Turbidimetric assays

Turbidimetric assays are used to determine the influence of an antibiotic on the growth rate of a target microorganism in a specific medium. All components are added to a tube. Then, the inhibition of growth of the microorganism due to the antibiotic is measured by the turbidity of the solution.

Potency estimation of a Test and Control sample in a turbidimetric assay

The following image shows the potency estimation of a Test and Control sample in a turbidimetric assay: Every rack contains the five concentration steps of the Standard (S1, S2, S3, S4, S5), and the Test (U3) and Control (CTL1) sample. Potency values of Test and Control Sample are interpolated based on the Reference step (S3).


  • Sample Report for a turbidimetric assay as recommended by USP <81> 54 KB
    Example turbidimetric assay based on USP <81>: Capreomycin is used as antibiotic substance on the test organism Klebsiella pneumoniae. The standard consists of five concentration steps (S1 to S5) with S3 being used as reference. The relative potency of the unkown sample U3 is estimated based on the standard. The control sample CTL1 contains the test diluent but no antibiotic. Note that the interpolated concentration value of CTL1 is beyond the regression line and therefore does not allow any conclusion about the true estimate (e.g. nonlinear trend for high response values). In accordance with USP <81>, the test system includes tests on the combined standard deviation of the standard, the coefficient of determination (r^2), and the relative potency. For a direct comparison, the response values are taken from USP <81>. Moreover, the concentration is log10-transformed as recommended by USP <81>.