Division 4.3.2.4 Determination of cut-off level and rate of false suspected results of blank samplesF1

The (relative) responses for the negative control and positive control samples are taken as basis for the calculation of the required parameters.

Screening methods with a response proportional with the mycotoxin concentration

For screening methods with a response proportional with the mycotoxin concentration the following applies:

Cut-off = R _STC – t-value _0,05 * SD _STC

R _STC =

mean response of the positive control samples (at STC)

t-value:

one tailed t-value for a rate of false negative results of 5 % (see table B)

SD _STC =

standard deviation

Screening methods with a response inversely proportional with the mycotoxin concentration

Similarly, for screening methods with a response inversely proportional with the mycotoxin concentration, the cut-off is determined as:

Cut-off = R _STC + t-value _0,05 * SD _STC

By using this specific t-value for establishing the cut-off value, the rate of false negative results is by default set at 5 %.

Fitness for purpose assessment

Results from the negative control samples are used to estimate the corresponding rate of false suspect results. The t-value is calculated corresponding to the event that a result of a negative control sample is above the cut off value, thus erroneously classified as suspect.

t-value

(cut off – mean _blank )/SD _blank

for screening methods with a response proportional with the mycotoxin concentration

or

t-value

( mean _blank – cut off)/SD _blank

for screening methods with a response inversely proportional with the mycotoxin concentration

From the obtained t-value, based on the degrees of freedom calculated from the number of experiments, the probability of false suspect samples for a one tailed distribution can either be calculated (e.g.. spread sheet function ‘ TDIST ’ ) or taken from a table for t-distribution.

The corresponding value of the one tailed t-distribution specifies the rate of false suspect results.

This concept is described in detail with an example in Analytical and Bioanalytical Chemistry DOI 10.1007/s00216 -013-6922-1.