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HAEMORHEOLOGY


Plasma viscosity: accuracy, precision and suitability – who wants it?


The diagnostic and prognostic value of measuring plasma viscosity has been highlighted recently in patients suspected or confirmed as having COVID-19; however, its use in other areas of UK medical practice has gained clinical acceptance and importance over the past seven decades since its first description in the literature.


Since 1947, when John Harkness published his seminal work on clinical plasma viscosity (PV) analysis in the British Medical Journal, the PV test has been generally misunderstood and certainly underutilised. With the infection of humans by SARS-CoV-2 since late 2019, the pandemic has forced scientists to take a fresh and detailed look at infections and PV; not just PV per se, but its values, trends and the fact it can be used in the


calculation of a possible predictor of severity. The world may well now see that Professor Harkness was a man before his time, and posthumously he will receive due recognition.


In May 2020, a paper published in The Lancet by a team at the Emory Medical Centre in Atlanta, Georgia, showed that COVID-19 patients who developed severe symptoms had markedly raised PV levels.1 This raised the profile of the test among


medical and scientific professionals trying to cope with the new pandemic. During more than 70 years since


Harkness’s work, there have been many publications confirming the clinical value of using PV for investigating patients with an acute-phase response or with paraproteins. In early publications plasma viscosity was compared against the established erythrocyte sedimentation rate (ESR), and later against both ESR and C-reactive protein (CRP). The PV test received favourable reports in many of the papers.


Acute-phase response In the acute-phase response, patients produce increased levels of immunoglobulins, the blood clotting protein fibrinogen, CRP and various inflammatory cytokines. Plasma viscosity is determined by two factors, the concentration of proteins and the shape of the molecules.


Clinical capillary viscometers have continued to develop, and now clinical viscosity analysers are available with advanced health and safety features, increased throughput, improved reliability, and most importantly improved precision.


WWW.PATHOLOGYINPRACTICE.COM DECEMBER 2020


Antibodies and fibrinogen are found at high concentrations in human plasma. The shape of their molecules has a high length to width ratio, which is the shape that has the most effect in increasing viscosity. This means that viscosity measurements are extremely sensitive to increased levels of these proteins. As long ago as 1988, leading haematologists recognised the value of the PV test for monitoring the acute- phase response. Published under the auspices of the International Committee for Standards in Haematology, they published a guidance paper on methods for investigating acute-phase response and stated that: “Measurement of plasma viscosity has several advantages over the ESR, including independence from the effects of anaemia; a single reference range that is independent of sex and less dependent on age; instruments that


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