© Clinical Chemistry

PEARLS OF LABORATORY MEDICINE

Complete Blood Count (CBC) Basics

Sarah Drawz, MD, PhD

Michael A Linden, MD, PhD

University of Minnesota

DOI: 10.15428/CCTC.2014.230268

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Intro to the CBC

• Blood = plasma (albumin, clotting factors) + cells

• CBC = complete blood count, quantifies 3 types of blood cells

• Red blood cells (RBCs)

• White blood cells (WBCs)

• Platelets

• Gives additional info, e.g.,

• % of blood composed of RBCs – “hematocrit”

• Different types of WBCs present – “differential”

• 3 to 10 ml of whole blood drawn into tube containing

anticoagulant

• EDTA

• Heparin

• Citrate

• Most common: purple-top tube with EDTA to chelate Ca2+

Collection

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Cell count analysis

• Sample dilution and even distribution of cells essential

to accurate counts

• Different solution for different cells

• RBC counts need isotonic solution

• WBC and platelet counts use RBC-lysis solution

• Performed by manual counts or automated analyzers

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Manual counts

• Rarely used for absolute count

• May use hemocytometer for platelet or low WBC counts

• Counting chamber with specific volume

• Viewed with microscope

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Automated analyzers

• Increased accuracy and speed

• Based on electrical impedance,

light scattering, radiofrequency

conductivity, or cytochemical

reactions

• Electrical impedance

• Change of voltage when

cells pass through aperture

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Automated analyzers

• Flow cytometry and light scatter

• Laser hits single stream of cells, light scatter interpreted into

info on size, structure, granularity

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CBC parameters

RBC

• Oxygen carrying cells

• Anucleate (unless immature)

WBC

• Immune function

Platelets

• Clotting

• Anucleate cell “fragments”

Images were originally published in ASH Image Bank. Maslak. Blood Cells. ASH

Image Bank. 2008; 8-00044 and 8-00067. © the American Society of Hematology

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Minimizing error

• All laboratory processes are subject to error

• Examples of sources of error in the CBC will

be discussed under each cell type

• Automated analyzer computers have

multiple programs to detect possible error

• If data meets possible error criteria, data is

“flagged” for operator review prior to release

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RBC parameters

• Quantitative

• Hemoglobin (Hgb, g/dL)

• Hematocrit (Hct, %)

• RBC count (per μL)

• Qualitative (averages)

• Mean corpuscular volume (MCV, fL)

• Mean corpuscular hemoglobin (MCH, pg)

• Mean corpuscular hemoglobin concentration (MCHC, g/dL)

• Red cell distribution width (RDW, %)

• Sometimes – reticulocyte count

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RBC parameters

Hemoglobin (Hgb, g/dL)

• Colored protein, measured by absorbance at 540nm

Hematocrit (Hct, %)

• Proportion of volume occupied by RBCs

• Manual – height of column after centrifugation

• Automated – RBC number/RBC volume

RBC count (cells per μL)

• Obtained via electrical impedance and/or light scatter

• RBCs and WBCs counted together by analyzer

- RBC outnumber WBC ~500:1, negligible error

Manual hematocrit tube

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RBC parameters

• Mean corpuscular volume (MCV, fL)

• Average volume of each RBC

• RBC volumes/RBC count

• Mean corpuscular hemoglobin (MCH, pg)

• Average Hgb in each RBC

• Hgb/RBC count

• Mean corpuscular hemoglobin concentration (MCHC, g/dL)

• Average Hgb concentration in each RBC

• Hgb/Hct

• These three are calculated averages – may not accurately

describe mixed population of cells

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• Red cell distribution width (RDW, %)

• Range of RBC sizes

• Reticulocytes

• Immature, anucleated cells containing RNA

• Reflect bone marrow’s ability to make new RBCs

• Manual – “supravital” stain of ppt RNA

• Automated – fluorescent dye stains RNA

RBC parameters

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RBC parameters – clinical scenarios

• Decreased Hgb, Hct, or RBC count – anemia

• 1° - iron deficiency anemia

• 2° - acute blood loss

• Increased Hgb, Hct, or RBC count – polycythemia

• 1° – bone marrow proliferative disease

• 2° – compensation due to chronically low oxygen from

smoking, sleep apnea, high altitude

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RBC parameters – examples of error

