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In a previous communication (1) a method was described for determining the plasma volume by injecting intravenously an azo dye "Evans Blue" and measuring the dye concentration of undiluted serum samples with the spectrophotometer. The cost of the spectrophotometer and the specialized training required for its successful operation limit the applicability of the method to the study of clinical problems. Studies with this method by one of us (2, 3) have indicated the desirability of employing a simpler and less expensive type of photometer.In this communication we will describe the adaptation of the plasma volume method of Gibson and Evans (1) to the photoelectric microcolorimeter of Evelyn and Cipriani (4,5). This adaptation has been accomplished without the introduction of any essential change in the technique, and with no sacrifice of accuracy, or restriction of the range of applicability of the method. In addition, the simplified technique described has the advantage of greater objectivity and rapidity of the photometric readings, as compared with the spectrophotometric method.In estimating the dye concentration of blood serum samples with the spectrophotometer, the absorption measurements are made in terms of optical density at the wavelength (620 millimicrons) of maximum absorption of the blue dye. In the photoelectric photometer, measurements of light transmission are made in a narrow spectral region isolated by a color filter which transmits light in the vicinity of 620 millimicrons.In order that the same mathematical formulae negative logarithm) into quantities called L values which are analogous to optical densities as measured on the spectrophotometer. Since an L value is merely the average optical density of the solution over the narrow band of wavelengths transmitted by the filter, it is possible, by making the filter sufficiently selective, to obtain the same linear relation between concentration and L value as exists between concentration and optical density. One may, therefore, employ standard spectrophotometric formulae by merely replacing optical densities by the corresponding L values.

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