Differential Blood Count

A differential blood count gives the relative percentage of each type of white blood cell and also helps to reveal abnormal white blood cell populations (eg, blasts, immature granulocytes, and circulating lymphoma cells in the peripheral blood).

Reference ranges for differential white blood cell counts are as follows ^[1] :

• Neutrophils - 2500-8000 per mm³ (55-70%)

• Lymphocytes - 1000-4000 per mm³ (20–40%)

• Monocytes - 100-700 per mm³ (2–8%)

• Eosinophils - 50-500 per mm³ (1–4%)

• Basophils - 25-100 per mm³ (0.5-1%)

Differential blood count gives relative percentage of each type of white blood cell and also helps reveal abnormal white blood cell populations (eg, blasts, immature granulocytes, or circulating lymphoma cells in the peripheral blood).

Differential blood count is also used along with leukocyte count (WBC) to generate an absolute value for each type of white blood cells (eg, absolute neutrophil count, absolute lymphocyte count, or absolute eosinophil count), which usually gives more meaningful information than the percentage of each, since relative percentage can be misleading. Expressing absolute values are also useful for monitoring (eg, monitoring neutropenia during chemotherapy or bone marrow transplantation). ^[2] Using absolute values, conditions such as neutropenia, neutrophilia, lymphopenia, lymphocytosis, monocytopenia, monocytosis, eosinophilia, and basophilia can be identified, whichcanhelp differential diagnosis of patient’s underlying disorders.

Differential blood count is not a part of complete blood count (CBC) but is interpreted together with CBC to help support or exclude a suspected diagnosis. For example, the presence of anemia along with thrombocytopenia with a low or high white blood cell count may suggest bone marrow involvement by leukemia.

Immature granulocyte (IG)

Immature granulocytes (IGs) encompass immature cells of granulocytic lineages, including metamyelocytes, myelocytes, and promyelocytes, which are easily recognized morphologically and are reported by automated analyzer as IG altogether. ^[3] IG normally absent from peripheral blood.

Increased IG occurs accompanied by an increase in neutrophils in the following conditions: ^[2]

• Bacterial infections

• Acute inflammatory diseases

• Cancer (particularly with marrow metastasis)

• Tissue necrosis

• Acute transplant rejection

• Surgical and orthopedic trauma

• Myeloproliferative diseases

• Steroid use

• Pregnancy (mainly during the third trimester)

Increased IG may occur without an accompanied increase in neutrophils in some conditions, particularly elderly patients, neonates, and patients with myelosuppression. In these situations, isolated increase in IGs (>2%) can be useful for recognizing an acute infection, even when not suspected. ^[2]

Collection and panel details are as follows:

• Specimen: Whole blood, usually collected by venipuncture

• Collection: EDTA tube (purple/lavender top; see image below) containing EDTA potassium salt additive as an anticoagulant

  Lavender-top tube.

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• Panels: Differential blood count, leukocyte differential

Description

Differential blood count can be performed by the following 2 methods:

• Automated differential blood count: Automated hematology instruments using multiple parameters and methods (such as fluorescence flow cytometry and impedance) are used to count and identify the 5 major white blood cell types in blood (so-called 5-part differential count): neutrophils, lymphocytes, monocytes, eosinophils and basophils. ^{[4, 5]}

• Manual differential blood count: This is performed by visual examination of peripheral blood smear (blood films) by trained personnel. ^[4]

The automated differential blood count is less time-consuming and less expensive than routine examination of blood smear. With the automated technique, thousands of white blood cells can be examined, whereas typically 100-200 white blood cells are examined by visual examination. ^[6]

Indications/Applications

Differential blood count is primarily needed in the 2 following reasons. ^[2]

• To look for quantitative abnormalities in morphologically normal WBC population such as in the diagnosis of infectious or allergic diseases and for therapeutic monitoring of cytotoxic or myelotoxic drugs (This requires a high level of precision and accuracy [ie, ability to provide consistent and correct results]).

• To look for morphologic abnormalities of white blood cells (eg, when circulating abnormal white blood cell population such as immature or atypical cells are suspected for diagnostic or monitoring reasons; this requires a high level of clinical sensitivity, [ie, ability to identify all patients who have circulating abnormal WBCs]).

Considerations

Accuracy, precision, and clinical sensitivity

The automated differential blood count provides a high level of accuracy and precision (correct and consistent results) for quantification and identification of normal white blood cells; however, this method is not sensitive at identifying abnormal or immature cells and is not able to accurately identifying and classifying all types of white blood cells. To overcome this problem, most automated analyzers will flag samples with possible abnormal white blood cell populations, indicating the need for peripheral smear examination to be examined by trained personnels to identify abnormal cells. ^{[6, 2]}

Monocyte count and basophil count are the most difficult population to count and have a low level of precision and accuracy. Moreover, automated analyzers tend to underestimate the basophil count during true basophilia. ^[2]

False negative

Both automated and manual methods may not detect small numbers of abnormal cells. The false negative rate for detection of abnormal cells varies from 1-20%, depending on the instrument and the detection limit desired (1-5% abnormal cells). The most difficult for both automated instruments and visual examination by human is identification of lymphoma cells and reactive lymphocytes. ^[5]

Band neutrophils and immature granulocytes (IGs)

The value of reporting band neutrophils is questionable. The measurement of the immature cells of the myeloid lineages, specifically “band,” has been considered clinically useful in the diagnosis of infections, especially neonatal sepsis. ^[7]

However, band neutrophils cannot be enumerated by automated analyzers and are reported together with segmented neutrophils as absolute neutrophil counts (ANC), which are used to defined neutropenia or neutrophilia. Identification of band neutrophils by visual examination (manual differential blood count) is neither precise nor consistent, as a high variability of morphologic classification or quantification of band neutrophils exists due to interobserver variability.

Some, therefore, advocated ceasing quantitative reporting of band neutrophils. ^{[5, 2]} The extended differential count includes reporting immature granulocytes (IG) can be used alternatively to help diagnosis neonatal sepsis. ^[2] For further reading, see Interpretation.