Differential blood count

A differential blood count (DKS) shows which types of white blood cells (leukocytes) are present in blood and how many there are. The basic differential is counted by an analyser and is already part of the complete blood count. The separate DKS test is a manual microscopic blood film examination, showing both the proportion of each white cell type and their appearance. It helps the doctor assess whether findings suggest bacterial or viral infection, allergy or another cause.

Who is this test for

• people whose blood count shows an altered number or proportion of white blood cells and whose doctor wants a closer look at the cells;
• people with fever or infection when the doctor assesses the likely cause, often alongside CRP;
• people with suspected infectious mononucleosis;
• people with allergies or asthma, and suspected parasites;
• people with blood diseases and those taking medicines that may affect bone marrow, for monitoring.

What a differential blood count is and what it shows

Blood contains five main white cell types. Each has its own role in defending the body, so changes in different types suggest different causes:

Type	Role	Most commonly elevated in
neutrophils	first defence against bacteria	bacterial infections, inflammation, after strenuous activity and stress, with smoking and corticosteroids
lymphocytes	defence against viruses, antibody production	viral infections such as mononucleosis; less commonly blood diseases
monocytes	removal of pathogens and damaged cells	recovery from infection, chronic infections and inflammatory diseases
eosinophils	allergic reactions, defence against parasites	allergies, asthma and eczema, parasitic infections, drug reactions
basophils	participate in allergic reactions	rarely elevated; allergies, chronic inflammation, certain bone marrow diseases

A reduced neutrophil count (neutropenia) is most often temporary, for example after a viral infection, and may result from medicines, less commonly severe infection or bone marrow disease. Lymphocytes temporarily fall during acute illness, stress and corticosteroid use; the doctor investigates persistently low counts further. Low eosinophil and basophil counts usually have no significance by themselves, while the doctor interprets markedly low monocytes with the rest of the blood count.

Some conditions also produce immature or abnormal cell forms in blood, such as immature neutrophils in severe bacterial infection or reactive (atypical) lymphocytes in infectious mononucleosis. These changes are best seen through microscopic blood film examination.

Preparation and sample

The differential is performed on venous blood, usually from the same sample as the complete blood count. Fasting is unnecessary. Blood collection in children is adapted to their age. Tell your doctor about infection, corticosteroids and other medicines, because these alter white cell counts.

How results are interpreted

Each white cell type is expressed as a percentage, but the absolute count matters more and also depends on the total white cell count: one type’s percentage may appear high simply because another is lower. For example, when neutrophils fall, the lymphocyte percentage rises even though there are no more lymphocytes in blood.

Reference intervals depend on age and method and are printed on your report. Young children normally have more lymphocytes than neutrophils, so their results are not compared with adult values.

Mild, temporary abnormalities are common, particularly during and after a cold or other infection. The doctor interprets results with symptoms, examination and other findings and repeats testing after several weeks if needed. If you have been told your neutrophil count is very low and develop a fever, contact a doctor or emergency service immediately.

How to arrange testing

You can come for blood collection without an appointment or referral at one of our branches throughout Croatia (listed on the Contactpage). Prices for all tests and panels are listed in the service price list. A separate differential is usually performed alongside complete blood count, when the doctor wants a closer look at cell appearance.

Frequently asked questions

What does it mean when lymphocytes are high and neutrophils low?

This pattern often accompanies viral infections and is usually temporary. The doctor interprets it according to absolute counts and symptoms and repeats the blood count if it persists.

What does a blood count show in mononucleosis?

In infectious mononucleosis, lymphocytes are usually increased and reactive lymphocytes are visible on the blood film. Diagnosis is confirmed by antibodies to Epstein-Barr virus, discussed in the article on infectious mononucleosis.

What do elevated eosinophils mean?

Most often an allergy, such as allergic rhinitis, asthma or eczema, a drug reaction or parasitic infection. The doctor may suggest allergy tests, such as IgE, or testing stool for cysts and parasites.

Can a differential blood count detect leukaemia?

A differential can show changes suggesting a blood disease, such as immature cells or a persistently very high lymphocyte count. This is not a diagnosis: confirmation requires further tests led by a haematologist. Most differential abnormalities have much more common and harmless causes, such as infection.