A full blood count (FBC) is the most frequently performed laboratory test worldwide and is usually one of the first tests a doctor requests. A single report contains numerous parameters identified by abbreviations: RBC, HGB, HCT, MCV, MCH, MCHC, RDW, WBC, PLT and MPV. Most patients focus only on the arrows marking results outside the reference range. However, the real value of a full blood count lies in understanding how these parameters relate to one another. This practical guide explains what each abbreviation means, which combinations of results are typical of common medical conditions and when a flagged result may not be significant. Final interpretation should always be carried out by a doctor.
Red blood cells: transporting oxygen
RBC, HGB, HCT
RBC (red blood cell count) indicates the number of red blood cells, or erythrocytes, per microlitre of blood. HGB (haemoglobin) measures the amount of oxygen-carrying protein in the blood. It is the most important individual parameter used to diagnose anaemia (approximately below 12 g/dl in adult women and below 13 g/dl in adult men; reference ranges vary between laboratories). HCT (haematocrit) represents the percentage of blood volume occupied by red blood cells. It increases with dehydration and decreases in anaemia.
MCV: red blood cell size
Mean corpuscular volume is a key parameter in classifying anaemia. Low MCV (microcytosis) most commonly indicates iron deficiency, and less frequently thalassaemia or anaemia of chronic disease. High MCV (macrocytosis) may indicate vitamin B12 or folate deficiency, liver disease, hypothyroidism, excessive alcohol consumption or medication effects. Normal MCV in someone with anaemia may suggest acute blood loss, kidney disease, inflammation or two simultaneous nutritional deficiencies.
MCH and MCHC: haemoglobin content of red blood cells
MCH is the mean mass of haemoglobin in an individual red blood cell, while MCHC represents its concentration. Both decrease in iron deficiency (red blood cells become small and pale, a condition known as hypochromia). Elevated MCHC is uncommon and may indicate spherocytosis or an analytical error, such as red blood cell agglutination at low temperatures.
RDW: variation in red blood cell size
Red cell distribution width (RDW) indicates how much red blood cells vary in size. It increases early in iron deficiency, as the bone marrow produces progressively smaller cells alongside older, normal-sized cells, even before haemoglobin levels fall. High RDW with normal MCV may indicate combined iron and vitamin B12 deficiency. Normal RDW with low MCV is more suggestive of thalassaemia than iron deficiency.
Reticulocytes
Reticulocytes are immature red blood cells and are not always included in a standard full blood count. A high reticulocyte count indicates increased bone marrow activity (following bleeding, during haemolysis or at the start of iron treatment). A low reticulocyte count in someone with anaemia suggests impaired red blood cell production, potentially caused by nutritional deficiencies or bone marrow or kidney disease.
White blood cells: immune defence
WBC and white blood cell differential
The total white blood cell count provides limited information without a differential, which divides white blood cells into five types: neutrophils, lymphocytes, monocytes, eosinophils and basophils. An automated differential reports both their percentages and absolute counts. The absolute count is generally more important for interpretation because percentages depend on the proportions of the other white blood cell types.
- Neutrophils increase during bacterial infections, inflammation, physical exertion, stress and corticosteroid treatment. They may decrease following viral infections, with certain medications and in bone marrow disorders. Significant neutropenia (below 1000/µl) requires prompt medical assessment.
- Lymphocytes increase during viral infections (including infectious mononucleosis), whooping cough and chronic lymphoproliferative disorders. They may decrease with stress, corticosteroid treatment, HIV infection and following chemotherapy.
- Monocytes increase during recovery from illness, chronic infections and inflammatory diseases.
- Eosinophils increase in allergies, asthma, parasitic infections, adverse drug reactions and skin conditions.
- Basophils are rarely clinically significant on their own. Persistently elevated levels require investigation to exclude myeloproliferative disorders.
Mild leukocytosis (an increased white blood cell count) after strenuous exercise, during pregnancy, in smokers and after eating can be physiological. Marked leukocytosis accompanied by immature or abnormal cells on a manual blood film requires haematological assessment.
