can be calibrated; convenience of quality control checks; and results that are available within 15 minutes.2 Some laboratories perform the PV test

in preference to the ESR, and often clinicians will be advised to use the PV test in preference.3

Plasma viscosity is no longer just a non-diagnostic support test and an alternative to ESR. It is much more than that and is probably a better test to reflect plasma protein changes in inflammatory conditions such as polymyalgia rheumatica. Feedback from laboratories suggest many benefits to PV testing, not just to patients, but also clinicians and from a laboratory management perspective, for example: n It is a cost-effective test and efficient to perform.

n Automatable, and can be performed on existing EDTA samples as it uses a small sample volume.

n Safer for high-risk samples such as COVID-19 as it can be performed on sealed tubes.

n Easy to perform and tests are completed in minutes with results reported to any LIMS.

n Modern clinical viscometers have the capability for serum viscosity (SV) to be performed without any changes to equipment or additional reagents.

n Clinical viscosity is a known measure of inflammatory response with the potential to be a measure of recovery and/or antibody response, especially if combined PV/SV is performed, as this will exclude the increased acute- phase response of fibrinogen.

n Clinical viscosity results become abnormal earlier in disease than ESR and are not affected by haematocrit.

n Plasma viscosity has a much lower instance of false normal values.

n Performing a PV test has practical advantages for a laboratory as it can be carried out on a sample up to seven days after blood being drawn, compared to an ESR which has to be processed within four hours. This makes PV ideal for rural areas or when the test is outsourced.

n Relatively small changes in PV are clinically significant for any individual.

n Clinical PV testing is technically reproducible and standardised.

n Plasma viscosity is more informative and stable than ESR. It has a definitive numerical value that can easily be referenced against condition charts.

It is expected that the PV value will be increased in any infection, but if clinicians look beyond that they can


True load-up and walkaway analysers now sample from a sealed cap on primary tubes to avoid potential biohazards, and incorporate software to perform auto-calibration and quality control checks between every few samples to monitor for drift.

follow trends through daily monitoring that will correlate with severity. Clinical viscosity can be used to calculate the rigidity (flexibility) of red cells in a way not practised previously. This has enormous potential for patient benefit. With the addition of SV, it adds another dimension of possibility. The PV:SV ratio could prove

remarkably interesting clinically. There remains the question of why some Black, Asian and minority ethnic (BAME) patients are more susceptible than their Caucasian (white European ancestry communities) equivalents. Can this be detected by PV, SV, or a combination of the two? One condition in which early diagnosis is vital is giant cell arteritis. A paper published by Gudmundsson in 1993 stated that PV was better than both the ESR and CRP for predicting flare ups in the condition.4

In 2011 Finke went even further, stating in his study that it appeared that the PV test measured a more specific inflammatory marker for giant cell arteritis than either CRP or ESR, and in patients outside the criteria suggested by the American College of Rheumatology… “plasma viscosity may significantly contribute to a reliable diagnosis early in the course of the disease”.5

Paraproteins Conditions that produce paraproteins include the myelomas and lymphomas. Paraproteins are immunoglobulins produced in abnormally large quantities by malignancies of the lymphoid B-cell lineage. Over the past 50 years in the UK, plasma viscosity has been, and still can be, used to screen for and monitor the paraproteinaemias.

Early diagnosis of myeloma is important as the malignancy is easier to treat the earlier it is diagnosed. However, early symptoms are not specific, and this rare condition is difficult to diagnose in primary practice. A paper published in the British Journal of General Practice in 2018 showed that in conjunction with a normal haemoglobin concentration, a normal PV level could be used to rule out the disease.6

Researchers also said

that GPs should use simple blood tests to support symptoms when diagnosing patients with multiple myeloma, in order to improve early detection rates, and that plasma viscosity was one of the best inflammatory markers to supplement symptoms.

As far back as 1974, four cases of IgA multiple myeloma with raised SV or PV and clinical features of hyperviscosity syndrome were reviewed. It was felt

Over the past seven decades, many publications have confirmed the clinical value of using PV for investigating patients with an acute-phase response or with paraproteins


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