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Purpose of Screening Tests

A Screening Test is a simple bioassay to determine if an effluent shows lethal toxicity to the test species during a specified period of time (usually 24 hours) at the 100% concentration of the effluent.  Screening tests are useful for rapid evaluation of the acute toxicity of a raw effluent, and are sometimes required with samples of overflow effluents or releases of effluents from holding ponds during heavy rainfall (stormwater samples).

Protocol

A screening test usually consists of just two treatments -- a 100% effluent sample, and a control.   Typically each treatment is tested with at least 4 replicates of 10 organisms for a total of 40 organisms per treatment.  At the end of the test, the data from the two treatments are analyzed statistically to determine if survival of the test organisms in the effluent differs from their survival in the controls.

Information Obtained

A screening test yields no information concerning sub-lethal toxicity, or the LC50 of an effluent.  The test simply reveals whether or not the raw effluent is lethally toxic to the test organisms  within the specified period of time.