Complete Blood Count (CBC) with Differential

Test Quick Guide

Red cell distribution width (RDW) is a measurement that describes the variation in the sizes of red blood cells (RBC) in a sample of blood. RBC carry oxygen throughout the body. The size of RBC can affect their ability to deliver oxygen to tissues and organs.

The RDW test is one part of a panel of tests called the RBC indices, which help to define the different physical characteristics of RBC. The RDW test can be used to diagnose and classify conditions such as anemia, diabetes, and heart disease.

About the Test

Purpose of the test

The purpose of the RDW test is to assess the range of sizes of RBC in a sample of your blood. An RDW test is performed in combination with other tests to diagnose and determine the cause of anemia, a condition in which there are not enough healthy RBC to carry enough oxygen to the rest of your body.

The RDW test is part of a complete blood count (CBC), a routine test used to diagnose and monitor a wide variety of health conditions. In a CBC report, RDW is grouped with a panel of tests called the RBC indices that provide information about certain features and qualities of RBC. The RBC indices include:

Collectively, the RBC indices play an important role in diagnosing and determining the underlying cause of anemia. An RDW test can be relevant to detect certain conditions that may cause anemia, including:

  • Iron deficiency
  • Vitamin B12 or folate deficiency
  • Thalassemia, an inherited blood disorder
  • Heart disease
  • Cancer
  • Kidney disease
  • Liver disease

What does the test measure?

The RDW test measures the variation in the sizes of your RBC or whether they are more similar or less alike according to their size. In this case, width does not refer to the actual size of the RBC. Rather, RDW is a calculation of the distance between the range of RBC sizes as they are displayed on a graph called a histogram.

Blood cells that are similar in size will appear as data points on a histogram clustered closely together, which will result in a lower RDW. When there is a greater variety of blood cell sizes, data points will appear to have a wider distribution across the graph and the RDW is higher.

When should I get an RDW test?

RDW is part of a CBC test that evaluates your overall health and is used to detect a range of disorders, including anemia. Your doctor might order a CBC as part of a routine health exam or if you are being assessed for a specific health issue, such as iron deficiency or diseases of the heart, liver, or kidney.

The RDW test is compared with others that evaluate RBC function to check for signs of anemia, a common condition triggered when your body does not have enough healthy RBC. There are many causes of anemia. You may be born with it. Or you can become anemic from bleeding faster than your body can make new healthy RBC, or if your body is destroying them.

Symptoms of anemia can be mild or severe depending on the underlying condition or disorder. Mild anemia may present with no signs and symptoms, develop slowly over time, or occur suddenly. Your doctor will advise whether a CBC with an RDW test is necessary.

Some signs of early or mild anemia that might cause your doctor to order an RDW test can include:

  • Headaches
  • Loss of appetite
  • Feeling unusually weak or tired, including excessive fatigue after exercise
  • Numbness or tingling in your hands and/or feet
  • Problems with concentrating or thinking
  • Irritability or feeling agitated

As anemia progresses, additional signs and symptoms can include:

  • Ulcers of the mouth
  • Being short of breath at rest or mild activity
  • Unusually pale skin
  • An unusually red or possibly sore tongue
  • Feeling dizzy
  • Brittle nails
  • Desire to eat dirt, ice, or other non-food items
  • Blue color in the whites of the eyes

Finding an RDW Test

How can I get an RDW test?

The RDW test is part of a CBC. A blood draw is generally required for the CBC to be accurate and is most often conducted by a licensed professional in a health care setting. The RDW test is also a component of the RBC panel that provides information about RBC, white blood cells, and platelets. You can purchase an RBC test online that will include RDW screening, then go to the assigned lab near you where a phlebotomist will perform the blood draw.

Testing.com offers an anemia package with a CBC that measures red and white blood cells, hematocrit, hemoglobin, and platelets. There is also a CBC test with differential and basic health check that includes CBC screening. The sample collection is performed at local labs. You can get this test without a doctor’s order, but always discuss testing and results with a health care professional.

Can I take the test at home?

CBC testing is available as an at-home test and can provide results for RDW and other RBC indices. At-home testing kits include instructions and equipment for collecting a blood sample. This is usually performed by finger prick.

Once complete, the blood sample can be sent to a laboratory for analysis. At-home test results are generally ready in about two to three business days after the sample arrives at the lab. Results may be reported through an online health portal or email.

Diagnosis or monitoring of a health condition cannot be performed using at-home CBC testing. For an accurate interpretation of your test results, they must be shared with a health care provider who is familiar with your medical history and condition.

