Plasma viscosity (PV) measures the viscosity, or the ‘thickness’ of the plasma (the liquid part of the blood). Plasma viscosity is affected by the amount of proteins in the blood. Protein levels in the blood can increase as part of the normal response to infection or inflammation. Plasma viscosity also increases in certain diseases where proteins (called ‘paraproteins’) are produced abnormally. Measuring the viscosity of blood may be used to detect and monitor inflammation. Plasma viscosity is therefore sometimes called an ‘inflammatory marker’ test. Other tests which measure inflammation include ESR and C-reactive protein.
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 monitor this condition, especially temporal arteritis or polymyalgia rheumatica. 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 examination of the eyes (‘fundoscopy’).
A blood sample taken from a vein in the arm.
No test preparation is needed.
Plasma viscosity is measured by calculating the force needed to send plasma down a thin tube in a given time. Normal plasma viscosity is around 1.3-1.7mPas (millipascal second). The higher the result, the more viscous (or “thicker”) the blood is. Increased levels of protein in the plasma (liquid) part of the blood make the blood thicker. Plasma viscosity can be used as an indirect measure of the amount of inflammation in the body. It can also detect the presence of abnormal paraproteins which can be produced by certain types of 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 it used?
Plasma viscosity is used to monitor the same conditions as the ESR test. Plasma viscosity has some advantages compared to ESR, but it is more difficult to perform and is not widely used. It can be used to monitor inflammation, due to infections, cancers, and autoimmune diseases. Plasma viscosity is a non-specific test, it does not tell the doctor what is causing the inflammation: it may be as simple as an infection or as serious as a cancer. 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) when a patient 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 cause a rise in PV.
Since the PV is a non-specific marker of inflammation, 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. Sometimes doctors never find a cause for the inflammation and the results go back to normal on their own.
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 suggest 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.