Hach DR/850, Procedures Manual, Page: 433 / 460

433
Interpreting Results
The Percent Inhibition results obtained are only a relative
measurement. They do not represent a true quantitative
measurement of toxic concentration. The Percent Inhibition does
not necessarily increase in direct proportion to the concentration
of toxins. To determine the minimum inhibition concentration of
a toxin, it is possible to make tenfold dilutions of the sample and
determine the Percent Inhibition for the dilutions. When the
sample is diluted so that no inhibition is observed, this is the No
Observed Effect Concentration (NOEC).
Due to the many variables involved in the test, the limits of
detection are on the order of 10% Inhibition. This would correlate
to the Lowest Observable Effect Concentration (LOEC). If a
sample shows less that 10% Inhibition, repeat the test. After
several repetitions, look at the series of data to determine the
likelihood of toxicity. Results below 10% are not reliable, but can
be used to surmise some presence of toxicity if they are
consistent. See the table below.
Some toxins will increase respiration and will give a negative
Percent Inhibition on this and all other respiration-based toxicity
tests. After repeated testing, samples that always give a Percent
Inhibition that is more negative than –10% should be considered
toxic.
Disposal of Test Cultures
Dispose of active bacterial cultures by using one of these
methods:
1. Autoclave used test containers at 121
°
C for 15 minutes at 15
pounds of pressure. Once the containers are sterile, pour the
contents down the drain with running water. The reaction
tubes may be washed
and re-used.
2. Sterilize test containers by using a 1:10 dilution of commercial
Toxicity Results
Data Points: Percent Inhibition Conclusion
7%, 9%, 5%, 8%, 5% May be slightly toxic
7%, –4%, 5%, 5%, 1% Most likely not toxic
–7%, –9%, 5%, –8%, –5% May be slightly toxic
TOXTRAK TOXICITY TEST,
continued