What Is a Complete Blood Count (CBC) and What Does It Measure?
Written by YiQiu Hu, NMD
Arizona-licensed naturopathic physician and virtual concierge medicine provider
Published August 2026 | Medically reviewed August 2026
A complete blood count, commonly called a CBC, is a blood test that measures the number and characteristics of the major cells circulating in your blood. It provides information about your red blood cells, white blood cells, and platelets, along with measurements such as hemoglobin and hematocrit.
A CBC is one of the most commonly ordered laboratory tests because it can provide useful information about many aspects of your health from a relatively small blood sample.
Physicians may use a CBC as part of routine health screening, to investigate symptoms such as fatigue or weakness, to look for signs of anemia or infection, or to monitor certain medical conditions and medications.
However, a CBC is not a diagnosis by itself. The individual measurements need to be interpreted in the context of your symptoms, medical history, medications, previous results, and other laboratory findings.
If you would like a broader introduction to blood testing, start with our Patient's Guide to Understanding Blood Work and Laboratory Testing.
What Does a CBC Measure?
A standard complete blood count provides information about three major groups of blood cells:
Red blood cells, which carry oxygen throughout the body
White blood cells, which are involved in immune defense
Platelets, which help control bleeding and form blood clots
The CBC also reports several measurements that help physicians understand the number, size, and characteristics of these cells.
Here are the major components you may see on your laboratory report.
Red Blood Cell Count (RBC)
The red blood cell count, usually abbreviated RBC, measures how many red blood cells are circulating in a specific volume of blood.
Red blood cells carry oxygen from your lungs to tissues throughout your body and return carbon dioxide to the lungs.
A low red blood cell count may occur in some forms of anemia, blood loss, nutritional deficiencies, kidney disease, certain chronic illnesses, medication effects, or disorders affecting blood-cell production.
A high red blood cell count may occur with dehydration, reduced oxygen availability, smoking, living at higher elevations, certain heart or lung conditions, or disorders that increase red blood cell production.
The RBC count is usually interpreted together with hemoglobin, hematocrit, and the red blood cell indices rather than by itself.
Hemoglobin (Hgb)
Hemoglobin is an iron-containing protein inside red blood cells that carries oxygen.
It is one of the primary measurements physicians use when evaluating for anemia.
A low hemoglobin level may suggest anemia, but the CBC alone often does not tell you why anemia is present.
Depending on the pattern, additional testing may be necessary to evaluate possible causes such as iron deficiency, vitamin B12 deficiency, folate deficiency, blood loss, kidney disease, chronic inflammation, or other medical conditions.
Hematocrit (Hct)
Hematocrit describes the percentage of your blood volume that consists of red blood cells.
Hemoglobin and hematocrit frequently move in the same direction.
For example, anemia may cause both values to decrease. Dehydration can sometimes make hematocrit and hemoglobin appear higher because there is less fluid in the bloodstream relative to the number of red blood cells.
Your physician therefore evaluates these measurements as part of a larger pattern.
Mean Corpuscular Volume (MCV)
Mean corpuscular volume, or MCV, measures the average size of your red blood cells.
MCV is particularly helpful when evaluating different patterns of anemia.
Smaller-than-average red blood cells may occur with conditions such as iron deficiency or certain inherited hemoglobin disorders.
Larger-than-average red blood cells may occur with vitamin B12 or folate deficiency, certain medications, alcohol use, liver disease, thyroid disorders, or other conditions.
A normal MCV does not automatically mean that anemia or a nutritional deficiency is absent.
MCH and MCHC
Your CBC may also include measurements called MCH and MCHC.
MCH, or mean corpuscular hemoglobin, estimates the average amount of hemoglobin contained in each red blood cell.
MCHC, or mean corpuscular hemoglobin concentration, estimates the concentration of hemoglobin within the red blood cells.
These values are generally interpreted together with MCV, hemoglobin, hematocrit, and the rest of the CBC rather than as independent screening tests.
Red Cell Distribution Width (RDW)
RDW measures how much variation exists in the size of your red blood cells.
A higher RDW means that there is greater variation in red blood cell size.
