CBC Blood Test: Measures and Results Explained
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Introduction
A complete blood count, or CBC, is one of the most common blood tests 2,3. It measures the number and features of several blood cells 1,4.
The test covers red blood cells, white blood cells, and platelets 1,3. It also reports hemoglobin, hematocrit, and several red blood cell indices 1,5.
This guide explains the CBC complete blood count test, what it measures, and understanding results. It also explains why providers order it and how the blood draw works.
CBC blood test results provide useful clues. However, a CBC cannot give all the answers needed for a diagnosis 4,6.
A provider reviews the CBC with your health history and other findings 4,6. Your laboratory’s own reference ranges also matter because ranges can differ between laboratories 1,5.
What Is a CBC and Why Is It Ordered?
Definition and purpose of a complete blood count
A CBC is a group of blood tests. It measures the number and size of different cells in the blood 4.
The main parts of a CBC include the following:
Red blood cells carry oxygen from the lungs to the rest of the body 3,4.
White blood cells are part of the immune system and fight infections and diseases 3,4.
Platelets help form clots and stop bleeding 3,4.
Hemoglobin is an oxygen-carrying protein found inside red blood cells 1,6.
Hematocrit shows how much of the blood is made up of red blood cells 2,4.
Red blood cell indices describe cell size and the amount or concentration of hemoglobin 5.
Together, these measurements give a broad view of blood cells. Results may show that one or more values fall outside the laboratory’s range.
Common reasons healthcare providers order a CBC
A provider may order a CBC as part of a routine checkup 3,4. The test can also help monitor overall health 4.
Providers may order it when they are looking into weakness, tiredness, bruising, or bleeding 1,6. It may also help assess unexplained weight loss or other health changes 1.
A CBC can help providers evaluate anemia, infections, and immune system disorders 2,4. It can also help assess blood disorders and blood cancers 2,4.
The test has an important role in ongoing care. Providers may use it to track conditions that affect blood cell counts 4,6.
Some medicines and radiation treatments can change blood counts 1,6. A CBC can help the care team watch for these changes 1,6.
What a CBC can and cannot diagnose
A CBC can show whether a value is above or below a reference interval. Such a result may point to an issue that needs more review 6.
However, many factors can change blood cell results 4. An abnormal result does not always mean that a medical condition needs treatment 4.
A CBC is only one tool used to assess health 4. A provider may consider your history, current health, and other test results 4,6.
Some results may lead to more tests. The ordering provider decides whether follow-up is needed based on the full clinical picture 6.
What Does a CBC Blood Test Measure?
Red blood cells, hemoglobin, and hematocrit
Red blood cells are also called RBCs or erythrocytes. They carry oxygen from the lungs to the rest of the body 3,4.
The RBC count shows the number of red blood cells in a set amount of blood 5,7. It does not directly measure the blood’s oxygen-carrying ability 5.
Hemoglobin may appear as Hb or Hgb. It is the protein in red blood cells that carries oxygen 1,6.
The hemoglobin test measures the amount of this protein in whole blood 5. Providers often review hemoglobin when assessing possible anemia 5,6.
Hematocrit may appear as Hct. It is the percentage of blood volume made up of red blood cells 5.
Hemoglobin and hematocrit provide related information. Both may change when plasma volume changes 5.
White blood cells and the CBC differential
White blood cells may appear as WBCs or leukocytes. They take part in immune responses and inflammation 1.
A standard CBC reports the total white blood cell count 4. This number combines the major white blood cell types.
A CBC with differential provides more detail. It counts the different types of white blood cells 4,7.
These types include neutrophils, lymphocytes, monocytes, eosinophils, and basophils 4,7. A differential may also report immature cells 7.
A CBC with differential may show each type as a percentage or an absolute count 7. An absolute count gives the number of that cell type in the sample.
Check the name of the ordered test. A standard CBC and a CBC with differential do not provide the same level of detail 4,7.
Platelets and platelet measurements
Platelets are small cell fragments that help blood clot 3. They gather at damaged blood vessels and help stop bleeding 3.
