The “Normal Range” of the Prothrombin Time Estimated by Quick's Method

In the communication by Yeoman (1955) it is stated that the proportion of cerebrospinal fluid to 3% aqueous salicylsulphonic acid is one to three when used to determine the protein content in the M.R.C. photometer. This method has been studied by com- paring results using his method and results obtained by the micro-Kjeldahl method (King, 1951) on the same specimens. Twenty specimens were examined in which the levels were representative of all levels found in practice, i.e., normal, moderate increase, and marked increase. The results showed that the M.R.C. photometer method was fairly accurate if 1 ml. of cerebrospinal fluid was added to 4 ml. (not 3 ml.) of 3% salicylsulphonic acid. One of us (J. G. A.) com- municated with Dr. Yeoman and he agreed that the proportions should have been one to four and not one to three. It was found, however, that the true protein level (x) was slightly higher than the reading on the photometer (y), the two being related by the equation x= 16y+20