If a preliminary stability test shows that the test substance concentration cannot be maintained (i.e. the measured concentration falls below 80 % of the measured initial concentration) over the test duration (7 days), a semi-static test regime is recommended. In this case, the colonies should be exposed to freshly prepared test and control solutions on at least two occasions during the test (e.g. days 3 and 5). The frequency of exposure to fresh medium will depend on the stability of the test substance; a higher frequency may be needed to maintain near-constant concentrations of highly unstable or volatile substances.
Kui sageli lastakse kultuuril puutuda kokku värske katsekeskkonnaga, see oleneb uuritava aine püsivusest; kui aine on väga ebastabiilne või lenduv, siis ligikaudu püsiva kontsentratsiooni hoidmiseks peab see sagedus olema suurem.