Complete Blood Count (CBC) — Your Blood Cells

A complete blood count, usually called a CBC, is one of the most commonly ordered blood tests. It measures the cells circulating in your blood and can provide important clues about anemia, infections, inflammation, and certain blood disorders.

Unlike tests that measure glucose, cholesterol, or hormones, a CBC examines three major types of blood cells:

Red blood cells, which carry oxygen throughout the body
White blood cells, which help protect against infections
Platelets, which help blood clot and prevent excessive bleeding

A CBC is often included in routine blood testing, particularly for older adults. However, an abnormal result does not necessarily indicate a serious illness.

Red Blood Cells — Carrying Oxygen

Red blood cells contain hemoglobin, the protein that transports oxygen from the lungs to tissues throughout the body.

A CBC provides several measurements related to red blood cells.

Hemoglobin (Hgb)

Hemoglobin measures the amount of oxygen-carrying protein in the blood.

Typical adult reference ranges are approximately:

Men: 13.5–17.5 g/dL
Women: 12.0–15.5 g/dL

Low hemoglobin indicates anemia, although its cause requires further investigation.

Hematocrit (Hct)

Hematocrit measures the percentage of blood volume occupied by red blood cells.

Typical ranges are approximately:

Men: 41–53%
Women: 36–46%

Hematocrit often changes alongside hemoglobin.

Red Blood Cell Count (RBC)

This measures the number of red blood cells in a given volume of blood.

The RBC count is useful, but hemoglobin and other measurements are generally more important when evaluating anemia.

MCV — Why Red Blood Cell Size Matters

Mean corpuscular volume (MCV) measures the average size of red blood cells.

A typical reference range is approximately 80–100 femtoliters (fL).

MCV helps doctors classify anemia.

Low MCV: Smaller-than-normal red blood cells, commonly associated with iron deficiency or thalassemia
Normal MCV: May occur with chronic inflammation, kidney disease, blood loss, or other causes
High MCV: Larger-than-normal red blood cells, sometimes associated with B12 or folate deficiency, alcohol use, liver disease, thyroid disorders, or certain medications

A high MCV does not automatically mean someone has a B12 deficiency, and a normal MCV does not rule it out.

RDW — Are Red Blood Cells Different Sizes?

Red cell distribution width (RDW) measures how much red blood cell sizes vary.

An elevated RDW means there is greater variation in cell size.

This can occur with:

Iron deficiency
Vitamin B12 or folate deficiency
Mixed nutritional deficiencies
Certain chronic illnesses

RDW is most useful when interpreted alongside hemoglobin and MCV.

Some observational studies have associated higher RDW with poorer health outcomes in older adults. However, RDW is not itself a diagnosis or a proven treatment target.

What Is Anemia?

Anemia means that the blood has less hemoglobin than normal, reducing its capacity to carry oxygen.

Symptoms may include:

Fatigue or weakness
Shortness of breath
Dizziness
Reduced exercise tolerance
Palpitations

Anemia becomes more common with age, but it should not automatically be dismissed as normal aging.

Possible causes include iron deficiency, B12 or folate deficiency, kidney disease, chronic inflammation, blood loss, and disorders affecting bone marrow.

Finding the underlying cause is important because treatment varies considerably.

White Blood Cells — The Immune System

White blood cells help defend the body against infections and other threats.

A typical white blood cell count is approximately 4,000–11,000 cells per microliter, although laboratory ranges vary.

A high white blood cell count may occur with:

Infection
Inflammation
Physical stress
Certain medications, including corticosteroids
Some blood or bone marrow disorders

A low white blood cell count may occur with:

Certain viral infections
Medication effects
Autoimmune conditions
Nutritional deficiencies
Bone marrow disorders

A mildly abnormal white blood cell count is not necessarily dangerous. Doctors consider the degree of abnormality, symptoms, and changes over time.

The White Blood Cell Differential

Many CBC reports include a differential, which identifies the major types of white blood cells.

Neutrophils: Particularly important in defending against bacterial and fungal infections
Lymphocytes: Include cells involved in antibody production and immune memory
Monocytes: Help remove damaged cells and participate in immune responses
Eosinophils: Often associated with allergies, asthma, and certain parasitic infections
Basophils: Participate in allergic and inflammatory responses

The absolute number of each cell type is often more informative than its percentage alone.

For example, a low absolute neutrophil count may increase infection risk, depending on how low it is and the underlying cause.

Platelets — Helping Blood Clot

Platelets are small cell fragments that help stop bleeding when a blood vessel is injured.

A typical platelet count is approximately 150,000–450,000 per microliter.

A low platelet count, called thrombocytopenia, may result from medications, infections, immune conditions, liver disease, or bone marrow disorders.

A high platelet count, called thrombocytosis, may occur with inflammation, iron deficiency, infection, or certain bone marrow diseases.

Mild abnormalities are often temporary, but substantial or persistent changes deserve evaluation.

Why Is the CBC Particularly Useful for Older Adults?

Several conditions that become more common with age can affect blood cell counts.

These include:

Iron deficiency from gastrointestinal blood loss
Vitamin B12 deficiency
Chronic kidney disease
Inflammatory illnesses
Medication effects
Bone marrow disorders

One condition worth knowing about is myelodysplastic syndrome (MDS), a group of bone marrow disorders that can cause persistently low red blood cells, white blood cells, or platelets.

MDS becomes more common with age, but most mildly abnormal CBC results are not caused by MDS.

Unexplained abnormalities that persist or worsen may require further testing.

Why Trends Matter More Than One Result

A CBC is a snapshot of blood cell counts at one point in time.

Results can vary with hydration, recent illness, medications, and normal biological fluctuations.

A value just outside the reference range may not be clinically significant.

However, a gradual decline in hemoglobin or a persistent change in white blood cells or platelets may deserve attention even if the numbers are only mildly abnormal.

Comparing current results with previous CBC tests can be more informative than examining one result in isolation.

The Bottom Line

A complete blood count provides a broad overview of the body's red blood cells, white blood cells, and platelets.

It can help identify anemia, infections, and blood disorders, but it rarely establishes a diagnosis by itself.

For older adults, the CBC is especially useful because it can reveal treatable nutritional deficiencies, chronic illnesses, and changes that warrant further investigation.

The most important questions are not simply whether a number is high or low, but how abnormal it is, whether it is changing, and what might explain it.