High monocytes on a blood test: what it means

Monocytes are one of the white blood cells that help the body respond to infection and inflammation. They are counted as part of the full blood count (FBC), one of the common blood tests the NHS describes for checking the number of blood cells in your blood. A result is flagged as high when it is above the reference range printed on the laboratory report. Ranges differ slightly between laboratories, so the range on your own result is the right comparison.

NHS Borders defines monocytosis simply as a monocyte count above the normal range. A single raised number is not a diagnosis. It is a prompt to look at the bigger picture: your symptoms, your other blood counts, previous results and any recent illness.

The York pathway lists causes in three broad groups. Infections include tuberculosis, brucella, malaria, syphilis and endocarditis. Autoimmune and inflammatory diseases, including sarcoidosis, can raise the count. Other causes include a stress response, for example after a heart attack, an absent or poorly working spleen (hyposplenism), chronic myelomonocytic leukaemia and solid cancers.

NHS Borders adds that monocytosis is most commonly caused by chronic inflammatory or infective problems and names tuberculosis, bacterial infection of the heart lining (endocarditis), systemic lupus erythematosus, rheumatoid arthritis and temporal arteritis, and that cigarette smoking is a common cause of mild monocytosis. Most people with a mildly raised count turn out to have one of the common, often treatable, explanations.

Taking medicines such as steroids, having a recent infection that has not fully settled, or being in recovery from illness can all be relevant, which is why your GP will usually ask about the last few weeks before deciding what to do.

The York pathway advises looking at the history and examination for signs of infection, asking about travel, weight loss, rashes and night sweats, examining for an enlarged spleen or liver, and comparing with older blood counts. Suggested investigations are a blood film, inflammatory markers and renal and liver function with calcium.

NHS Borders gives a simple schedule: when first identified, if the monocyte count is above 2.0 repeat at 6 weeks; if it is above 0.80 but below 2.0 repeat at 3 months; if it has settled, no further tests are needed. If it persists, a blood film and an ESR are suggested. These figures come from NHS Borders guidance for GPs, and local pathways differ, so your own surgery may follow a different timetable.

The pathways describe a haematology opinion for a raised count that persists with no explanation, or that comes with other changes in the blood. NHS Borders suggests referral for unexplained persistent monocytosis over 3 months above 1.5 (above 2.0 in cigarette smokers, including e-cigarettes), or above 1.5 with another blood count abnormality such as anaemia or low neutrophils or platelets. The York pathway suggests referral or advice for a persistent count over 5, or over 1.2 with other cytopenias, an enlarged spleen or an abnormal blood film.

NHS Borders names myelodysplasia, including chronic myelomonocytic leukaemia, as the main alternative diagnosis to consider, and notes that it generally requires a bone marrow test for confirmation. It also points out that this includes many subtypes, from very benign to very serious, and that much of the internet literature concentrates on the more aggressive ones. In other words, being referred does not mean a serious diagnosis has been made.

The York pathway adds that persistent monocytosis with no obvious cause, particularly with normal inflammatory markers, may represent chronic myelomonocytic leukaemia, that a blood film is the first investigation, and that some cases behave indolently, particularly in the very elderly.

Tell your GP about fevers, drenching night sweats, unexplained weight loss, new rashes, a lump or swelling, or persistent tiredness, as these help to guide the work-up. A high monocyte count with these symptoms is more likely to be followed up promptly. If you feel suddenly very unwell, have a high temperature with shivering, severe breathlessness or chest pain, use NHS 111 or call 999 rather than waiting for a routine appointment.

The NHS says blood test results can be complicated and that you should ask to talk to a healthcare professional if you have questions or do not understand them. Results are usually available after a few days and normally within a few weeks.

In the NHS, the full blood count is done through your GP or hospital and a raised monocyte count is interpreted in the context of your health. Some people have a full blood count through private health checks and then bring the result to a GP. A single private result is best repeated through your own GP, who can compare it with previous results and the reference range of the laboratory used.