RDW can sometimes provide additional clues when evaluating anemia, particularly when it is interpreted alongside MCV and other red blood cell measurements.
White Blood Cell Count (WBC)
White blood cells are an important part of your immune system.
The white blood cell count, or WBC, measures the total number of white blood cells circulating in your blood.
A high white blood cell count can occur with:
Infection
Inflammation
Physical stress
Certain medications
Smoking
Tissue injury
Some immune disorders
Certain blood or bone marrow disorders
A low white blood cell count can occur with:
Certain viral illnesses
Some medications
Autoimmune conditions
Nutritional deficiencies
Bone marrow disorders
Treatments that suppress blood-cell production
Other medical conditions
An abnormal WBC count does not automatically diagnose an infection, immune disorder, or blood disease. The degree of abnormality, symptoms, previous results, and specific types of white blood cells involved all matter.
What Is a CBC With Differential?
You may notice that your physician ordered a CBC with differential rather than simply a CBC.
A differential provides additional information by separating white blood cells into their major categories.
These typically include:
Neutrophils
Lymphocytes
Monocytes
Eosinophils
Basophils
Each type of white blood cell has somewhat different functions within the immune system.
For example, neutrophils play an important role in responding to many infections and inflammatory processes, while eosinophils may increase with allergies, asthma, medication reactions, certain parasitic infections, and other conditions.
The differential can therefore help your physician better understand an abnormal total white blood cell count, but the pattern still needs to be interpreted within your complete clinical picture.
Platelet Count
Platelets are small cell fragments that help your blood clot and control bleeding.
The platelet count measures how many platelets are circulating in your bloodstream.
A low platelet count is called thrombocytopenia. Potential causes range from temporary infections and medication effects to immune conditions, liver disease, nutritional deficiencies, and problems involving blood-cell production.
A high platelet count is called thrombocytosis. Platelets may temporarily increase because of inflammation, infection, iron deficiency, blood loss, or surgery. Persistent elevations sometimes require additional evaluation.
The degree of the abnormality is important. A platelet count that is only slightly outside the reference range may have a very different clinical significance from a severely abnormal result.
What Can a CBC Help Evaluate?
Because a CBC measures several types of blood cells, physicians may use it to help evaluate a wide range of medical concerns.
A CBC may provide useful information when evaluating:
Anemia
Fatigue or weakness
Fever or suspected infection
Unexplained bruising
Abnormal bleeding
Immune-system activity
Certain nutritional deficiencies
Medication effects
Chronic medical conditions
Blood loss
Changes in blood-cell production
Certain blood or bone marrow disorders
It may also be ordered as part of appropriately selected routine laboratory testing.
Importantly, the CBC usually provides clues rather than a final diagnosis.
A physician may need additional laboratory tests, examination findings, imaging, medical history, or other diagnostic information to determine why a particular CBC measurement is abnormal.
What Does a CBC Not Measure?
Despite the word "complete" in its name, a complete blood count does not provide a complete evaluation of every aspect of your health.
A CBC does not directly measure:
Blood sugar
Hemoglobin A1C
Cholesterol
Triglycerides
Kidney function
Liver enzymes
Electrolytes
Thyroid function
Vitamin D
Vitamin B12
Iron or ferritin
Most hormone levels
These require separate laboratory measurements.
For example, a comprehensive metabolic panel evaluates measurements related to blood sugar, electrolytes, kidney function, liver-related markers, proteins, and other areas that are not included in a CBC.
This is why a routine blood draw may contain several different laboratory panels even though all of the samples are collected during the same appointment.
Does a CBC Test for Iron Deficiency?
Not directly.
A CBC can reveal a pattern that suggests iron deficiency, such as anemia accompanied by smaller red blood cells, but it does not measure the body's iron stores.
If iron deficiency is suspected, your physician may order tests such as ferritin, serum iron, transferrin, or total iron-binding capacity depending on the clinical situation.
Can a CBC Detect an Infection?
A CBC can sometimes provide evidence that supports the possibility of infection, particularly when the white blood cell count or specific components of the white blood cell differential change.
