Whole blood viscosity extrapolation formula: Note on appropriateness of units

BACKGROUND: A series has recently been done on whole blood viscosity. The first on the series proposed extrapolation chart with conventional reference range. Since the publication, two concerns have been received in personal communications. The first expressed concern over the use of serum proteins values in ‘g/L’ instead of ‘g/dL’ that was contained in the referenced material. The second enquired on suitability of the formula for determination of blood viscosity at low shear rate. AIM: This work sets out to compare different units-converted-modifications of the extrapolation with a view to develop a statement of comparison and suitability of units. MATERIALS AND METHODS: The values of haematocrit and serum proteins were used in different unit-conversions on the mathematical formula to derive four modifications, which were compared. Five clinical cases that were specifically tested for whole blood viscosity, and had results for haematocrit and serum proteins were evaluated. The appropriateness of modifications for determination of viscosity at low shear rate was reviewed. RESULTS: Except as modified and used in the series, determination of whole blood viscosity at low shear rate using other unit conversions on the formula yields negative values for all five cases. CONCLUSION: Given that it is unexpected for blood viscosity level to be zero let alone less, it is logical that negative values are invalid. A formula that gives the least invalid results may be most appropriate. Therefore, extrapolation modification used in the series is most appropriate.