The platelet count shows the total number of platelets in the sample 7. It may appear as PLT on a laboratory report.
Some reports also include mean platelet volume, or MPV. MPV describes the average size of platelets 7.
A platelet count does not explain why a value is high or low. The ordering provider must review it with the rest of the CBC 6.
Red blood cell indices
Red blood cell indices describe key features of red blood cells. They can help providers study patterns linked with anemia 5.
Common red blood cell indices include the following:
Mean corpuscular volume, or MCV, describes average red blood cell size 5,7.
Mean corpuscular hemoglobin, or MCH, describes the hemoglobin amount in each red blood cell 5.
Mean corpuscular hemoglobin concentration, or MCHC, describes hemoglobin concentration within red blood cells 5,7.
Red cell distribution width, or RDW, describes how much red blood cell sizes vary 5,7.
These measurements work best when viewed together 5. One index cannot show the full cause of an abnormal pattern 5.
Understanding CBC Results and Reference Ranges
How to read the laboratory report
Begin with the test name. Check whether the report says CBC or CBC with differential.
Next, find the measured result. Then compare it with the reference interval shown beside it.
Use the units printed on that same report. Laboratories may use different measurements or methods 1.
Use the reference interval from the laboratory that tested your sample. CBC reference intervals can vary between laboratories 1,5.
A report may flag a result that falls outside its range. The flag shows a comparison with that laboratory’s interval. It does not establish a diagnosis.
Common parts of a CBC report
This table can help you identify each part. Always use the units and reference interval printed on your own report 1,5.
CBC component: Abbreviation | What it describes | How to use the report
Red blood cell count: RBC | The RBC count shows the number of red blood cells in a set blood volume 5. | Compare the result with the laboratory’s printed interval 1,5.
Hemoglobin: Hb or Hgb | Hemoglobin is the oxygen-carrying protein in red blood cells 1,6. | Use the unit and interval shown on the report 1,5.
Hematocrit: Hct | Hematocrit shows the percentage of blood made up of red blood cells 5. | Compare the percentage with the laboratory’s interval 1,5.
White blood cell count: WBC | The WBC count shows the total number of white blood cells 4. | Use the report’s stated unit and reference interval 1,5.
Platelet count: PLT | The platelet count gives the total number of platelets in the sample 7. | Use the report’s stated unit and reference interval 1,5.
Mean corpuscular volume: MCV | MCV describes the average size of red blood cells 5,7. | Compare it with the laboratory’s printed interval 1,5.
Mean corpuscular hemoglobin: MCH | MCH describes the hemoglobin amount in each red blood cell 5. | Compare it with the laboratory’s printed interval 1,5.
Mean corpuscular hemoglobin concentration: MCHC | MCHC describes hemoglobin concentration within red blood cells 5,7. | Compare it with the laboratory’s printed interval 1,5.
Red cell distribution width: RDW | RDW describes differences in red blood cell size 5,7. | Compare it with the laboratory’s printed interval 1,5.
Mean platelet volume: MPV | MPV describes average platelet size 7. | Use the unit and interval shown on the report 1,5.
Why complete blood count normal ranges vary
There is no single range that applies to every report. Laboratories may use different reference intervals and measurements 1.
Age and sex can affect the expected range for some CBC values 3,5. Pregnancy and altitude can also affect interpretation 3,5.
Hydration can influence measured blood values 4,5. Diet, activity, medicines, and menstrual periods may also affect results 4.
These factors do not explain every abnormal value. They show why the ordering provider needs context.
Why one result may not tell the full story
A CBC offers a view of the blood sample taken at that time. A provider may compare it with earlier CBC blood test results 6.
The full pattern often matters more than one highlighted number. For example, hemoglobin may be reviewed with hematocrit, MCV, and other red cell indices 5.
The ordering provider decides whether an abnormal result needs follow-up 6. That decision may depend on the full report, other tests, and overall health 4,6.
