Joint Fluid and Tissue Tests for Arthritis

By Linda Rath | June 27, 2024

No single test can diagnose any rheumatic or autoimmune disease, although researchers around the world are trying to develop one. For now, the diagnosis mainly centers on a patient’s medical history and symptoms. Blood tests may help support the diagnosis but can be inaccurate. That’s why doctors may analyze a small sample of joint fluid or certain tissues in some cases.

Joint Fluid Test (Arthrocentesis): Gout

Symptoms of gout are often unmistakable: intense pain or swelling, usually in the big toe, that goes away in a few days, with or without treatment. In addition to noting symptoms, doctors usually also test the blood for higher-than-normal levels of uric acid, which forms the sharp crystals that lodge in joints and other tissues in gout. But the blood test can be misleading; many people have high uric acid levels and never develop gout. Others who have gout can have normal uric acid, even during a severe flare. Withdrawing a small amount of fluid from an affected joint and checking it for uric acid crystals is not only more accurate, but definitive for gout. Your doctor may suggest this test when it’s hard to distinguish gout from other forms of arthritis. This simple in-office procedure is generally painless and takes just a few minutes. Here’s how it’s done:

• The affected joint is cleaned and numbed.
• Your doctor inserts a thin needle into the joint and withdraws a small amount of fluid.
• The sample is sent to a lab for testing.
• You can go home immediately afterward, although you may be asked to limit strenuous activity for a day or two.

In addition to checking for gout, arthrocentesis can help differentiate gout from pseudogout, another type of painful, inflammatory arthritis that comes on with no warning, usually in the knee, wrist or ankle. Both involve the buildup of crystals in the joint. Needle-shaped uric acid crystals are associated with gout and rhomboid-shaped calcium pyrophosphate crystals occur with pseudogout. Distinguishing between the two diseases is important because pseudogout isn’t well studied and less is known about how to manage it. Also, unlike gout, certain foods and alcohol don’t seem to trigger pseudogout symptoms.

Arthrocentesis can also:

• Help determine if osteoarthritis (OA) may be causing joint pain and swelling. Although OA is now considered to have an inflammatory component, it usually results in non-inflammatory joint fluid, which is clear or straw-colored, with relatively few white blood cells. Fluid from a joint affected by inflammatory arthritis, like gout, is yellow and contains tens of thousands of white blood cells – a sure sign of inflammation.
• Relieve symptoms by removing excess fluid that puts pressure on the joint in inflammatory forms of arthritis, including rheumatoid arthritis (RA).

Tissue Tests

Around a quarter of people with psoriasis eventually develop psoriatic arthritis – often a decade or more after skin symptoms. A skin biopsy is rarely needed to diagnose psoriasis in white patients, but psoriasis can be harder to identify in people of color. This may partly but not entirely explain why they’re four times more likely to have a skin biopsy and wait three times longer for a psoriasis diagnosis than whites.  Another reason: People of color are less likely to be referred to a dermatologist who can diagnose them accurately.

There are several types of skin biopsies. A punch biopsy, which removes several layers of skin with a pencil-sized device, is commonly used for psoriasis. A newer method uses tape strips – plastic discs with an adhesive side to remove cells from the surface of the skin. Unlike biopsies, tape strips are minimally invasive, don’t leave a scar and are relatively painless. On the flip side, a dermatologist may need to perform the procedure 20 times or more to get a sample and it’s been hard to standardize the number of strips and amount of pressure needed. If your dermatologist recommends a skin biopsy, you might want to ask about tape strips, but chances are you’ll be steered to a punch biopsy.

Some people with the autoimmune disease systemic lupus erythematosus (SLE, lupus) may have a biopsy, either to diagnose cutaneous lupus, which only affects the skin, or to diagnose and guide treatment for a potentially fatal kidney complication called lupus nephritis. Half of adults – mainly women of color – and 80% of kids who have lupus develop lupus nephritis, usually within five years of their diagnosis.

Some of them, depending on many factors, may have a procedure called a percutaneous kidney biopsy. It involves guiding a thin needle through the skin into the kidney and removing a small bit of tissue with a spring-loaded instrument. Patients are awake during the biopsy, but the area where the needle is inserted is thoroughly numbed.

Although kidney biopsies are generally considered safe, they can cause complications. The most common is blood in the urine, which usually goes away in a day or two. Many people also have some pain at the biopsy site that lasts a few hours. Kidney biopsies are controversial, with some experts arguing that the benefits of an invasive procedure don’t outweigh the risks, especially for children.

New clinical guidelines from the nonprofit organization Kidney Disease Improving Global Outcomes (KDIGO) recommend a biopsy when a urine test shows more than 500 mg a day of protein in the urine – a sign of kidney damage. This is half the amount of urinary protein that older guidelines from the American College of Rheumatology — scheduled to be updated in 2025 — use as a kidney biopsy benchmark.