Haematologic Techniques: Absolute Cell Values and Red Cell Indices

The assessment of blood counts relies on two distinct measurement types: relative values and absolute values.
Relative values represent the percentage of a specific leukocyte type based on a count of $100$ quantified cells. Historically, these are used significantly in settings where automated counting technology is unavailable.
The advent of automated blood counting machines, such as Sysmex, Micros, and Pentra, has enabled the precise determination of absolute values and cell counts for every specific cell type.
Absolute values are critical for clinical accuracy because they provide the exact number of cells per microliter ( $/\mu L$ ) or liter ( $/L$ ), rather than just a proportion.
The importance of absolute values is intrinsically clear in specific pathologies, such as Chronic Myeloid Leukemia (CML). In CML, an increase in basophil concentration is a vital diagnostic criterion. A patient might have a relative basophil value of $1\%$ , which appears normal within a general percentage framework. However, if the aggregate White Blood Cell (WBC) count is $100,000\,\text{cells}/\mu L$ , the absolute value reveals a pathological basophilia of $1,000\,\text{cells}/\mu L$ .

Total WBC Count: $4,500 - 10,000\,\text{cells}/\mu L$
Agranulocytes:
- Lymphocytes: Relative value = $25 - 35\%$ ; Absolute value = $1,700 - 3,500\,\text{cells}/\mu L$ .
- Monocytes: Relative value = $4 - 6\%$ ; Absolute value = $200 - 600\,\text{cells}/\mu L$ .
Granulocytes:
- Bands or Stabs: Relative value = $3 - 5\%$ .
- Basophils: Relative value = $0.4\% - 1.1\%$ ; Absolute value = $40 - 110\,\text{cells}/\mu L$ .
- Eosinophils: Relative value = $1 - 3.1\%$ ; Absolute value = $100 - 310\,\text{cells}/\mu L$ .
- Neutrophils: Relative value = $50 - 70\%$ ; Absolute value = $2,500 - 7,000\,\text{cells}/\mu L$ .
A precise assessment requires considering both values simultaneously. For example, a $70\%$ relative neutrophil count seems normal. However, if the total WBC count is $20,000\,\text{cells}/\mu L$ , the absolute count is $14,000\,\text{cells}/\mu L$ ( $70\% \times 20,000$ ), which is an abnormally high and alarming count.

Lymphocytes are unique in their clinical classification into relative and absolute categories.
Normal lymphocyte range: $20\% - 40\%$ of total WBCs.
Relative Lymphocytosis: Occurs when the percentage of lymphocytes exceeds $40\%$ .
Absolute Lymphocytosis: Identified when the total lymphocyte count exceeds specific age-based thresholds:
- Adults: $> 4,000/\mu L$
- Adolescents: $> 7,000/\mu L$
- Infants: $> 9,000/\mu L$

Patient: A $57$ -year-old male presenting with fever and fatigue.
Initial CBC (WBC = $2.0 \times 10^9/L$ ) using Relative Percentages:
- Basophils: $0\%$ (Reference: $0 - 2\%$ , Normal)
- Eosinophils: $0\%$ (Reference: $0 - 7\%$ , Normal)
- Monocytes: $10\%$ (Reference: $0 - 12\%$ , Normal)
- Lymphocytes: $50\%$ (Reference: $15 - 45\%$ , High/Relative Lymphocytosis)
- Neutrophils: $40\%$ (Reference: $40 - 70\%$ , Normal/Low Normal)
Converted CBC using Absolute Values ( $\times 10^9/L$ ):
- Basophils: $0.00$ (Reference: $0.0 - 0.1$ , Normal)
- Eosinophils: $0.00$ (Reference: $0.0 - 0.4$ , Normal)
- Monocytes: $0.20$ (Reference: $0.2 - 0.8$ , Normal)
- Lymphocytes: $1.00$ (Reference: $1.0 - 3.4$ , Low Normal)
- Neutrophils: $0.80$ (Reference: $1.8 - 6.8$ , Moderate Neutropenia)
Conclusion: The relative values suggested lymphocytosis, which was misleading. The absolute values reveal that the patient actually has moderate neutropenia, putting them at risk for infection, while the lymphocyte count is actually at the lower end of the normal range.

An index serves as a quantitative measure. Red cell indices specifically quantify red cells and the amount/concentration of haemoglobin within them.
History: These indices were first described by Wintrobe in $1929$ to define size and haemoglobin concentration.
Indices can be calculated provided that Haemoglobin, PCV/Haematocrit, and Red Cell count are known.
The primary indices used to diagnose specific types of anemia based on red cell morphology include MCV, MCH, MCHC, and RDW.

Definition: A measure of the average size or volume of Red Blood Cells.
Units: Expressed in femtolitres ( $fl$ , where $1\,fl = 10^{-15}\,litres$ ) or cubic microns ( $\mu m^3$ ).
Normal Value: $87 \pm 7\,fl$ (Standard range: $80 - 100\,fl$ ).
Morphological Classifications:
- Normocyte: Normal MCV range.
- Macrocyte: High MCV (e.g., Pernicious and Megaloblastic anaemia).
- Microcyte: Low MCV (e.g., Iron deficiency anaemia).

Mean Corpuscular Haemoglobin (MCH):
- Definition: The amount of haemoglobin per individual RBC.
- Measurement Unit: Picograms ( $pg$ ).
- Normal Value: $29 \pm 2\,pg$ per cell.
Mean Corpuscular Haemoglobin Concentration (MCHC):
- Definition: The amount of haemoglobin per unit volume, correlating haemoglobin content to the cell volume.
- Measurement Unit: Expressed as $g/dL$ or as a percentage.
- Normal Value: $34 \pm 2\,g/dL$ .

Definition: A measurement of the variation in RBC size, referred to as Anisocytosis.
Usual physiological cell size: $6 - 8\,\mu m$ .
Measurement Unit: Displayed as a percentage.
Normal Value: $13 \pm 1.5\%$
Diagnostic Interpretations:
- Increased: Typical of Iron Deficiency.
- Normal: Typical of Thalassemia trait.
- Normal or Raised: Seen in Anemia of chronic disease.
- Very High: Characteristic of Megaloblastic Anaemias.

Haematologic analysis categorizes cells based on color and size:
- Color classifications: Hyperchromic, Normochromic, and Hypochromic.
- Size classifications: Macrocytes, Normocytes, and Microcytes.