To confirm results from the at-home test, your health care provider may recommend retesting using a standard blood draw. At-home CBC testing to gather RDW information is useful to gain insight into whether you should seek further testing from your doctor.

How much does the test cost?

The cost of a CBC, which includes RDW results, depends on whether you choose an at-home kit, order the CBC test online to take in a lab, or get the test from your health care professional in a clinical setting. For example, a test ordered online can cost as little as $37 for a CBC, but if you select a more comprehensive panel, you can pay up to $114.

Again, the cost varies. Your health insurance may cover the test, but take into consideration copays and/or deductibles.

Taking an RDW Test

The RDW test is part of the CBC performed by a licensed health care professional who will draw a blood sample from a vein, usually from the inside crook of your elbow or the top of your hand. At-home CBC tests are generally performed with a finger prick to gather a blood sample to send to a certified lab.

Before the test

No preparation is required before a blood draw for a CBC unless specified by your doctor. Some at-home tests advise performing the test before breakfast, but fasting is not necessary.

During the test

If you are having your blood drawn for an RDW in a medical facility, a health care professional will determine whether to take blood from a vein in your arm or hand. Then, the area is prepped and cleansed using an alcohol wipe. Sometimes, an elastic band is placed around your upper arm to make the vein easier to access by needle. After inserting the needle, blood is gathered into a tube.

The process is relatively painless. You might feel a pinch or a bit of pain when the needle is inserted or removed. The blood draw will only take a few minutes.

If you take an at-home CBC test, you will receive detailed instructions and necessary equipment that will include a lancet and collection device. You will clean the area with a provided antiseptic wipe, prick your finger with the lancet, and then squeeze a drop of blood into the container or vial. Test kit instructions vary, so be sure to read carefully before starting the process.

After the test

When the blood draw is finished, a bandage or cotton swab will be placed on the area where the needle was inserted. The health care provider may recommend that you keep this in place for an hour or more to help prevent bleeding.

After a blood draw, it is common to experience some slight bruising. Other less common side effects from a blood draw may include persistent bleeding, tingling of hands or feet, nausea, or fainting.

Your health care provider may ask you to stay and be monitored for these side effects for a few minutes after the blood draw to make sure you are able to walk and/or drive safely.

If you take the test at home using a finger-prick kit very little blood is collected for the test and side effects are uncommon.

RDW Test Results

Receiving test results

Depending on the laboratory instrumentation and procedures, the results for your RDW test can be available within a few days of the blood sample arriving at the laboratory.

Generally, you can access your RDW test results on an online health portal or by email.

  • If you purchase a test online or choose an at-home CBC test, the company may provide a smartphone app you can download to securely review results.
  • If you order the test from Testing.com, results are available on the website in your personal, secured account.

Always discuss results with a health care provider, who can explain what they mean and determine any next steps. Your doctor will take into account your health history and present condition. The test is one aspect of your overall health.

Interpreting test results

RDW is usually reported as a percentage that describes the amount of variability in the size of RBC. The RDW result is interpreted by comparing it to a reference range, or a range of values that the laboratory conducting the test has established as an expected RDW if you are healthy.

Healthy blood cells are about the same size and can range from 6.2 to 8.2 micrometers. If your RBC vary significantly in size from each other, this can be a sign of anemia.

RDW results are interpreted by comparing other RBC indices to find out the size, quality, and function of RBC. That includes looking at MCV, which is the average size of RBC.

If RDW is within the reference range, it is considered normal. If RDW is outside the reference range, it is considered high or elevated.

Normal RDW means that the RBC in the sample that was tested are similar to each other in size or volume. Normal RDW can occur even when someone has a health problem that causes RBC to be abnormally large or small.

Elevated RDW indicates that the sizes of the RBC in the sample have a greater degree of variation. Again, with RDW, RBC size matters. But even if your RDW test shows elevated results, this information alone is not enough for a doctor to make a full diagnosis.

Depending on the results of the rest of your CBC, both normal and abnormal RDW test results may require follow-up testing. It is important to always discuss testing with your doctor. Additional blood testing, other laboratory tests, and imaging tests may all be considered based on the outcome of your initial RDW.

If you discuss your results with your doctor, here are some questions you may want to ask:

  • What do my RDW levels indicate about my health?
  • Can any diagnosis be made based on my RBC indices?
  • Will I need any follow-up tests based on my RDW results?
  • If my test results are abnormal, is there anything I can do to improve my health?

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