The Estimation of Plasma Protein Concentration from Plasma Specific Gravity

In 1930 Moore and Van Slyke (1) reviewed the various physical measurements, refractometric and others, that might serve as measures of the protein concentration of plasma, and concluded that specific gravity was the most reliable if application was required to pathological as well as normal plasmas. For plasma of nephritic patients they found that the relation between protein concentration and specific gravity, Diz, was expressed by the linear equation, P = 343 (G 1.0070), P representing protein concentration in gm. per 100 ml. and G the specific gravity, 0% The constants 343 and 1.0070 were based on gravity measurements by pycnometer and on proteins calculated from Kjeldahl nitrogen as 6.25 X N. A number of authors have since published modifications of the equation P = a(G b), with differing constants based on gravities measured by several procedures, and on micro-Kjeldahl analyses also by various procedures. The procedures and the constants arrived at for the equation are indicated in Table I. The gravity method indicated as CuSO4 is the procedure described in the preceding paper (2), but without the correction of 0.0007 for the difference between 0:: of copper sulfate standards and the plasma drops that balance in them. The gradient column method is that of Linderstrgm-Lang in the modification described by Lowry and Hunter (3). The Harbour method is the “falling drop” method described by Barbour and Hamilton (4). In order to compare calculations by the different equations the protein concentration calculated by each equation for plasma which has a gravity of 1.0270 determined by pycnometer is indicated in the column “P calculated.” From a recently published experimental review of Kjeldahl procedures (13) it appears probable that the differences in the relation between specific gravity and Kjeldahl-determined protein nitrogen found by different investigators (last column of figures, Table I) may in part be due to the fact