Abnormal Full Blood Count: When Imaging Helps

A full blood count (FBC) is one of the most commonly ordered blood tests in the UK. It measures your red cells, white cells, and platelets — and abnormalities in any of these can sometimes point to conditions where imaging provides the next piece of the puzzle. Here's when an abnormal FBC warrants an MRI, and what the scan is looking for.

What's in a Full Blood Count?

An FBC measures several components of your blood:

Red blood cells

• Haemoglobin (Hb): The oxygen-carrying protein. Low = anaemia. High = polycythaemia.
• Haematocrit (Hct): The percentage of blood volume occupied by red cells.
• MCV (mean corpuscular volume): The average size of your red cells. Small cells (low MCV) suggest iron deficiency. Large cells (high MCV) suggest B12/folate deficiency or liver disease.
• Red cell count: The total number of red cells per litre of blood.

White blood cells

• Total white cell count: High can indicate infection, inflammation, or rarely leukaemia. Low can indicate bone marrow problems, viral infections, or immune conditions.
• Differential: The breakdown into neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each type has specific conditions associated with abnormal levels.

Platelets

• Platelet count: High platelets can occur with inflammation, iron deficiency, or myeloproliferative disorders. Low platelets can indicate bone marrow problems, autoimmune conditions, or liver disease.

When Abnormal Red Cells Point to Imaging

Unexplained anaemia

When haemoglobin is low and the cause isn't obvious from initial blood tests (iron studies, B12, folate, reticulocyte count), imaging may be needed to look for:

• Hidden bleeding: An abdominal MRI can identify tumours in the stomach, bowel, kidneys, or bladder that might be causing slow, chronic blood loss.
• Splenic enlargement (splenomegaly): An enlarged spleen can trap and destroy red cells, causing anaemia. MRI shows the spleen's size and any underlying cause.
• Bone marrow infiltration: When the bone marrow is replaced by cancer, fibrosis, or other abnormal tissue, it can't produce red cells effectively. MRI of the spine and pelvis can show bone marrow signal changes that suggest infiltration.

Polycythaemia (too many red cells)

Persistently high haemoglobin and haematocrit can indicate polycythaemia vera (a myeloproliferative disorder) or secondary polycythaemia (caused by something stimulating red cell production). Imaging may be useful to:

• Check for kidney tumours (renal cell carcinomas can produce erythropoietin, stimulating red cell production)
• Assess for liver tumours (hepatocellular carcinoma can also produce erythropoietin)
• Measure spleen size (splenomegaly is a feature of polycythaemia vera)

When Abnormal White Cells Need Imaging

Persistently raised lymphocytes

A raised lymphocyte count (lymphocytosis) in an older adult can be a sign of chronic lymphocytic leukaemia (CLL) — the most common leukaemia in the UK. While CLL is primarily diagnosed through blood tests and bone marrow biopsy, imaging (CT or MRI) is used to:

• Check for enlarged lymph nodes throughout the body
• Assess the spleen and liver for enlargement
• Stage the disease (determining how widespread it is)

Raised white cells with lymph node enlargement

If you have abnormal blood counts and your GP or a specialist has found enlarged lymph nodes (in the neck, armpit, or groin), MRI of the abdomen and pelvis can check for:

• Deep lymph node enlargement that can't be felt on examination
• Organ involvement (liver, spleen, kidneys)
• The overall disease burden — important for staging lymphoma

Low white cells (leucopenia/neutropenia)

Persistently low white cells, especially neutrophils, can indicate bone marrow problems. MRI of the spine and pelvis can show bone marrow signal abnormalities that suggest:

• Aplastic anaemia (bone marrow failure)
• Myelodysplastic syndrome
• Bone marrow infiltration by cancer
• Marrow fibrosis

When Abnormal Platelets Need Imaging

Low platelets (thrombocytopenia)

If platelets are low and the cause isn't clear from blood tests alone, imaging helps by:

• Assessing the spleen. An enlarged spleen is one of the most common causes of low platelets — the spleen sequesters and destroys platelets. MRI accurately measures spleen size and can often identify why it's enlarged (portal hypertension, lymphoma, infection).
• Liver assessment. Chronic liver disease causes portal hypertension, which enlarges the spleen and lowers platelets. Liver MRI shows the degree of liver disease, cirrhosis, and any focal lesions.
• Bone marrow assessment. As with low white cells, MRI of the spine and pelvis can evaluate the bone marrow.

High platelets (thrombocytosis)

Reactive thrombocytosis (high platelets due to inflammation, infection, or iron deficiency) is much more common than primary thrombocytosis. When iron deficiency is excluded and inflammation is ruled out, persistently high platelets warrant investigation for myeloproliferative disorders. MRI may be used to assess spleen size and look for abdominal vein thrombosis.

Bone Marrow MRI

One of MRI's underappreciated strengths is its ability to evaluate bone marrow without biopsy. Normal bone marrow has a characteristic signal on MRI — when this signal is abnormal, it can indicate:

• Infiltration: Cancer cells (leukaemia, lymphoma, myeloma, metastases) replacing normal marrow
• Fibrosis: Scar tissue replacing normal marrow (myelofibrosis)
• Reconversion: Yellow (fatty) marrow converting back to red (active) marrow — a sign the body is trying to produce more blood cells
• Oedema: Marrow swelling from stress fractures, infection, or inflammation

MRI of the lumbar spine and pelvis provides excellent views of the bone marrow and is often requested alongside bone marrow biopsy when blood count abnormalities suggest a marrow problem.

Blood Tests to Combine

If your FBC is abnormal and imaging is being considered, additional blood tests help narrow the differential diagnosis:

• Reticulocyte count (is the marrow responding appropriately to anaemia?)
• Blood film (manual review of cell shapes and sizes)
• LDH and haptoglobin (markers of haemolysis — red cell destruction)
• Iron studies, B12, and folate (nutritional causes of anaemia)
• CRP and ESR (inflammatory markers)
• Liver function tests
• Protein electrophoresis (screening for myeloma)

Our Peak 70 Blood Test includes many of these markers and pairs well with abdominal imaging when investigating abnormal blood counts.

When to Seek Imaging for Abnormal FBC

Not every slightly abnormal FBC needs an MRI. Guidelines suggest imaging when:

• Abnormalities persist on repeat testing (2+ occasions, weeks apart)
• Multiple cell lines are affected (e.g., low haemoglobin AND low platelets — called bicytopenia)
• There are accompanying symptoms — unexplained weight loss, night sweats, fevers, lymph node enlargement, abdominal fullness
• Initial blood tests can't explain the abnormality
• Your GP or haematologist suspects an underlying structural cause

An abnormal FBC is a signpost, not a diagnosis. When blood tests point to a problem but can't identify its source, MRI provides the detailed structural information needed to find the answer.