Red blood cells in a test tube

Blood Tests and Sed Rate in Psoriatic Arthritis

Blood tests are commonly used as a tool to diagnose psoriatic arthritis (PsA). Specifically, a type of blood test called a “sed rate test” is used to determine the level of inflammation in the body. Because PsA causes joint inflammation, sed rate tests are an important diagnostic tool since they indicate the severity of inflammation.1

While a sed rate test alone cannot diagnose PsA, understanding this test can help you and your doctor either diagnose or monitor its progress.

What to expect in a blood test

Blood tests are common procedures where a small sample of blood is taken from the body. This is typically done by inserting a needle into a vein in the arm or by a finger prick. The person taking your blood may ask you to make a fist or tie a band around the upper part of your arm. This is to make inserting the needle easier since it makes the veins pop out.

After the tube is full, the person taking your blood will remove it, and the tube will seal by itself. A bandage will then be applied to the area. The person taking your blood will instruct you on how long to keep this on. Generally, you will have to keep it on for a few hours.1

Results from your blood test can take anywhere from a few minutes to a few weeks. You will follow up with your doctor after the test to discuss the results.1

What is a sed rate test?

Sed rate (erythrocyte sedimentation rate) is a type of blood test that checks for inflammation in your body. Red blood cells, also called erythrocytes, settle to the bottom of the test tube when blood is drawn during a blood test. Inflammation, like the inflammation caused by PsA, can cause these cells to clump together. These cell clumps are denser than normal cells and settle to the bottom of the test tube more quickly.2

The sed rate test measures the distance that the red blood cells fall in the tube in 1 hour. A higher sed rate means that the cells have moved farther, and a lower sed rate means they have moved less. This is correlated to the amount of inflammation in the body.2

A sed rate test alone cannot pinpoint the problem that is causing the inflammation. However, it is an important diagnostic tool to determine the degree of inflammation.

What do the results of a sed rate test mean?

Results from a sed rate test will be reported to you in the distance in millimeters that the cells have descended in 1 hour (mm/hr). The upper range of what is considered “normal” varies somewhat between medical practices. However, normal values are roughly 0 to 22 mm/hr for men and 0 to 29 mm/hr for women. Check with your doctor about what is considered normal for you.2

Numbers above this threshold are a sign of increased levels of inflammation in the body. Since PsA causes joint inflammation, sed rate is a helpful diagnostic tool.3 It provides a metric of the severity of inflammation that could be caused by PsA.3

A higher sed rate is considered one of the best predictors of damage progression caused by PsA. Lower sed rate also seems to be protective against disease progression.4

It is important to remember that sed rate is just 1 diagnostic tool for PsA. A number of different conditions can affect the blood, which can affect sed rate. These include:2

  • Age
  • Pregnancy
  • Kidney problems
  • Anemia
  • Thyroid disease
  • Infection
  • Some cancers

Because of this, your doctor will also take these factors into account when discussing your sed rate test results. They will also consider other symptoms and the results of other diagnostic tests.

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