BLOOD SCIENCES
Plasma and serum viscosity ratio in diagnostic medicine
The relationship between plasma viscosity and serum viscosity can reveal much about the body’s response to infection and inflammation. David Norcliffe, David Manuel and Bernie Benson explore the clinical significance of this metric in diagnostic medicine.
Plasma viscosity (PV) and serum viscosity (SV) are well understood as rapid, cost- effective, non-specific tests. Despite their simplicity, they can demonstrate diagnostic capability when used to diagnose and monitor complex disorders that are difficult to detect, diagnose or quantify.1,2
Studies of blood and serum viscosity
date back to the middle of the 19th Century by the physician Poisueille3 but with the introduction of in vitro anticoagulation in the 1920s the emphasis moved towards plasma.4
It was measured
using an Ostwald device consisting of a glass U-tube, where the time taken for a
1-2 mL sample of fluid to travel from one arm to the other was measured, giving a reflection of viscosity. Today a fully automated capillary
viscometer will use only 70 µL of patient sample per test giving repeatable, reproducible and reliable results.5 The measurement of serum viscosity is not a new practice; it has been used in medical diagnostics for many years. Early haemorheology research concentrated mainly on serum viscosity rather than plasma.6
In recent years, however, plasma
viscosity has shown a greater potential to be utilised for a wider range of clinical conditions.
Plasma and serum preparation One of the critical differences between plasma and serum samples are the presence of cloting factors in plasma. Serum is obtained from a blood sample which has been collected into a plain non-anticoagulated tube, gently mixed and allowed to clot. After a minimum of 15 minutes the sample is then centrifuged and serum obtained.7
Hence serum
does not contain any cloting factors or platelets.
Plasma is obtained from blood
Plasma is obtained from blood collected into a tube containing an anticoagulant, gently mixed and then centrifuged. Unlike serum, the plasma contains all the cloting factors, which will marginally increase the viscosity.
collected into a tube containing an anticoagulant, gently mixed and then centrifuged. For rheological purposes any dry anticoagulant can be used but most commonly it is potassium EDTA (ethylenediaminetetraacetic acid),8
which
is also used for a full blood count. The EDTA works by removing calcium from the
April 2026
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