About the Test
Purpose of the test
The purpose of a synovial fluid analysis is to evaluate the cause of joint pain and inflammation. A synovial fluid analysis can help doctors narrow down or diagnose one of the many potential causes of joint pain. A synovial fluid analysis may be ordered to diagnose or rule out the following conditions:
- Autoimmune disorders Autoimmune disorders are conditions in which a person’s immune system mistakenly attacks healthy tissue. In some autoimmune disorders, including rheumatoid arthritis, lupus, and psoriatic arthritis, the immune system damages healthy tissues of the joints.
- Septic arthritis: Septic arthritis is an infection caused by microbes such as bacteria or fungi that lead to acute inflammation in one or more joints.
- Gout: or pseudogout: Gout and pseudogout are painful forms of arthritis, caused by the accumulation of uric acid or calcium crystals in the joint.
- Bleeding in the joint: Also called hemarthrosis, an accumulation of blood in the joint can be caused by an injury to the joint, cancer, hemophilia, and other health conditions.
What does the test measure?
A synovial fluid analysis can consist of several tests which may be performed on the fluid extracted from the joint. Broadly, the tests conducted as part of the synovial fluid analysis fall into three categories: gross assessment, chemical analysis, and microscopic assessment.
A gross assessment of synovial fluid describes the process in which a doctor examines the synovial fluid for characteristics such as volume, clarity, color, and viscosity. A gross assessment can help doctors begin to understand the cause of a patient’s symptoms.
The chemical analysis of synovial fluid involves testing a patient’s synovial fluid for its chemical composition. Chemical assessment of synovial fluid often includes the following tests:
|Chemical Analysis of Synovial Fluid|
|Test Name||What It Measures||Why|
|Glucose Testing||The amount of glucose||Glucose levels are lower in certain conditions, including autoimmune disorders or septic arthritis.|
|Lactate||The amount of lactate||High levels of lactate are associated with infection.|
|Total Protein||The amount of protein||Elevated protein concentration can indicate an inflammatory or bleeding disorder.|
|Uric Acid Testing||The amount of uric acid||The presence of uric acid indicates gout.|
A microscopic assessment of synovial fluid includes several tests to further evaluate a patient’s symptoms. During a microscopic assessment, the synovial fluid is examined using a microscope with a special filter to detect the presence of crystals, called a crystal analysis. If crystals are detected in the synovial fluid, their structure and composition can provide additional support for certain diagnoses.
Other tests performed as part of a microscopic assessment vary depending on the suspected cause of a patient’s symptoms. The microscopic assessment of synovial fluid routinely includes the following tests:
|Microscopic Assessment of Synovial Fluid|
|Test Name||What It Measures||Why|
|White Blood Cell (WBC) Count and Differential||Presence and type of WBCs||WBC levels become elevated in response to infections, septic arthritis, gout, and pseudogout.|
|Gram Stain||Presence and type of microbes||Positive Gram stains can help diagnose septic arthritis and infection.|
|Bacterial Culture||Presence and type of microbes||The presence of certain microbes may indicate infection.|
When should I get synovial fluid analysis?
A synovial fluid analysis may be ordered by your doctor in a number of situations where the cause of joint problems is unknown. The following symptoms may indicate the need for a synovial fluid analysis:
- Joint swelling, often referred to as joint effusion
- Sudden pain and warmth in a joint
- Redness at a joint
- Difficulty bending a joint
- Joint discomfort accompanied by fever
Joint fluid analysis is not performed as often in children with joint swelling and pain unless they have symptoms that suggest a joint infection, such as fever and the appearance of being very ill.
Your doctor can tell you how synovial fluid analysis can confirm or rule out diseases that may be causing your symptoms. They can also explain why the benefits of testing outweigh the small risks involved with the procedure.
Finding a Synovial Fluid Analysis
How to get tested
A synovial fluid analysis is ordered by a doctor. The test may be ordered by your primary care provider, a specialist such as a rheumatologist, who specializes in treating joint diseases, or an orthopedist or orthopedic surgeon, who specializes in bone and connective tissue disorders.
A synovial fluid analysis is conducted on joint fluid, which is extracted from the joint through a procedure called arthrocentesis. Arthrocentesis is typically performed in the office of the provider who orders the test, or in another medical setting, like a hospital. Arthrocentesis is administered by a medical professional who has been trained in the procedure, such as a physician, physician’s assistant, or nurse practitioner.
Can I take the test at home?
A synovial fluid analysis cannot be conducted at home. The process by which joint fluid is removed, arthrocentesis, is only performed in a professional medical setting.
How much does the test cost?
When a doctor orders arthrocentesis and joint fluid analysis, your insurance will typically cover the procedure and lab costs. The cost of arthrocentesis varies depending on the joint’s size and the complexity of the procedure. Laboratory costs depend on how many and what type of tests are performed on your joint fluid.
You may have to pay a deductible or a copay. Your doctor, lab, and health plan can provide more information about your out-of-pocket costs. It’s also a good idea to ask whether it is necessary to use an in-network physician and lab.
If you do not have medical insurance, you can ask the doctor’s office and lab about costs and whether payment arrangements or cash discounts are possible.
Taking a Synovial Fluid Analysis
The process of obtaining a synovial fluid sample, arthrocentesis, involves inserting a needle into the joint and withdrawing some of the liquid from the joint space. Arthrocentesis takes place in a hospital, doctor’s office, or other medical setting. If the patient has an artificial joint, testing is usually done in the office of an orthopedic surgeon.
Before the test
Typically, no special preparation for arthrocentesis is needed. Some patients may be instructed to avoid eating or drinking for several hours before the test.
