Sample viscosity as a source of analytical error: The role of protein structure and mass concentration

Dear Editor,

I read with interest the case report highlighting the laboratory’s pivotal role in leading to a diagnosis of Waldenstrom’s macroglobulinemia (WM). ¹ Laboratory staff may infrequently encounter samples that cause analytical difficulties owing to raised sample viscosity due to the concentration or type of plasma proteins as well as very high cell counts that may be encountered with clonal haematological disease. In the case report, the persistent ‘clotting’ flags on a chemistry automated analyser were identified as aspiration errors due to cryoprotein precipitate which was characterized as a type 1 cryoglobulin (IgM kappa) of 27 g/L which was associated with a markedly high serum viscosity.

I report a similar case however with some dissimilarity. A 79-year-old male patient with relapsed lymphoplasmacytic lymphoma (WM) presented to the haematology day ward for treatment with venetoclax. Biochemical analysis of his serum samples was not possible due to repeated aspiration errors, and further investigation showed a markedly raised paraprotein (IgM kappa isotype) of 74.8 g/L. Investigation for a possible cryoprotein observant to local preanalytical practices to avoid protein loss was negative. Clinically hyperviscosity syndrome (HVS) was suspected as the patient became lethargic and confused, prompting urgent plasmapheresis which reduced his paraprotein levels to 26.9 g/L. Serum viscosity was not formally measured as sample volume was insufficient for analysis. No further analytical flags were recorded subsequently and clinically the patient was alert and back to his usual self.

Sample viscosity should be considered as a source of analytical error when analyser-generated flags indicate aspiration errors in the sampling system. Other analytical errors leading to spurious results in the context of WM should also be considered in the interpretation of results since cases of pseudohyponatraemia and pseudohypercalcaemia have been reported in the literature.^2,3 The source of sample viscosity may not necessarily be due to the temperature-dependent gelling phenomenon that occurs in both type 1 and type 2 cryoglobulinaemia. Raised immunoglobulins should be considered as the source of clinically significant hyperviscosity, the likelihood of which is related to the specific isotype and its serum concentration. Pentameric IgM (970 kDa) has a high axial length-to-width ratio which raises plasma viscosity at levels far below those of other plasma proteins. Significant hyperviscosity may be observed with IgM levels as low as 30 g/L, and IgM levels of 60 g/L or higher are associated with rapid development of hyperviscosity, with a median time to symptomatic HVS of 3 months.^4,5 Dimeric IgA (320 kDa) is associated with increased viscosity at levels of 60 g/L or greater. ⁵ Monomeric IgG (180 kDa) often requires levels as high as 100 g/L to produce significant changes in plasma viscosity, with the exception of the IgG3 subtype which can increase viscosity at lower levels due to an inherent tendency for aggregation.^5,6 Other recognized causes of serum or plasma hyperviscosity include marked polyclonal hypergammaglobulinaemia which can be seen in several conditions such as IgG4-related disease. ⁷