Note: This type of test, sometimes referred to as a "threshold" or "incipient" lethality test, is allowed to continue until acute toxicity (mortality or a defined sublethal effect) has ceased or nearly ceased and the toxicity curve (median survival time vs. test material concentration) indicates a threshold or incipient concentration. With most test materials, this point is reached within \(\pu{7\!-\! 10 d}\), but it may not be reached within \(\pu{21 d}\). Practical or economic reasons may dictate that the test be stopped at this point and a test be designed for longer duration.