Neutrophil percentage: what high and low results mean

Neutrophils are the white blood cells that arrive first at a site of bacterial or fungal infection. Lab Tests Online UK describes them as phagocytes that eat foreign organisms and kill them with internal poisons, and notes that they are the most abundant white blood cells. On a laboratory report you may see them called granulocytes, polymorphs or PMNs, which are alternative names for the same family.

The neutrophil percentage is the proportion of all counted white cells that are neutrophils. In a healthy adult it is the largest component of the differential. That has a practical consequence: when neutrophils rise or fall they pull the other percentages in the opposite direction, so a change in lymphocyte or monocyte percentage can be a mirror image of a neutrophil change.

Alongside the percentage, the analyser reports an absolute neutrophil count. Doctors rely on this absolute figure, particularly when judging the risk of infection or deciding whether chemotherapy can safely go ahead.

The differential is run automatically with the full blood count, which is ordered for a very wide set of reasons. Neutrophils receive special attention when someone has a fever, an unexplained infection, mouth ulcers that keep returning, a medicine that can lower blood counts, or cancer treatment that suppresses the bone marrow.

They are also checked for people taking antipsychotic medicines or treatments for an overactive thyroid, because the NHS lists those among the common causes of a low white blood cell count.

No fasting or preparation is needed. Blood is taken from a vein in the arm. Lab Tests Online UK points out that age, eating, physical activity and stress may alter white cell differential values.

Neutrophil levels also depend on background factors. The laboratory guide notes that the normal level of neutrophils varies between ethnic groups, so a result that sits slightly under a general reference range may be a normal variant for the individual. This is one reason doctors interpret results alongside your background and your previous counts rather than using a single cut-off.

Lab Tests Online UK says neutrophils can increase in response to bacterial infection, inflammatory disease, steroid medication or, more rarely, leukaemia. Its full blood count guide adds physical stress on the body. Everyday examples include a chest or urine infection, an inflamed appendix, a flare of an inflammatory condition, a course of prednisolone, and the aftermath of surgery or injury.

A high percentage in someone who is clearly unwell with a bacterial illness is expected, and the number will usually fall as they recover. A persistently raised count without an explanation, or one with abnormal cells or other raised cell lines, is a reason for a blood film and sometimes a haematology opinion.

Lab Tests Online UK lists severe infection, liver disease, enlarged spleens and responses to various medications or chemotherapy as reasons for lower levels. The NHS describes low white cell counts as potentially increasing the risk of infections, with causes that include cancer treatment, antipsychotic medicines, medicine for an overactive thyroid, some cancers such as leukaemia, infections such as HIV or hepatitis, and autoimmune conditions like rheumatoid arthritis.

A low neutrophil percentage does not always mean a low neutrophil count. If lymphocytes are high, as in many viral illnesses, the neutrophil share looks small even though the absolute number is acceptable. The absolute count is the figure that determines the infection risk.

Most abnormalities are checked against symptoms and then repeated. If your count is low, the NHS suggests seeing a GP if you know you are at risk of a low white cell count and you get an infection, or if you keep getting infections. Signs of infection it lists include a high temperature or feeling hot, cold or shivery, a sore throat, recurring mouth sores, toothache, skin rashes, tiredness and flu-like symptoms.

Practical precautions suggested by the NHS for people with a low white cell count include avoiding close contact with people who are sick, preparing food properly, washing hands regularly and using an electric shaver rather than a razor. The treatment team will tell you if you need specific advice, particularly while having chemotherapy.