SUMMARY: Streptomycin may be estimated in body fluids by allowing the fluid to diffuse through nutrient agar seeded with a culture of contained in small-diameter (3 mm. internal) glass tubes. The zone of inhibition produced is affected neither by the anaerobic conditions within the agar nor by the volume of the test fluid. Acidity of the test fluid invalidates the results, since the depth of the zone of inhibition is decreased. Increase in the size of the inoculum of the test organism also decreases the depth of the zone of inhibition. Small errors arise from variations in timing during the setting-up of the test. The zones produced in the presence of different human sera differ slightly and to a degree similar to that found in the presence of various urines.

The available mathematical expressions suggest that the square of the depth of the zone of inhibition is linearly related to the log of the concentration of an antibiotic in the test fluid. Experimentally this relationship does not hold for low concentrations of streptomycin, probably due to the assumption of boundary conditions which cannot be defined with certainty; but it is a better approximation than the assumption of a linear relationship between the depth of the zone of inhibition (unsquared) and the log of the streptomycin concentration. A method of statistical analysis is given in which a weighted regression line is fitted to the squared values of the zones of inhibition in a manner analogous to probit analysis. The routine method may be refined without undue labour to the point where assays are accurate to within + 5%.


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