Diagnostic Sensitivity Test Agar is recommended for diagnostic as well as testing of susceptibility of organisms to antibiotics. The diagnostic role is supported by the nutritional amino-acid base with glucose to encourage early growth. The inclusion of the buffers (disodium phosphate and sodium acetate) help prevent excessive movements of pH values which could result from utilization of glucose or amino acids. Such pH movements would interfere with haemolytic reactions¹ and the MIC values of pH-susceptible antimicrobials².

The agar used in the formulation has been specially processed to allow unimpeded diffusion of antimicrobials from discs³.

Diagnostic Sensitivity Test Agar is now primarily used for susceptibility tests and its role in diagnostic microbiology, i.e. the primary isolation of organisms from clinical samples, has diminished.

An essential requirement for satisfactory antimicrobial susceptibility media is that the reactive levels of thymidine and thymine must be sufficiently reduced to avoid antagonism of trimethoprim and sulphonamides⁴. Diagnostic Sensitivity Test Agar meets this requirement and in the presence of lysed horse blood (or defibrinated horse blood if the plates are stored long enough to allow some lysis of the erythrocytes) the level of thymidine will be further reduced. This is caused by the action of the enzyme thymidine phosphorylase which is released from lysed horse erythrocytes⁵. Thymidine is an essential growth factor for thymidine-dependent organisms and they will not grow in its absence or they will grow poorly in media containing reduced levels⁶.