Reading a Complete Blood Count Through a Functional Lens

The complete blood count is one of the most frequently ordered tests in medicine and one of the most under read. Often only the haemoglobin and white cell count are glanced at before the result is filed as normal. Yet the red cell indices, white cell differential and platelet count together contain clues to nutritional deficiencies, inflammation, alcohol use, liver and thyroid function, allergy and more. Reading a full blood count carefully, in the context of the history, is one of the simplest ways to add value in functional practice.

The red cells

Small red cells

A low MCV most commonly reflects iron deficiency, typically with a high RDW. Thalassaemia trait, common in people of Mediterranean, Middle Eastern, South Asian, Southeast Asian and African ancestry, also causes small cells, usually with a normal RDW, a relatively high red cell count and normal ferritin. A simple ratio of MCV to red cell count below about 13 suggests thalassaemia trait rather than iron deficiency, though it is only a guide. Distinguishing the 2 matters: iron supplements do not help thalassaemia trait, and the diagnosis has implications for family planning. Anaemia of chronic disease can also cause mildly small cells.

Large red cells

A raised MCV has several causes:

• vitamin B12 or folate deficiency, often with a high RDW;
• alcohol, a common and often unrecognised cause;
• liver disease;
• hypothyroidism;
• medicines, including methotrexate, some antiepileptics, hydroxycarbamide and some antiretrovirals;
• a high reticulocyte count, as after bleeding or haemolysis;
• bone marrow disorders, particularly in older adults.

A raised MCV with normal haemoglobin is an early clue worth investigating, particularly for B12 deficiency and alcohol.

Rising RDW

An increasing RDW with a normal haemoglobin can be an early sign of developing iron, B12 or folate deficiency, or of mixed deficiencies in which small and large cells coexist and the MCV appears normal.

The white cells

Benign ethnic neutropenia

Many healthy people of African and Middle Eastern ancestry have neutrophil counts below the reference ranges derived mainly from European populations, associated with a variant called Duffy null. This is not a disease and does not increase infection risk. Recognising it avoids unnecessary investigation and inappropriate withholding of treatment.

Eosinophilia in practice

Mild eosinophilia is common in allergic conditions. Higher or persistent eosinophilia, particularly in people who have lived in or travelled to regions where parasites are endemic, warrants investigation for parasitic infection such as Strongyloides, especially before any steroid treatment.

Platelets

A raised platelet count is most often reactive, occurring with iron deficiency, inflammation, infection, bleeding or after splenectomy. Persistently high counts without an obvious cause need medical assessment. A low platelet count may reflect viral infection, medicines, alcohol, liver disease with an enlarged spleen, B12 or folate deficiency, immune thrombocytopenia or marrow disorders.

Ratios and derived measures

The neutrophil to lymphocyte ratio is a marker of systemic inflammation associated with outcomes in many research studies. It is influenced by many factors and is not yet a validated clinical tool for individual decisions, but a very high ratio in an unwell patient is a signal to look for infection or inflammation.

Findings that need prompt medical review

• Haemoglobin that is very low or falling rapidly
• Abnormal cells such as blasts reported on the film
• Low counts in 2 or 3 cell lines at once
• Very high white cell counts or unexplained persistent lymphocytosis
• Very low neutrophil counts, particularly with fever
• Very high or very low platelet counts
• Anaemia with weight loss, bleeding or other red flags

5 common patterns

An illustrative case

A 42 year old woman with fatigue is told her blood count is normal. On closer reading, her haemoglobin is at the lower end of normal, her MCV is slightly low, her RDW is raised and her platelets are at the upper limit. Her ferritin is 9 micrograms per litre. She has heavy periods. The “normal” blood count had contained every early sign of iron deficiency. Treating her iron deficiency and addressing her heavy bleeding improves her energy substantially.

More than a haemoglobin

A complete blood count read in full, and in context, can reveal early deficiencies, unrecognised alcohol use, allergy, parasites and inherited traits. It costs nothing extra to read carefully, and the information it contains is often already sitting in the patient’s records.