BLOOD SCIENCES
author has found that the accuracy of the dilution with trisodium citrate was of particular importance. Automated ESR analysers, which considerably shorten the duration of testing, will only provide an approximation of the results obtained by a traditional Westergren test. The ICSH indicated in a report21
that
while the Westergren method remains the gold standard, the results produced by new instruments may vary from those obtained using the Westergren method by as much as 142%. Furthermore, the ICSH report highlighted that various non-Westergren methods exhibited discrepancies among themselves of up to 42%.
Plasma viscosity Plasma viscosity is renowned as a quick, non-specific test, which is accurate, reliable, repeatable, is very useful for screening and eminently suitable for condition monitoring. The viscosity (a fluid’s resistance to flow) of plasma is determined by size of the protein, its concentration and importantly, molecular symmetry. Fibrinogen has a very significant effect on the viscosity of plasma. This is due to fibrinogen having a length to diameter ratio of approximately 18:1.22
So, elevation in the total levels
of acute-phase proteins and fibrinogen contributes to an increase in plasma viscosity. Therefore, the viscosity of plasma is significantly influenced by changes in the size and the levels of protein and importantly the volumes of fibrinogen molecules, which occur as a consequence of infection, inflammation, and physical trauma.
In 1963 he designed a semi- automated capillary viscometer for PV measurement which was later developed, manufactured and marketed as the Coulter Harkness Plasma Viscometer. This instrument eventually (1972) then became the gold standard for clinical plasma viscosity measurement. The analyser test temperature was chosen to be 25°C, room temperature. It was considered easier and more economical to manufacture the viscometer testing and reporting at 25°C rather than the more obvious and physiological human body temperature of 37°C. Reporting at 25°C is still widely used today, though modern automated clinical viscometers will test precisely at 37°C but can report at a choice of 37°C or 25°C depending on the user’s preference.
Plasma viscosity measurement in clinical practice has been used since 1946. Following studies referencing the mechanism of the ESR, John Harkness published what was to be a seminal paper.23
16
Note the slight streaming effect in sample 4 which is often
greater than this
Illustration of ESR tests, reflecting the tendency of red blood cells to settle more rapidly in the face of some disease states.20
The PV measurement is most accurately achieved using a capillary viscometer. The theory is quite simple, thin, low-viscosity fluids will travel faster through a capillary than thick high- viscosity fluids. The time taken for an aliquot of plasma (under the influence of air pressure) to travel through a calibrated capillary is detected by two measured infra-red optic points and recorded. The recorded viscometer ‘run time’, is used by an algorithm which will determine the viscosity of the fluid. A modern automated clinical viscometer aspirates only 50µL of sample therefore, adult and paediatric repeat tests are feasible. The PV test can be carried out using the residue from the full blood count tube, eliminating the need for additional blood samples. Another method that has been used historically to measure plasma viscosity is the cone and plate viscometer, which measures the resistance (drag) in an aliquot of plasma when a cone or plate is rotated in the fluid. However, this method is far less sensitive than the capillary method. The cone and plate instrument required a much larger sample volume, a greater assay time and manual pipetting. Another early viscosity test method once used for determining PV was the falling ball viscometer. This viscometer is often used in industry, and it determines the viscosity by measuring time for a uniform ball to fall through a vertical column of liquid. Both these methods are now generally considered inappropriate in a modern clinical laboratory environment. The viscosity of plasma within the human body will automatically increase as part of the normal response to infection,
inflammation, and due to the effects of traumatic physical injury. A PV will give an accurate reflection of the intensity of the patient’s condition at the time of test. The plasma viscosity test is cost effective, quick and available in many haematological laboratories. The PV test has the capability to diagnose and monitor many clinical conditions. It has been exclusively used in the diagnosis and detection of Waldenström’s macroglobulinaemia and hyperviscosity syndrome. It exhibits significantly greater versatility in behaviour across various other disease states.
Hence plasma viscosity is considered to have a dramatic diagnostic impact leading to the most appropriate treatment for conditions such as, multiple myeloma, polymyalgia rheumatica, rheumatoid arthritis, temporal arteritis, cardiovascular disease, COVID-19, Waldenstrőm’s macroglobulinaemia, hyperviscosity syndrome, diabetes, sepsis, meningitis, vascular dementia and Alzheimer’s disease. Most notably in a 2021 paper, by Gleghorn et al24 who reported an increase in viscosity and a predictive value of greater than 90% of patients with SARS COVID-19 developing complications and requiring hospitalisation. Further interesting, published papers indicate that PV can be used to diagnose and differentiate between vascular dementia and Alzheimer’s disease.25
It has been long
established that PV can be used to great advantage as a predictor of cardiovascular diseases.26
The PV normal reference range for individuals aged three years and above
MAY 2025
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