A complete blood count (CBC) test is one of the most commonly ordered blood tests. It is used to evaluate your overall health and detect a wide range of disorders, including anemia, infection, and leukemia. The test provides key information about the kinds and numbers of cells in your blood, including red blood cells, white blood cells, and platelets.
What is a CBC test tube?
To perform a CBC test, a blood sample is collected in a special tube called a CBC tube or vacutainer. These tubes contain additives designed to prevent clotting and preserve the blood cells until testing can be performed. The most common additives used are:
- EDTA – Ethylenediaminetetraacetic acid prevents clotting by binding calcium.
- Heparin – Prevents clotting by binding to clotting factors.
The tube contains a vacuum that draws the correct amount of blood into the tube when it is inserted into the vein. After collection, the tube is inverted several times to distribute the additive throughout the blood to prevent clotting.
Why is a special tube required?
Using a specially designed CBC collection tube is important for several reasons:
- Provides accurate results: The additives preserve the blood cells in their original state.
- Prevents clotting: Clotting can damage cells and alter results.
- Standard volume: The vacuum draws the optimal volume of blood required for testing.
- Barcoded: Allows each sample to be accurately tracked.
If a non-additive tube was used, the blood would clot and results would become unreliable. Insufficient or excess blood volume could also lead to inaccurate results.
What color are CBC tubes?
CBC tubes come in several different colors, with each color coded to contain specific additives and for particular types of tests:
| Tube Color | Additive | Tests Performed |
|---|---|---|
| Lavender | EDTA | CBC, Hematology |
| Light Green | Lithium Heparin | CBC, Chemistry |
| Dark Green | Sodium Heparin | CBC, Chemistry |
The lavender-top EDTA tube is the tube specifically designated for CBC tests. The light green lithium heparin and dark green sodium heparin tubes can also be used.
Are there any special requirements for CBC collection?
There are some important requirements that must be followed when collecting blood for a CBC test:
- Patient should be well hydrated before sample collection.
- Tourniquet should not be applied for longer than 1 minute before drawing.
- Blood should be drawn in the morning before food intake.
- Tube must be gently inverted multiple times immediately after collection.
- Sample should be transported to lab promptly at room temperature.
These precautions help obtain a high-quality non-hemolyzed blood sample and maximize the accuracy of CBC results.
What are the components of a CBC test?
A CBC provides detailed information about the main cell types in your blood:
- Red blood cells (RBC) – Carry oxygen throughout the body.
- White blood cells (WBC) – Fight infection.
- Platelets – Help blood clot.
Key CBC parameters examined include:
| Test | What it measures | Normal range |
|---|---|---|
| Hemoglobin | Oxygen-carrying protein in RBCs | Male: 13.5-17.5 g/dL Female: 12.0-15.5 g/dL |
| Hematocrit | Percentage of blood volume made up of RBCs | Male: 40-54% Female: 36-48% |
| RBC count | Number of RBCs | Male: 4.7-6.1 million cells/mcL Female: 4.2-5.4 million cells/mcL |
| WBC count | Number of WBCs | 4,500-11,000 cells/mcL |
| Platelet count | Number of platelets | 150,000-450,000 cells/mcL |
What does the CBC results mean?
CBC test results provide an overview of your general health status and can indicate a variety of conditions:
- Anemia – Low hemoglobin, hematocrit, or RBC counts.
- Infection – Elevated WBC count.
- Leukemia – Abnormal WBCs and blast cells present.
- Bleeding disorders – Low platelet counts.
- Dehydration – Increased hemoglobin and hematocrit.
However, most results fall within the normal range. Normal values can sometimes vary between labs, so it is important to compare your results to the particular reference range used.
What are hematology analyzers?
After collection, CBC tubes are taken to the laboratory where specialized machines called hematology analyzers are used to measure the different cell parameters:
- Impedance counters – Count cells by detecting changes in electrical resistance when they pass through an aperture.
- Photometers – Use light absorption to evaluate hemoglobin levels.
- Flow cytometers – Use lasers and fluorescence to analyze cell size, shape, and markers.
- Automated microscopy – Take digital images of cell morphology.
These sensitive instruments provide fast, accurate analysis of the numbers and properties of blood cells from a CBC sample.
Why order a CBC test?
Some common reasons a doctor may order a CBC test include:
- Part of a routine physical examination
- Monitor ongoing medical conditions
- Diagnose the cause of symptoms like fatigue, bruising, or fever
- Screen for diseases such as anemia, leukemia, and infection
- Monitor bone marrow function and response to treatment
- Check for blood loss or dehydration
The CBC provides an overview of your blood cell composition and often gives the first indication that further testing may be needed to diagnose a condition.
How often should a CBC be performed?
For healthy individuals with no symptoms, a CBC test is often recommended annually as part of a routine wellness checkup. Those with ongoing medical conditions may need the test more frequently to monitor blood cell counts and check for treatment response, such as:
- Every 3-6 months for stable medical conditions
- Monthly or weekly to monitor chemotherapy
- Before and after surgery
- Weekly follow-up when critically ill
Your doctor will advise how often you need the test based on your age, health status, and medical history.
Conclusion
CBC testing provides valuable information about red and white blood cell counts and overall blood health. It is performed using a specialized lavender-top tube containing EDTA additive. Precise collection and handling procedures help ensure accurate results, which are obtained rapidly using hematology analyzers. The CBC is one of the most commonly ordered blood tests to screen for anemia, infection, and other conditions. For many patients it is recommended as part of an annual physical, but may be required more frequently to monitor ongoing medical treatment.