What Does a CBC Blood Test Measure?

A Complete Blood Count (CBC) test is one of the most commonly ordered blood tests by healthcare providers. Whether you're at your routine check-up or experiencing unexplained symptoms, the CBC provides valuable insights into your overall health. But what exactly does a CBC test measure? This article will explain the various components of a CBC, why it’s important, and how it can help detect potential health issues.

What Is a CBC Blood Test?

A Complete Blood Count (CBC) is a comprehensive blood test that evaluates the different components of your blood. The test measures key elements in your blood, including red blood cells, white blood cells, hemoglobin, hematocrit, and platelets. Each of these components plays an essential role in your body's health and function, and their levels can provide crucial information about various health conditions.

What it Measures

1. Red Blood Cells (RBC)

Red blood cells are responsible for carrying oxygen from your lungs to the rest of your body and bringing carbon dioxide back to your lungs for exhalation. Low and high levels can mean a variety of things, including:

• A low RBC count (anemia) might suggest iron deficiency, vitamin B12 deficiency, chronic disease, or other conditions. [1]
• A high RBC count (polycythemia) could indicate dehydration, lung disease, or other disorders that affect oxygen delivery to tissues. [2]

2. White Blood Cells (WBC)

White blood cells are part of your immune system and help fight off infections. [3]

• A high white blood cell count could indicate an infection, an inflammatory disease, or even a bone marrow disease. [4]
• Conversely, a low white blood cell count (Leukopenia) count may make you more susceptible to infections. [4]

3. Hemoglobin (Hb)

Hemoglobin is a protein found in red blood cells that carries oxygen. If hemoglobin levels are low, it may indicate anemia or a blood disorder.

• A high hemoglobin level could indicate dehydration, lung disease, or heart disease, as the body compensates for low oxygen levels by producing more red blood cells. It can also be a sign of polycythemia vera, a rare blood disorder where the body makes too many red blood cells, potentially leading to blood thickening and increased risk of clots. [10]
• A low hemoglobin level could indicate iron deficiency, vitamin B12 deficiency, chronic disease, or bone marrow disorders, leading to reduced oxygen delivery to tissues. It may cause symptoms like fatigue, weakness, shortness of breath, and pale skin. [11]

4. Hematocrit (Hct)

Hematocrit measures the proportion of red blood cells in your blood. This is important because it helps assess your body’s ability to transport oxygen.

• A high hematocrit level could indicate dehydration, lung disease, or heart disease, as the body compensates for reduced oxygen levels by producing more red blood cells, making the blood thicker. It may also signal polycythemia vera, a condition where the body produces too many red blood cells.
• A low hematocrit level could indicate anemia, blood loss, malnutrition, or bone marrow disorders, resulting in less oxygen being carried throughout the body. This can cause symptoms like fatigue, dizziness, and weakness.

5. Platelets

Platelets are small cells that help with blood clotting. [6]

• A high platelet count (thrombocytosis) could indicate a bone marrow disorder, an inflammatory condition, or an increased risk of blood clots, which may lead to conditions such as stroke or heart attack. It can also be a response to infection or injury. [7]
• A low platelet count (thrombocytopenia) could indicate a bone marrow disorder, autoimmune disease, or liver disease, and it increases the risk of bleeding and bruising easily. It can also be a side effect of certain medications or chemotherapy.

6. Mean Corpuscular Volume (MCV)

MCV measures the average size of your red blood cells. This can help diagnose types of anemia, such as iron-deficiency anemia (small RBCs) or vitamin B12 deficiency (larger RBCs). It’s a valuable indicator of the health and functionality of red blood cells.

• A high Mean Corpuscular Volume (MCV) could indicate macrocytic anemia, which may be caused by vitamin B12 or folate deficiency, liver disease, or alcoholism. It can also be a sign of hypothyroidism or certain medications.
• A low MCV could indicate microcytic anemia, which is commonly caused by iron deficiency or chronic blood loss. It can also be associated with thalassemia, a genetic blood disorder.

7. Mean Corpuscular Hemoglobin (MCH)

MCH tells you how much hemoglobin is in the average red blood cell.

• A high Mean Corpuscular Hemoglobin (MCH) could indicate macrocytic anemia, which is often caused by vitamin B12 or folate deficiencies, liver disease, or hypothyroidism.
• A low MCH could suggest microcytic anemia, commonly due to iron deficiency or thalassemia, a genetic blood disorder affecting hemoglobin production.

8. Red Cell Distribution Width (RDW)

RDW measures the variation in the size of your red blood cells. [8]

• A high Red Cell Distribution Width (RDW) could indicate various conditions, such as anemia, vitamin deficiencies, or chronic liver disease, where there is a wide variation in red blood cell size. [8]
• A low RDW is generally not a concern and typically indicates uniformity in red blood cell size, which is usually a sign of healthy red blood cell production. [8]

9. Differential White Blood Cell Count

This part of the CBC breaks down the total WBC count into specific types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Abnormal levels of any of these can indicate different types of infections, immune system issues, or allergic reactions. [9]

• A high differential white blood cell count could indicate an ongoing infection, inflammation, or a more serious condition like leukemia or an autoimmune disorder. [9]
• A low differential white blood cell count could make you more vulnerable to infections and may be caused by conditions such as bone marrow disorders or the use of certain medications like chemotherapy. [9]

Why Is a CBC Blood Test Important?

