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plasma, thus preventing cloting. Unlike serum, the plasma contains all the cloting factors – including fibrinogen – which will marginally increase the viscosity of plasma. Liquid anticoagulants, such as sodium citrate should not be used due to their dilutional effect.


Viscosity differences The relative components of plasma and serum account for their difference in viscosities. At 37°C it has been reported that plasma viscosity is about 1.8 times greater than that of water due to its plasma proteins,9 1.6 times greater.10


and serum is about The difference is


primarily due to the proteins, enzymes and substrates involved in cloting, with fibrinogen accounting for approximately 22% of the effect on plasma viscosity.11


It


has been reported that serum viscosity is about 93% that of plasma viscosity.12 Fibrinogen has a very significant effect on the viscosity of plasma, due to its high molecular weight and asymmetry with a length to diameter ratio of approximately 18:1 thus offering the resistance to flow.


Plasma viscosity


The viscosity of plasma changes in proportion to the quantity and molecular size of plasma proteins, particularly fibrinogen, immunoglobulins and paraproteins when present.14


These will


increase as part of the body’s normal response to infection, inflammation,15


and


due to the effects of traumatic injury. The plasma viscosity (PV) test result


can provide critical information which can be utilised by medical professionals to aid diagnosing and monitoring complex clinical conditions14


such as:


polymyalgia rheumatica multiple myeloma temporal arteritis Waldenström’s macroglobulinaemia hyperviscosity syndrome diabetes COVID-19 sepsis


meningitis cardiovascular disease rheumatoid arthritis vascular dementia and Alzheimer’s disease.


The PV test has advantages over the other inflammatory markers, C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR) in terms of stability, direct measurement, and minimal external factor interference. This makes it an excellent choice in detection of elevated proteins, and also abnormal paraproteins in many clinical scenarios. Plasma viscosity samples are tested and reported at 37°C, but can also be


reported at 25°C (room temperature). The results are presented in a clear, easy to interpret numerical format, displayed at two decimal places. The unit of clinical viscosity


measurement most commonly reported is the SI unit of milliPascal-second (mPa.s). The previous non-SI viscosity unit centipoise (cP) has now been largely superseded by mPa.s. Interestingly, the values are interchangeable. Hence 1.60 mPa.s is equivalent to 1.60 cP. Plasma viscosity normal reference


range:16 1.12 - 1.27 mPa.s, tested and reported at 37°C.


1.50 - 1.72 mPa.s, tested at 37°C reported at 25°C.


Serum viscosity


The viscosity of serum is dependent on the size and concentration of blood proteins, paraproteins and immunoglobulins present in a patient sample. These


18 WWW.PATHOLOGYINPRACTICE.COM April 2026


Plasma viscosity was first measured using an Ostwald device.


protein changes will increase in the body as a natural reaction to infection and inflammation. This mechanism is an automatic body response and is affected proportionally by the severity of the patient’s clinical condition. The presence of a large concentration monoclonal proteins can be


of IgM17


the most common cause of serum hyperviscosity and it is commonly observed in conditions such as Waldenström macroglobulinaemia18 and multiple myeloma. The tendency for these monoclonal proteins to cause hyperviscosity is determined by the concentration, molecular weight, and aggregation of the proteins. Serum viscosity (SV) testing is routinely


used by medical professionals to support clinical monitoring and to identify complex clinical conditions19


that are


difficult to diagnoses, such as: multiple myeloma Waldenström’s macroglobulinaemia hyperviscosity syndrome rheumatoid arthritis.


Serum viscosity testing has been found to be useful in autoimmune disorders, connective tissue diseases and in hyperviscosity syndromes due to the abnormal presence of paraproteins (IgM, IgG and IgA). The SV test offers significant


advantages over other inflammatory markers in terms of stability, direct measurement, and minimal interference from external factors. These features make it an excellent tool for detecting elevated protein levels and abnormal paraproteins across a range of clinical contexts. Serum viscosity samples are tested and


reported at 37°C, but can also be reported at 25°C, room temperature. The results are presented in a clear, easy to interpret numerical format, displayed and reported at two decimal places. Since the SV result is reported in a


numerical format, regardless of the unit, whether it’s 1.20 mPa.s or 1.20 cP, both milliPascal-second and centipoise are universally recognised, accepted, and easily understood. Serum viscosity typical normal


reference range:13 1.09 - 1.23 mPa.s, tested and reported at 37°C.


1.40 - 1.60 mPa.s, tested at 37°C reported at 25°C.


The PV:SV ratio Plasma and serum viscosity ratio refers to the relationship between the viscosity of plasma and that of serum from the same patient. The normal plasma viscosity is slightly higher than serum viscosity. In


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