∗ Comparison of Inorganic Phosphate Contents of Serum,Fluoride Plasma and Native Plasma. †

In the course of work on the clearances of various urinary constituents (White and Monaghan 1 ) we noted a difference between the inorganic phosphate values of serum and of fluoride plasma, where 0.25 to 0.3% NaF was used as anticoagulant. A few weeks later Gaebler 2 reported that 4.2% NaF inhibited or abolished the color production with the Benedict-Theis 3 or the Fiske-Subarrow 4 method and that this interference could be avoided by adding AlCl₃. Gaebler gives no data on the extent to which 1% NaF, the concentration which he used as anticoagulant, influences color production in the phosphate methods. Since, however, the lower phosphate content of fluoride plasma as compared with serum corresponded roughly with the extent to which the total nitrogen of the plasma was less than that of serum, it might be inferred that 1% NaF interferes little and that the lower phosphate content of the plasma is due in the main to its dilution by the osmotic effect of the fluoride. His results may have been somewhat complicated by the facts that the serum lacked fibrinogen nitrogen and that 1% NaF caused some hemolysis. Gaebler also reported that in 7 cases out of 8 serum separated after the blood had stood 30 minutes at about 25° showed a higher inorganic phosphate content than did native plasma separated immediately, while oxalate plasma always showed a lower phosphate content than native plasma. He suggests that serum is less suitable than native plasma for inorganic phosphate determinations.

We have (1) investigated the effect upon the color production of the addition of 0.3, 0.6, 0.9, and 1% NaF to known phosphate solutions and of 0.3% NaF to serum, (2) compared the inorganic P (Benedict-Theis method) and total N (macro Kjeldahl) contents of 0.3% fluoride plasma and of native plasma, both separated immediately, and in another set of experiments added hematocrit readings, substituting heparin or hirudin for native plasma, and (3) compared the inorganic P contents of serum and of native plasma, both separated immediately.

Suummary. A concentration of 0.3% NaF does not influence the color production with the Benedict-Thesis method for inorganic phosphates; an inhibiting effect is beginning to manifest itself with 1% NaF.

Plasma obtained by adding 0.3% NaF to blood shows a lower content of inorganic P than does native plasma or hirudin plasma. This difference is usually but not always accounted for by the osmotic dilution due to the added salt, the extent of dilution being measured by total N hematocrit findings. In the cases where the drop in plasma P is greater than can be accounted for by dilution, the absolute amount of inorganic P in the plasma is apparently decreased.

The inorganic P content of serum is the same as that of native plasma.