However, it usually cannot determine by itself:
Whether an infection is definitely present
Where an infection is located
Whether the cause is bacterial, viral, fungal, or another organism
Which treatment is appropriate
Your symptoms, examination, and additional testing may be necessary.
Does an Abnormal CBC Mean Something Is Wrong?
Not necessarily.
A laboratory value falling outside its reference range does not automatically mean you have a disease.
CBC measurements can be affected by factors such as:
Hydration
Recent illness
Exercise
Medications
Menstruation
Pregnancy
Smoking
Altitude
Recent blood loss
Normal biological variation
Laboratory reference intervals also vary somewhat between laboratories and patient populations.
This is why a result highlighted as "high" or "low" should not be interpreted in isolation. You can learn more in Understanding Laboratory Reference Ranges: Why "Normal" Results Still Need Context.
For a more detailed discussion of what individual CBC abnormalities may mean, read our physician article, Understanding a Complete Blood Count (CBC): What Your Results May Mean.
Do You Need to Fast for a CBC?
A CBC by itself generally does not require fasting.
However, your physician may order a CBC together with other tests that have specific preparation requirements.
For example, other blood tests collected during the same appointment may occasionally require fasting or other preparation.
Always follow the instructions provided for your specific laboratory order rather than assuming you should or should not fast.
How Often Should You Have a CBC?
There is no single schedule that applies to everyone.
How often a CBC is appropriate depends on factors including:
Your age
Symptoms
Medical history
Current medications
Chronic medical conditions
Previous laboratory abnormalities
Treatment being monitored
Individual risk factors
Some people may require regular CBC monitoring, while healthy individuals may need the test much less frequently.
For more information, read How Often Should I Have Routine Blood Tests?.
Why Are Previous CBC Results Important?
A single blood test provides a snapshot of your health at one point in time.
Previous results provide something different: a trend.
A value may still fall within the laboratory's reference range while gradually changing from your previous baseline. Conversely, a mildly abnormal measurement that has remained unchanged for many years may have different significance from a sudden new abnormality.
When reviewing a CBC, your physician may consider:
Your current results
Previous CBC results
The direction and speed of any change
Whether several measurements changed together
Your symptoms
Recent illnesses
Medications and supplements
Your overall medical history
For a broader explanation of laboratory interpretation, see How to Interpret Common Blood Test Results: What Every Patient Should Know.
The Bottom Line
A complete blood count is a common laboratory test that measures red blood cells, white blood cells, platelets, hemoglobin, hematocrit, and several other characteristics of your blood cells.
It can provide valuable information about anemia, immune activity, infection, bleeding concerns, medication effects, and other medical conditions.
However, a CBC should not be interpreted as a collection of isolated high and low numbers. The most useful interpretation considers how the different measurements relate to one another, how they have changed over time, and how they fit with your symptoms, medical history, medications, and other laboratory findings.
If you are interested in understanding how laboratory testing fits into a broader approach to your health, explore our Routine Laboratory Testing Resource Center or learn more about Preventive Care & Health Optimization at Ask Dr. Hu.
Medical Review & Trusted Sources
This article was written for patient education and reviewed for medical accuracy using established clinical laboratory principles and trusted medical resources.
For additional information about complete blood counts, patients can review:
Complete Blood Count (CBC) — MedlinePlus, U.S. National Library of Medicine
Blood Tests — National Heart, Lung, and Blood Institute, National Institutes of Health
Medical information on Ask Dr. Hu is intended for general educational purposes. Laboratory results should be interpreted in the context of your individual medical history, symptoms, medications, physical examination, previous results, and other clinically appropriate testing.
About the Author
YiQiu Hu, NMD is an Arizona-licensed naturopathic physician and virtual concierge medicine provider at Ask Dr. Hu. His clinical approach emphasizes personalized medical care, preventive healthcare, laboratory interpretation, chronic disease management, and helping patients better understand the reasoning behind their medical care.
Ask Dr. Hu provides personalized concierge medicine and naturopathic healthcare for patients throughout Arizona, including physician-guided laboratory evaluation and preventive health planning.
If you would like individualized guidance regarding laboratory testing or your blood test results, contact Ask Dr. Hu to learn more about becoming a patient.