What High or Low CBC Results May Mean
Abnormal red blood cell results and indices
Low hemoglobin or hematocrit is consistent with anemia 1,5,6. A low red blood cell count may also occur with anemia 1,6.
Anemia can have many causes 5,6. A CBC pattern cannot identify the cause by itself 5.
High red blood cell, hemoglobin, or hematocrit values can also have several explanations 1. Changes in plasma volume may affect hemoglobin and hematocrit measurements 5.
Low plasma volume can make measured hemoglobin appear higher 5. Higher plasma volume can make it appear lower 5.
MCV helps sort anemia by average red blood cell size 5. A low MCV pattern is called microcytic, and a high MCV pattern is called macrocytic 5.
A pattern within the laboratory’s MCV range is called normocytic 5. These terms describe cell size rather than the cause of anemia 5.
MCH, MCHC, and RDW give more information about the red cells 5. A blood smear may also help show how cells look under a microscope 7.
High or low white blood cell counts
A high white blood cell count is called leukocytosis 1,7. A low white blood cell count is called leukopenia 1,7.
High and low WBC counts can have several causes 1,3,4. The total count alone cannot show which cause applies.
A CBC with differential may add useful detail. It shows how the different white blood cell types contribute to the total count 4,7.
The provider can compare those results with the rest of the CBC. Other tests and overall health may also guide the interpretation 4,6.
High or low platelet counts
A low platelet count is called thrombocytopenia 6,7. A high platelet count is called thrombocytosis 6,7.
Either result may point to a medical condition or a medicine effect 6. Platelet levels outside the standard range may also relate to bleeding or clotting disorders 3.
The platelet count does not identify the cause. A result outside the range may lead to more testing 6.
The ordering provider selects the next step. That choice depends on the full report and the person’s overall health 6.
Preparing for a CBC and Next Steps
Preparing for the blood draw
A CBC alone requires no special preparation 1,4. You can eat and drink as usual before a CBC 6.
Other blood tests may be ordered at the same time. Some of those tests may require fasting 3,4.
Follow the instructions from your healthcare provider or laboratory. Ask before the test if the directions are unclear.
What happens during the test
A healthcare professional takes a blood sample from a vein in the arm 4,6. The blood goes into a tube or vial for testing 4.
The collection generally takes less than five minutes 4. Afterward, the sample is sent to a laboratory 6.
Questions to ask about abnormal results
Prepare a few questions before talking with the ordering provider. Useful questions may include the following:
How does this result compare with this laboratory’s reference interval?
Is this result different from my earlier CBC blood test results?
How do the other CBC values affect its meaning?
Could medicines or personal factors affect this result?
Does the result need to be checked again?
Would another test help explain the CBC pattern?
Does this result affect my current care plan?
The ordering provider should decide what follow-up is appropriate 6. Results outside the standard range may need other tests 6.
Possible follow-up tests
A CBC does not provide every answer needed for diagnosis 4,6. Follow-up testing depends on the CBC pattern and other health information 4,6.
A peripheral blood smear is one possible follow-up test. It gives more detail about how blood cells look under a microscope 7.
A provider may also compare the CBC with other test results 4,6. Repeat testing may be considered when the provider reviews the overall picture 6.
Do not change medicines or treatment based only on a CBC report. Discuss the result with the healthcare professional who ordered the test.
Conclusion
Understanding the CBC complete blood count test, what it measures, and understanding results can make a report feel less confusing. A CBC measures red blood cells, white blood cells, platelets, hemoglobin, hematocrit, and cell indices 1,4.
A CBC with differential also counts different white blood cell types 4,7. This gives the ordering provider more detail about immune cell groups.
CBC blood test results do not confirm most diagnoses by themselves 4,6. The provider reviews them with overall health and other test results 4,6.
Always use the reference interval and units printed on your own report. Laboratory ranges can differ 1,5.
One high or low result should not be used for self-diagnosis. The ordering provider can explain whether monitoring, repeat testing, or further evaluation is appropriate 4,6.
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