Before arthrocentesis, tell your doctor about all medications you’re taking. Certain medications, including blood thinning medications like aspirin, warfarin (Coumadin) or clopidogrel (Plavix), may affect your ability to take the test as well as the test results. If you notice a rash or skin infection in the area of the joint prior to being tested, be sure to point it out to the medical team as well.
Arthrocentesis is done using a sterile technique, which involves taking certain steps to prevent the spread of germs. The team that performs the procedure will clean your skin with antiseptics and gather all the equipment and supplies needed before beginning. Often, patients will be given a local anesthetic to numb the area before the needle is inserted. Children may be given a sedative, which is a medication to make them feel relaxed and calm.
During the test
During arthrocentesis, you will be lying down or sitting in a position that makes it easy for the health care provider to access your joint. A needle attached to a sterile syringe is then carefully inserted into the joint space.
In some cases, the arthrocentesis may be accompanied by imaging, such as an ultrasound or fluoroscopy. These imaging techniques can help guide the provider to more easily access joints that are deep or difficult to reach, such as the hip or shoulder joint.
Once the needle is properly positioned, a small amount of joint fluid will be extracted into the syringe.
Arthrocentesis may cause some discomfort. Although rare, the procedure has some risks associated with it, such as fainting, bleeding into the joint, or infection. Arthrocentesis usually takes less than two minutes.
After the test
After the joint fluid sample has been extracted, you should not immediately stand up. You will be advised to rest for a few moments.
You will probably be able to resume your usual activities shortly after the procedure. Ask your doctor if you should take any special precautions following the arthrocentesis. In some cases, limited activity or joint immobilization is recommended after the test.
If you are experiencing discomfort after arthrocentesis, your doctor may recommend applying ice or cold packs to the area. It may also be helpful to elevate the joint, if possible. Doctors may also recommend the use of anti-inflammatory medications to help with pain. In some cases, the joint may be wrapped in an elastic bandage after arthrocentesis to reduce pain and swelling.
Complications are rare after arthrocentesis, but it is important to watch out for signs of infection. If any of the following symptoms occur after the procedure, contact your doctor:
- Increased redness
- Worsening swelling
- More intense pain
Synovial Fluid Analysis Results
Receiving test results
Because synovial fluid analysis involves multiple tests, you may get different results at different times, or your doctor may wait to share results until all tests are complete. Your doctor may be able to share the results of a gross assessment right away, as the gross assessment is conducted at a patient’s bedside.
Your test results may be reported to you by your doctor, the laboratory, or through an online portal.
Interpreting test results
Understanding the results of a synovial fluid analysis involves interpreting the findings of several tests. The results of a gross assessment of synovial fluid describe the appearance of the fluid extracted from your joint. This visual inspection of the synovial fluid often allows your joint swelling to be categorized as non-inflammatory, inflammatory, infectious, or related to bleeding in the joint.
The following table provides information about the results of a gross assessment of synovial fluid and their possible significance:
|Gross Assessment of Synovial Fluid|
|Quality Analyzed||Normal Result||Abnormal Result||Possible Causes of Abnormal Result|
|Clarity||Clear||Cloudy||Presence of blood, crystals, or inflammation|
|Color||Colorless||Yellow, yellowish-green, pink, red, or rusty||Inflammation, infection, or bleeding in the joint|
|Viscosity or thickness||Thick fluid that forms a string when stretched||Thin, watery fluid||Inflammation|
The results of a chemical analysis of synovial fluid are compared to the results of blood testing. Normally, the composition of synovial fluid is similar to the composition of a person’s plasma, the liquid component of blood. Certain differences between the plasma and synovial fluid can indicate the presence of disease:
|Chemical Analysis of Synovial Fluid|
|Test||Normal Result||Abnormal Result||Possible Causes of Abnormal Result|
|Glucose Testing||Similar to blood glucose||Lower than blood glucose by more than 10 mg/dL||Inflammation, infectious conditions|
|Lactate||Similar to blood lactate||Higher than 5-10 mmol/L||Bacterial infection|
|Total Protein||About one-third lower than the protein level of blood||Higher than 3 g/dL||Inflammation, bleeding|
|Uric Acid Testing||Similar to blood level||Significantly greater than blood level||Gout|
Finding evidence of crystals and the nature of crystals in the joint fluid during a crystal analysis can support the diagnosis of gout and pseudogout. Monosodium urate (MSU) or uric acid crystals are indicative of gout, while calcium pyrophosphate dihydrate (CPPD) crystals support a diagnosis of pseudogout. Other types of crystals are sometimes also found during a crystal analysis.
The microscopic content of synovial fluid provides evidence of different diseases and conditions. The following table provides information about the results of routine tests included in a microscopic analysis of synovial fluid:
|Test||Normal Result||Abnormal Result||Possible Causes of Abnormal Result|
|White Blood Cell Count and Differential||Small number of white blood cells, normal proportion of polymorphonuclear (PMN) cells||High number of white blood cells, elevated PMN and/or eosinophils||Infection, septic arthritis, Lyme disease, parasites|
|Gram Stain||Negative or normal||Bacteria or fungus detected||Presence of bacterial or fungal infection|
|Bacterial Culture||Bacteria or fungus not detected||Bacteria or fungus detected||Type of bacterial or fungal infection|
Questions for your doctor about test results
- What are the results of my synovial fluid analysis?
- Are you able to make a diagnosis based on my synovial fluid analysis results?
- Have any diseases been ruled out through these results?
- Are any test results still pending?
- Will I be able to begin treatment right away, or is additional testing necessary?
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