Platelets: blood clotting
PLT (platelet count) may decrease during viral infections, liver disease, vitamin B12 deficiency, autoimmune disorders and following certain medications. It may increase with inflammation, iron deficiency, following significant blood loss and after removal of the spleen. MPV (mean platelet volume) and PDW (platelet distribution width) provide additional information: large platelets may indicate increased platelet production by the bone marrow. A common cause of falsely low platelet counts (pseudothrombocytopenia) is platelet clumping in an EDTA blood collection tube. In such cases, the laboratory may repeat the test using a citrate tube.
Common patterns in blood test results
| Result pattern | Most common explanation | What to do next |
|---|---|---|
| Low HGB, low MCV and MCH, high RDW, high PLT | Iron deficiency | Ferritin – laboratory test, Iron – laboratory test, TIBC; investigate the source of blood loss |
| Low HGB, high MCV, often low PLT and WBC | Vitamin B12 or folate deficiency | Vitamin B12 – laboratory test, folate, homocysteine |
| Low HGB, normal MCV, low reticulocyte count | Chronic disease, kidney disease, inflammation | C-reactive protein (CRP), Creatinine – laboratory test, Ferritin – laboratory test, Thyroid-stimulating hormone (TSH) – laboratory test |
| High WBC with neutrophilia, high CRP | Bacterial infection | Clinical assessment, with a culture test if indicated |
| High WBC with lymphocytosis | Viral infection or chronic lymphocytosis | Repeat after 2–4 weeks; consult a haematologist if the abnormality persists |
| High HGB, HCT and RBC | Dehydration, smoking, sleep apnoea or, less commonly, polycythaemia | Repeat the test when well hydrated; consider EPO and JAK2 testing if the abnormality persists |
When a flagged result may not matter
A reference interval covers 95% of healthy individuals, meaning that statistically, one in twenty results from healthy people may fall outside the reference range. Minor deviations in a single parameter, without symptoms, usually require no immediate action beyond repeating the test after a few weeks. More significant findings include haemoglobin below 10 g/dl, platelets below 100 000/µl or above 600 000/µl, neutrophils below 1000/µl, white blood cells above 20 000/µl, and any abnormal result accompanied by symptoms such as bleeding, fever, weight loss, night sweats or enlarged lymph nodes. These findings require medical assessment without delay.
Preparing for a full blood count
A full blood count can be performed at any time of day. However, for consistent results, it is best to have your blood taken in the morning, after resting, without strenuous exercise the previous day and while adequately hydrated. Fasting is not necessary for a full blood count alone, although it is often performed alongside tests that do require fasting. When discussing your results, it is helpful to mention any medication you take, infections during recent weeks and whether you are pregnant.
A full blood count with differential does not require a referral (a formal request for testing from a doctor); the current price is shown on BadamySie.pl. Ready-made preventive screening packages:
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A full blood count also has practical value when a wound is not healing properly: anaemia means less oxygen is delivered to the tissues, while an elevated white blood cell count may suggest infection. We explain which tests to consider for a wound that has remained open for more than 4 weeks in our article A wound that won’t heal: causes and tests.
Frequently asked questions
What is the difference between a full blood count with differential and a standard full blood count?
A white blood cell differential (5-diff) reports the absolute counts and percentages of five types of white blood cells. Without a differential, the result includes only the total white blood cell count. A differential is useful when investigating immune function, infections or allergies.
How often should I have a routine full blood count?
Once a year for healthy individuals; more frequently during pregnancy, in people with chronic conditions and while receiving treatment for nutritional deficiencies.
Does high MCV always mean vitamin B12 deficiency?
No. Alcohol consumption, liver disease, hypothyroidism and certain medications (such as methotrexate and hydroxyurea) can also increase red blood cell size. Vitamin B12 and folate testing helps identify the underlying cause.
Sources
- National Heart, Lung, and Blood Institute, NIH, Anemia: Diagnosis, 2025. nhlbi.nih.gov
- Maner BS et al., StatPearls, NCBI Bookshelf, Mean Corpuscular Volume, 2024. ncbi.nlm.nih.gov
- Snook J. et al., Gut, British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults, 2021. gut.bmj.com
Where can I get blood tests without a referral?
Blood collection centres with addresses and opening hours: Warszawa · Wrocław · Łódź · Kraków · Poznań · Gdańsk · Szczecin · Bydgoszcz · all cities.
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