Plasma viscosity measurement can be used to assess the viscosity, or the ‘thickness’ of the blood. The viscosity of plasma (the liquid part of the blood) is affected by the presence of proteins that can be produced in normal response to infection or inflammation, or proteins that are produced abnormally in certain diseases (paraproteins). Measuring the viscosity of blood may be used to indirectly detect and monitor inflammation and can aid in the diagnosis and monitoring of many conditions.
Plasma viscosity is a specialised test that is usually only available in larger, specialist laboratories. If your doctor thinks that you might have a condition that causes inflammation they may use this test to help diagnose and follow the course of this condition, especially temporal arteritis or polymyalgia rheumatica. Production of paraproteins is associated with some disease states, including Waldenström macroglobulinaemia. Production of high levels of paraproteins may lead to a condition called ‘hyperviscosity syndrome’. Measurement of plasma viscosity maybe used to help diagnose this condition along with clinical presentation and fundoscopic examination to assess the retinal veins.
A blood sample taken from a vein in the arm.
No test preparation is needed.
Plasma viscosity measurement is performed by analysing the force needed to send a fixed amount of plasma flowing along a thin tube in a given time, at a standard temperature. Typically, distilled water at 20oC will have a viscosity of 1.00mPas (millipascal-second), whereas normal plasma at 37oC may have a viscosity of around 1.3-1.7mPas . The higher the result, the more viscous (or “thicker”) the blood is. Plasma viscosity can be used as an indirect measure of the amount of protein present in the plasma (liquid) part of the blood. It can reflect the degree of inflammation present in the body. It can also detect the presence of abnormal paraproteins which can be produced by benign or malignant tumours. Increased blood levels of certain proteins, such as fibrinogen (which is increased in inflammation) or immunoglobulins (which are increased in inflammation or secreted by some tumours) cause the plasma viscosity to rise.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in your arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
Plasma viscosity is used to monitor the same conditions as the ESR test. Although it can be considered a technically superior test to the ESR, it is more difficult to perform and is not widely used. It can be used to monitor acute and chronic (that is, short or long duration) inflammation, due to infections, cancers, and autoimmune diseases. Plasma viscosity is said to be non-specific because increases do not tell the doctor exactly whether the cause of hyperviscosity is due to a paraprotein or inflammation; nor where inflammation is occurring in your body or what is causing it. For this reason, an abnormal plasma viscosity is typically followed by other tests to investigate the specific cause of the abnormal result.
Plasma viscosity (PV) is helpful in diagnosing specific inflammatory diseases, temporal arteritis, giant cell arteritis and polymyalgia rheumatica. A high PV is one of the main test results used to confirm the diagnosis; PV remains high in those patients who have started anti-inflammatory treatment, while the ESR result could be reduced to normal by this. Plasma viscosity measurement may also be used to monitor disease activity and response to therapy in both of these diseases.
When is it requested?
A healthcare professional usually requests a PV test (along with other tests) to evaluate a patient who has symptoms that suggest polymyalgia rheumatica, giant cell arteritis or temporal arteritis, such as headaches, neck or shoulder pain, pelvic pain, anaemia, unexplained weight loss, and joint stiffness. There are many other conditions that can result in a temporary or sustained elevation in PV.
Since the PV is a non-specific marker of inflammation and is affected by other factors, the results must be used with the doctor’s other clinical findings, the patient’s health history, and results from other appropriate laboratory tests. If the PV and clinical findings match, the doctor may be able to confirm or rule out a suspected diagnosis. A single elevated PV, without any symptoms of a specific disease, will usually not give the doctor enough information to make a medical decision.
Before doing an extensive investigation looking for disease, a doctor may want to repeat the PV test after a period of several weeks or months. If a doctor already knows the patient has a disease like temporal arteritis (where changes in the PV mirror those in the disease process), they may use the PV at regular intervals to assist in monitoring the course of the disease.
What does the test result mean?
Medical decisions will not be made solely on the results of a plasma viscosity measurement, normal results may not exclude a clinical condition and an abnormally high result can be found in association with many causes.
A significantly high PV usually has an obvious cause, such as an infection. Further follow-up tests, such as microbiological cultures, may be requested depending on the patient’s symptoms.
Extremely high PVs are unusual in infections or inflammation and may point more specifically to the presence of paraproteins associated with myeloma or lymphoma. Again, further tests will be performed before arriving at a diagnosis.
Moderately elevated PV occurs with inflammation, but also with smoking.
A rising PV can mean an increase in inflammation or a poor response to a therapy; a decreasing PV can mean a good response.
Unlike the ESR, the plasma viscosity is not raised in anaemia or decreased with polycythaemia (too many red cells). It increases slightly with age but less so than the ESR. The ESR may occasionally decrease again with very high levels of some paraproteins, leading to misleading results, while PVs accurately reflect protein levels.
Is there anything else I should know?
Plasma viscosity, ESR and C-reactive protein (CRP) are all markers of inflammation. Generally, PV and ESR do not change as rapidly as does CRP. CRP is not affected by as many other factors as the PV or ESR, making it a better marker of some types of inflammation. PV, however, is more sensitive and more specific than either ESR or CRP when monitoring the activity of rheumatoid arthritis.
Because PV is technically more challenging, many NHS laboratories only offer ESR and CRP. Smokers have slightly higher PV than non-smokers.
What other tests might my doctor be requesting besides plasma viscosity?
Your doctor may request a CRP test as well as other biochemistry tests, FBC and blood film, at the same time they request plasma viscosity. They may also request additional tests based on your symptoms, such as the ANA (antinuclear antibody) and RF (rheumatoid factor) tests suggestive of autoimmune diseases, or cultures to investigate infection. Serum electrophoresis can look for evidence of diseases such as multiple myeloma and lymphoma.
What do changes in my plasma viscosity mean?
Plasma viscosity results give an indication of the amount of protein in the blood. The amount of protein in the blood can become elevated for many reasons and in most cases, a raised plasma viscosity will decrease over time once the underlying inflammation or other cause is addressed. If you have a chronic (longstanding) inflammatory disease, plasma viscosity may fluctuate with the degree of activity your condition.