A new method to measure viscosity in saliva

Objectives: Small sample volumes of saliva, e.g. a single drop collected from children or patients suffering xerostomia, do not allow viscosity determination with commercial viscosimeters. A micro-viscosimeter for analysing 10 µl of saliva was developed. Methods: A pressure sensor was installed between a micropipette (10 µl) and the pipette tip. A vacuum of 7 µl was produced with the piston of the pipette. The speed of the flow into the tip depended on viscosity of the respective liquid, with higher viscosity causing slower flow. Changes in vacuum pressure, corresponding to the flow rate, were recorded by a computer. The flow ended with accomplishment of pressure equilibrium. The decreasing flow rates showed an exponential function, with typical exponent for each viscosity. A standard curve was obtained from the viscosity of known standards at 37°C, and used to calculate the viscosity of salivary samples of 20 volunteers after homogenisation (centrifugation and mixing). Additionally, samples were frozen/defrosted and subsequently homogenized also. Results: The coefficients of variation (CVs) of standard viscosity samples were found between 0.27% and 1.09%. Results of salivary samples showed kinematical viscosities between 5 and 1200 mm2/s. 90% of the samples ranged between 10 and 130 mm2/s. The sample viscosities were quite homogeneously reproducible (CV: 6.5%). Multiple freezing-defrosting cycles did not influence the viscosity of the tested salivary samples. Conclusion: The micro-viscosimeter is able to distinguish between different viscosities in fresh saliva samples. This is also true for frozen/defrosted samples. The required volume of only 10 µl of saliva allows determination of saliva viscosity in small volumes.