Symptomatic Threshold Of Viscosity

Serum viscosity is often measured in centipoises (cp), which is a measure of viscosity relative to water. The normal range of serum viscosity is between 1.2 and 1.8 cp. Patients with nonmalignant inflammatory disorders can display elevated serum viscosities above 1.8 cp, but generally are under the symptomatic threshold leading to HVS. Fahey et al.16 first reported on the "symptomatic threshold" of viscosity in 1965 in patients with monoclonal gammopathies. While acknowledging in their studies that the serum viscosity at which symptoms occurred varied between patients, no patient exhibited symptoms at a serum viscosity of less than 4 cp. Similarly, Crawford et al. reported in a series of 126 patients with monoclonal gammopathies that patients usually became symptomatic at serum viscosity levels of >4 cp, whereas, none of the patients in this series who had a serum viscosity of <3 cp exhibited HVS.17 In a study of 56 patients with malignant paraproteinemias, retinopathy was always associated with serum viscosities >3.8 cp, though in some patients with serum viscosity levels between 2 and 3 cp, bleeding and neurologic symptoms were observed.18 Numerous additional studies have also confirmed the appearance of symptoms at serum viscosity levels of 4-5 cp.19-22

While many investigators have focused on the use of serum viscosity levels to evaluate HVS, others have emphasized the use of whole blood viscosity wherein both immunoglobulin and hematocrit levels are taken into account.23 However, in follow-up studies, MacKenzie et al. demonstrated that serum viscosity levels were as reliable as whole blood viscosity levels in identifying HVS in patients with monoclonal gam-mopathies.24 This is probably due to the high concentration of immunoglobulins present in serum. However, HVS was reported in a patient with low immunoglobu-lin levels, in whom a high whole blood viscosity level was detected, which likely was due to extensive red cell paraprotein interactions. It therefore appears that evaluation of the whole blood viscosity may reveal in certain individuals the presence of HVS.

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