Plasma Viscosity

Plasma viscosity measures the resistance of plasma to flow and depends mainly on fibrinogen, immunoglobulins and, to a lesser degree, albumin. Unlike the ESR it is independent of haematocrit, red cell shape, age and sex, and is not affected by anaemia.

It is used as an acute phase marker and, importantly, to assess suspected hyperviscosity in paraproteinaemia. It is used mainly in the United Kingdom, and many laboratories rely on CRP and ESR instead.

Specimen. EDTA or citrate blood, centrifuged, with the plasma tested in a capillary or cone-and-plate viscometer. Test within 24 hours.

Reference range. About 1.50 to 1.72 mPa.s at 25 °C. The range varies with the instrument and the temperature of measurement, so use the local laboratory range.

Clinical thresholds. Raised values correlate with disease activity. Hyperviscosity symptoms in paraproteinaemia typically arise when serum viscosity relative to water exceeds about 4, though this varies between patients.

Raised.

• Acute phase response from infection, inflammation, malignancy and tissue injury.
• Paraproteinaemia: Waldenström macroglobulinaemia (IgM), and multiple myeloma (especially IgA and IgG3).
• Giant cell arteritis and polymyalgia rheumatica.
• Connective tissue disease and vasculitis.
• Hypergammaglobulinaemia and pregnancy.

Lowered. Hypoproteinaemia and severe liver disease. A falling value over time reflects treatment response in inflammatory disease.

Critical findings needing immediate action. Headache, visual disturbance, mucosal bleeding, confusion or heart failure with a high viscosity in a patient with paraproteinaemia indicates hyperviscosity syndrome and needs urgent specialist management.

Pitfalls.

• Marked lipaemia raises the result.
• Haemolysed, clotted or fibrin containing samples give inaccurate values.
• Temperature differences between laboratories prevent direct comparison.
• It is a non-specific marker and cannot localise disease.

Special groups. Pregnancy raises values modestly. Children have a range similar to adults.

Follow up tests. Serum protein electrophoresis, immunofixation, serum free light chains, C-reactive protein, erythrocyte sedimentation rate, immunoglobulin levels.