A CBC is often one of the first tests ordered by doctors because it provides a broad overview of your general health. It helps doctors detect a wide range of conditions, including:

• Anemia: Low levels of red blood cells or hemoglobin.
• Infections: Elevated white blood cell count could indicate an infection.
• Blood disorders: Abnormal levels of platelets or RBCs can point to blood clotting issues or bone marrow diseases.
• Nutritional deficiencies: Low levels of hemoglobin or red blood cells can be linked to a lack of iron, folate, or B12.

How Is a CBC Test Performed?

A Complete Blood Count (CBC) test is a routine and straightforward procedure that provides important information about your overall health and can help diagnose various conditions. Below is a breakdown of the steps involved in how a CBC test is typically performed:

Step 1: Preparation

Before the CBC test, no special preparation is usually needed. You may be asked to fast (avoid eating or drinking) for a few hours if your healthcare provider needs additional tests like blood glucose levels, but fasting is not required for a basic CBC test. It’s always a good idea to inform your healthcare provider of any medications or supplements you are taking, as they may affect the results.

Step 2: Blood Sample Collection

The healthcare provider will begin by cleaning the skin over a vein in your arm, typically in the crook of your elbow, with an antiseptic wipe. A small needle is then inserted into the vein to collect a blood sample. While this process might cause a quick pinch or slight discomfort, it is generally well tolerated and takes only a few minutes.

Step 3: Sending the Sample to the Lab

Once the blood sample is collected, it is sent to a laboratory where it is processed for analysis. The sample is placed into a test tube, and the laboratory staff will separate the blood components (plasma, red blood cells, white blood cells, and platelets) for further analysis.

Step 4: Analyzing the Sample

In the laboratory, an automated machine performs the majority of the analysis. The machine measures the quantities and types of blood cells (red blood cells, white blood cells, and platelets), along with additional parameters like hemoglobin levels, hematocrit, and MCV. Some laboratories may also examine the blood sample manually, especially if there are abnormalities in the results that require closer inspection.

Step 5: Reviewing Results

The results of the CBC test are typically available within a few hours to a few days, depending on the laboratory's processing time. A healthcare provider will review the results, interpreting the findings to determine if any values fall outside the normal range. A CBC test gives a detailed overview of your red and white blood cells, platelets, and overall blood health.

Step 6: Receiving Your Results

Once your CBC results are ready, your healthcare provider will discuss them with you. If any values are out of the normal range, they may suggest further testing or provide recommendations for treatment or lifestyle changes. The results can help diagnose a variety of conditions, from anemia and infections to more complex blood disorders.

Step 7: Follow-Up (if necessary)

If abnormal results are detected, your healthcare provider may request follow-up testing, such as additional blood tests or imaging studies, to help identify the underlying cause. Depending on the findings, further steps in the diagnosis and treatment process will be outlined.

What Can You Learn From Your CBC Results?

Once your results are in, your doctor will review the numbers and compare them to the normal ranges. If any of your values are outside the normal range, your doctor may recommend further testing or follow-up appointments to determine the cause. A CBC test by itself can't diagnose specific conditions, but it can help pinpoint areas that require further investigation.

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Learn more

1. Red blood cell (RBC) count. MedlinePlus. Accessed April 29, 2025. https://medlineplus.gov/lab-tests/red-blood-cell-rbc-count/.
2. White blood cell (WBC) count. MedlinePlus. Accessed April 29, 2025. https://medlineplus.gov/lab-tests/white-blood-count-wbc/.
3. Low white blood cell count (leukopenia). Cleveland Clinic. Accessed April 29, 2025. https://my.clevelandclinic.org/health/diseases/17706-low-white-blood-cell-count.
4. Platelet information. American Red Cross. Accessed April 29, 2025. https://www.redcrossblood.org/donate-blood/dlp/platelet-information.html.
5. Thrombocytosis. Mayo Clinic. Accessed April 29, 2025. https://www.mayoclinic.org/diseases-conditions/thrombocytosis/symptoms-causes/syc-20378315.
6. RDW blood test. Cleveland Clinic. Accessed April 29, 2025. https://my.clevelandclinic.org/health/diagnostics/22980-rdw-blood-test.
7. Blood differential. MedlinePlus. Accessed April 29, 2025. https://medlineplus.gov/lab-tests/blood-differential/.
8. High hemoglobin count. Cleveland Clinic. Accessed April 29, 2025. https://my.clevelandclinic.org/health/symptoms/17789-high-hemoglobin-count.
9. Low hemoglobin count. Cleveland Clinic. Accessed April 29, 2025. https://my.clevelandclinic.org/health/symptoms/17705-low-hemoglobin.