In employing the above-named useful test all observers have encountered a difficulty which makes accuracy of reading impossible, namely, the yellowish color of the diluted urine as compared with the standard solution. To obviate this difficulty Cabot1 suggests using the patient's urine in proper amount in preparing the standard solution. A simpler method which will be found perfectly satisfactory is to interpose a piece of fairly vivid yellow glass between the eye and the solutions to be compared, since viewed through such glass urine and clear water are almost indistinguishable. This applies, of course, both to Cabot's system of test-tubes and to any more complicated colorimeter (Geraghty-Rowntree).2
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Whitney JL. THE PHENOLSULPHONEPHTHALEIN TEST OF RENAL SUFFICIENCY. JAMA. 1912;LVIII(13):936. doi:10.1001/jama.1912.04260030334016