A full blood count is one of the most commonly requested laboratory tests, and one of the most frequently misunderstood. The report arrives with a column of numbers, reference ranges, and occasional flags, and it is not always obvious what to do with them. This article explains what each component measures, why values fall outside the reference range, and which findings typically require further investigation.

Haemoglobin and Red Cell Indices

Haemoglobin is the protein in red blood cells that carries oxygen. A low haemoglobin (anaemia) has many possible causes: iron deficiency, B12 or folate deficiency, chronic disease, or less commonly a bone marrow problem. The red cell indices, particularly MCV (mean cell volume), help narrow the cause. A low MCV with low haemoglobin points toward iron deficiency or thalassaemia trait. A high MCV points toward B12 or folate deficiency, or alcohol excess. A normal MCV with low haemoglobin is common in anaemia of chronic disease.

White Cell Count and Differential

The white cell count measures the total number of immune cells in circulation. The differential breaks this into subtypes: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. A raised neutrophil count is common in bacterial infections and inflammatory conditions. A raised lymphocyte count is more typical of viral infections. Eosinophilia, a raised eosinophil count, can indicate allergy, parasitic infection, or, less commonly, a haematological condition. A single mildly elevated or mildly low result in an otherwise well person often requires no more than a repeat test in four to six weeks.

Platelets and Clotting

Platelets are the small cell fragments responsible for initiating clot formation. A low platelet count (thrombocytopenia) can result from viral illness, certain medications, autoimmune conditions, or bone marrow suppression. A high platelet count (thrombocytosis) is often a reactive finding, seen after infection, surgery, or iron deficiency. Isolated mild thrombocytosis in an otherwise normal FBC rarely requires urgent investigation. Severe thrombocytopenia, particularly below 50 x10^9/L, warrants prompt clinical review.

When to Act on an Abnormal Result

Not every result outside the reference range requires immediate action. Reference ranges are set to include 95% of healthy individuals, which means 5% of normal people will have at least one value outside range on any given test. Context matters: a mildly low haemoglobin in a menstruating woman is a different clinical situation from the same value in a 70-year-old man. If you receive a result you are uncertain about, the most useful next step is to discuss it with your GP or the requesting clinician rather than interpreting it in isolation.

At Pulse Path Lab Core, every FBC with an abnormal flag receives a manual slide review and a brief interpretive comment in the report. If you have questions about a result, our scientific team is available by phone during operating hours.