The differential white blood cell count

Written by Evgeny Zaytsev, physician in clinical laboratory diagnostics.

What the differential count is #

The differential white blood cell count — the leukogram, «leukoformula» in Russian — is the ratio of the different classes of white cells in the blood. Human white cells, like those of many animals, fall into five main types:

• Neutrophils — the largest fraction of white cells in adults.
• Lymphocytes — the group of blood cells that predominates in children.
• Monocytes — third by how often they are met in the blood.
• Eosinophils — cells everyone should have, at least in small numbers.
• Basophils — the rarest fraction, which healthy people may have too.

Reading the differential count #

Neutrophils #

A raised neutrophil count (neutrophilia) is common in fever. The higher the body temperature, the higher the number of neutrophils in the blood.

A left shift is a rise in immature band neutrophils. It is a sign of an inflammatory reaction, most often of infectious origin. In bacterial infections the band count is usually above 1500/µl. The German medical society considers a band proportion above 10 % to be a criterion of bacterial infection on its own. Band neutrophils are often seen in the blood of children and teenagers with infectious mononucleosis, alongside reactive lymphocytes (atypical mononuclear cells).

A right shift is a rise in mature segmented neutrophils while the immature forms stay within the reference interval (up to 5 %). It is often seen in chronic kidney failure, in vitamin B12 deficiency anaemia and when older people lack folic acid in their diet.

A low neutrophil count (neutropenia) may be caused by medication, by a thyroid disorder or by viral infections.

Lymphocytes #

The causes of a raised lymphocyte count (lymphocytosis) differ with age. In children and teenagers it is most often a sign of a viral infection — the Epstein — Barr virus, which causes infectious mononucleosis.

In older people an absolute lymphocyte count above 5000/µl is an important reason to investigate for chronic lymphocytic leukaemia. Lymphocytes may also rise in allergic conditions.

In infants with a long barking cough who missed their vaccinations (DTP, Pentaxim), lymphocytosis is an important symptom of whooping cough (Bordetella pertussis).

A low lymphocyte count (lymphopenia) is most often linked to an immunodeficient state, and HIV infection has to be ruled out. It is frequently caused by glucocorticoids — methylprednisolone, prednisolone, dexamethasone — and is seen in cancer and in severe infections.

Monocytes #

A raised monocyte count (monocytosis) is most often a reactive sign of a bacterial infection, but it also occurs in viral, fungal and parasitic infections. Before prescribing an antibiotic it is therefore essential to read the other symptoms as well, including how the temperature behaves over time.

A monocyte count that stays raised for more than three months is an important criterion for diagnosing chronic myelomonocytic leukaemia, and juvenile myelomonocytic leukaemia in children.

Eosinophils #

A raised eosinophil count (eosinophilia) has several causes. The most common is an allergic reaction. Depending on how severe the disease is, the count may be slightly raised (allergic rhinitis) or high — in bronchial asthma hypereosinophilia is possible.

The second most common cause is a parasitic infection: ascariasis, opisthorchiasis and others. Sometimes eosinophils rise temporarily after recovery from an infection or after taking medication, antibiotics for example.

If severe eosinophilia persists for a long time, chronic eosinophilic leukaemia has to be ruled out.

A low eosinophil count (eosinopenia) is a sign that a severe infection is progressing.

Basophils #

A raised basophil count is a sign of mastocytosis, chronic myeloid leukaemia, allergic reactions and chronic iron deficiency anaemia; it is also seen in atopic dermatitis and diabetes.

Pathological cells in the differential count #

In a healthy person’s peripheral blood the differential count never contains blasts or promyelocytes. Finding these cells in the blood is critical diagnostic information that has to be reported to the treating doctor at once.

Blasts in the peripheral blood #

Blasts in the peripheral blood are the very first sign of leukaemia. Unlike other blood cells, blasts have a delicate net-like nuclear chromatin that is spread evenly. Straight-edged nuclei and cytoplasm are met. Nucleoli are present, although they are sometimes hard to see. The nuclear-to-cytoplasmic ratio is high. Blasts also tend to form clusters of cells.

There are many kinds of blasts: granular blasts, blasts with Auer rods, cup-like blasts, monoblasts, myeloblasts, lymphoblasts, megakaryoblasts, proerythroblasts, «hand mirror» blasts and blasts with Phi bodies.

Promyelocytes in the peripheral blood #

Promyelocytes are more mature than blasts, but their presence in the blood is in many cases more critical for the patient. They are an important sign of acute promyelocytic leukaemia. In this disease promyelocytes often carry Auer rods, which do great damage to the patient’s haemostatic system.

Myelocytes and metamyelocytes #

Large numbers of myelocytes and metamyelocytes are a sign of chronic myeloid leukaemia. Low and moderate numbers of these cells may occur in severe inflammation caused by a bacterial infection.

Reactive lymphocytes (atypical mononuclear cells) #

Atypical mononuclear cells in the differential count are a sign of infectious mononucleosis — infection with the Epstein — Barr virus. They may also be met in small numbers in allergic conditions. Infections such as cytomegalovirus and dengue fever lead to atypical mononuclear cells as well.

Normoblasts (erythroblasts) in the peripheral blood #

Erythroblasts in the blood are a sign of an anaemic or inflammatory syndrome. Normoblasts may also be a symptom of hyposplenism — reduced function of the spleen.

Large numbers of normoblasts in a newborn are a sign of haemolytic disease.

What else the smear shows during the count #

Counting the differential means more than identifying the main classes of white cells: the laboratory physician also assesses the morphology of the red cells and the platelets.

In a fever of unknown origin, finding malaria plasmodia inside red cells is the single most important step towards saving the patient.

Other infectious agents are occasionally found in the blood as well:

• leishmania;
• histoplasma;
• Candida albicans;
• Loa loa;
• relapsing fever borrelia;
• streptococci in sepsis;
• trypanosomes.

Plasma cells — a few may be present in the blood after recovery from an infection; moderate and large numbers occur in plasma cell leukaemia.

Sézary cells are specific for T-cell lymphoma.

How the differential count is done #

Manual counting #

Manual counting is the economical method, but it takes a lot of effort. It needs highly qualified staff: a technician who prepares and stains the blood smear. By law the count itself may only be performed by a physician in clinical laboratory diagnostics or by a biologist with a higher medical or biological degree.

Manual counting takes from 5 to 30 minutes of working time, depending on how difficult the case is.

Automated counting #

The count can be performed on an automatic blood cell scanner — digital microscopy.

It is important to understand that automatic digital scanning microscopy will not replace the physician in the laboratory, but it speeds the work up considerably.

The count can also be done on haematology analysers, which come in two kinds: 3-diff and 5-diff. A 3-diff analyser cannot tell monocytes from eosinophils and basophils. A modern 5-diff analyser, for instance the Sysmex XN series, can measure 125 parameters in a single full blood count.