Evaluation of the US pharmacopeia adsorption tests for activated charcoals and proposals for changes

The current USP adsorption tests for activated charcoals, involving methylene blue (MB) and strychnine sulfate (SS), were conducted for 6 activated charcoals having surface areas ranging from 600 to 2000 m2/g. The MB test is relatively complex and tedious, and uses a pass/fail criterion which is of such small magnitude that substantial uncertainties are likely. The SS test is only qualitative. Modification, using quantification of the turbidities, permitted quantitative interpretation. The test was failed by 2 charcoals of surface area 660 m2/g or less, and was passed by 4 charcoals of 720 m2/g surface area or higher. Neither test was able to reflect the substantial drug adsorption differences which exist for charcoals of average, above average, and high surface areas. Alternative tests for MB and SS were developed and evaluated using 6 charcoals. These tests, which are proposed for consideration by the US Pharmacopeial Convention, are simpler and faster than the current tests and yield precise values for the percentages of the 2 compounds adsorbed. Moreover, they clearly reflect the wide differences in adsorption performance of charcoals having widely different surface areas. Minimum percent adsorption values that must be attained for a charcoal to qualify as "USP grade" are proposed for both alternative tests (these would not exclude any current USP grade charcoal). Performance classifications of charcoals into categories of Low, Average, Above Average, and High are also proposed for both tests. These are shown to correspond to surface area ranges of < 700, 700-950, 950-1200, and > 1200 m2/g, respectively.