A method for measuring plasma viscosity and a comparison of plasma viscosity with blood sedimentation rate in rheumatoid arthritis.

In the study of the rheumatic diseases the measurement of blood sedimentation rate has an established value in estimating the activity of the disease and in evaluating the effects of treatment. It is generally agreed, however, that the erythrocyte sedimentation rate is not always a reliable index even if a correction is made for anaemia. The study of the viscosity of blood was first described over a hundred years ago. Hess in the early part of the present century used a well-known type of viscometer. Evans (1942) gave a detailed survey of the subject. Whittington (1942) called attention to the measurement of plasma viscosity as a clinical index and claimed diagnostic value for it. Miller and Whittington (1942), Houston and others (1945), and Harkness and others (1946) applied the test in pulmonary tuberculosis and reached the conclusion that plasma viscosity is a reliable laboratory test in assessing the clinical condition and prognosis in tuberculosis and certain other diseases. In the present paper a simplified method of measuring plasma viscosity is described, and its application in comparison with the erythrocyte sedimentation rate in interpreting cases of rheumatoid arthritis is discussed. Viscosity is a physical property of fluids which, in the case of pure liquids, may be treated mathematically. Protein and similar colloidal solutions show slight divergencies from the equation, probably due to deformation of the large molecules or to adsorption effects. For practical purposes these slight anomalies are unimportant, and it is sufficient to compare the rate of flow of plasma through a capillary tube with the rate of flqw of distilled water under the same conditions. Strictly speaking, as the plasma is slightly heavier than water the specific gravity should be taken into account, but this factor may be omitted for simplicity.