Parameter Falsely increased Falsely decreased

Total RBC count High WBC count Hemolysis (in vitro),

clotting

Hct Giant platelets Hemolysis (in vitro),

clotting

MCV Cell clumping Giant platelets

Giant platelet

Hemolyzed Non-hemolyzed

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WBC parameters

• Quantitative cell count (cells per μL)

• Differential (cells per μL and % of total WBCs)

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WBC parameters

• Quantitative cell count (cells per μL)

• Dilution of blood in RBC lysis buffer, usually acid or

detergent

• Total count of nucleated cells obtained by electrical

impedance or flow cytometry (automated) or

hemocytometer (manual)

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WBC parameters

• Differential (cells per μL and % of total WBCs)

Immature neutrophil

“band form”

Neutrophil Basophil Eosinophil

Lymphocyte Monocyte

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WBC differential

• Automated

• Individual cells analyzed by flow cytometry

• Light scatter

• Forward (cell size)

• Side (complexity, granularity)

• Cells identified based on expected profile

• E.g., neutrophils – larger than

lymphocytes with granular complexity,

monocytes – fewer granules than

neutrophils and therefore, less SSC

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Image was originally published in ASH Image Bank. Maslak

and Rose. White cell differential -1. ASH Image Bank. 2008;

00003658. © the American Society of Hematology.

• Abnormal cells that do not identify as WBCs are

tagged for manual review

• E.g., atypical lymphocytes, immature blasts

• Manual review

• Drop of blood smeared on glass slide

• Dyes

• Basic (nuclei, basophilic)

• Acidic (eosinophilic)

WBC differential

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WBC parameters – clinical scenarios

• Decreased total WBC count – leukopenia

• 1° - HIV, bone marrow disease, e.g., aplastic anemia

• 2° - immunosuppressants

• Increased total WBC count –

leukocytosis

• 1° - acute leukemia

• 2° - neutrophilia and

lymphocytosis from infection

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This image was originally published in ASH Image Bank. Maslak. Neutrophilia-1.

ASH Image Bank. 2008; 00003785. © the American Society of Hematology.

WBC parameters – examples of error

• Total WBC count – falsely increased

• Nucleated RBCs (counted as WBCs)

• Antibodies that cause RBC clumping

• Differential

• Automated –

- Abnormal cells incorrectly identified

• Manual –

- Poor staining leading to cell

recognition errors

- Larger cells pushed to edge,

missed in count

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Platelet parameters

• Platelet count

• Obtained by electrical impedance or light scatter

(automated) or hemocytometer (manual)

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Platelet parameters – clinical scenarios

• Decreased platelet count – thrombocytopenia

• 1° - decreased bone marrow production

• 2° - immune-mediated destruction/sequestration

• Increased platelet count – thrombocytosis

• 1° - proliferative bone marrow disease

• 2° - acute phase reactant, e.g., inflammation

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Platelet parameters – examples of error

Parameter Falsely increased Falsely decreased

Platelet count Hemolysis (in vitro),

microcytic (small) red

cells

Giant platelets,

platelet clumping

Microcytes Giant platelet

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Summary

• CBC is an important screening and diagnostic tool

• Majority of specimens run on automated analyzers which

use impedance and flow cytometry for cell identification

and enumeration

• Manual review and count reserved for abnormal

specimens or patients with clinical history

• Knowledge of error sources essential to accurate

interpretation

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References

1. Perkins SL. Examination of the Blood and Bone Marrow. In:

Wintrobe’s Clinical Hematology. 12th ed. Philadelphia: Lippincott,

Williams & Wilkins; 2009, p. 1-20.

2. Vajpayee N, Graham SS, Bem S. Basic Examination of Blood and

Bone Marrow. In: Henry’s Clinical Diagnosis and Management.

22nd ed. Philadelphia: Elsevier/Saunders; 2011, p. 509-535.

3. Images created by authors except those reproduced with

permission as noted from American Society of Hematology Image

Bank, www.ashimagebank.org

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Disclosures/Potential Conflicts of Interest

Upon Pearl submission, the presenter completed the Clinical Chemistry

disclosure form. Disclosures and/or potential conflicts of interest:

▪ Employment or Leadership: None declared

▪ Consultant or Advisory Role: None declared

▪ Stock Ownership: None declared

▪ Honoraria: None declared

▪ Research Funding: None declared

▪ Expert Testimony: None declared

▪ Patents: None declared