Handling of Zero and Non-Numeric Results in Microbiological Proficiency Testing
Reporting non-numeric results for microbiological tests is a relatively common occurrence. It is good microbiological practice to express results in such a way as not to mislead the customer, meaning that any result provided expresses the limits of detection and/or the volume of sample tested.
When too many organisms were present, one would be expected to report "greater than XXXX/test volume".
Expression of a negative result would normally not be "0". The usual method of expressing that the target organism was not found would be either:
Expressing "0" as the result of a microbiological test is not strictly correct, as there could be an implication that the result is absolute rather than a reflection of the result today, the method used and the sensitivity of the test.
Note that "not detected" does not mean zero - it means that the target organism was not found in the sample on that occasion. The volume expression afterwards indicates how much sample was tested. The sample volume gives the customer the indication of the probable risk they face should some target organisms exist in the test system.
However, statistical evaluation cannot occur on non-numeric results in proficiency testing programs. Similarly, since microbiological counts are transformed to log values prior to assessment, and the log of zero is undefined, zero results also cannot be correctly assessed.
Other forms of assessment can be made on such results provided the limitations of testing undertaken is clear to IFM. Such assessments are simply made by allocating pass or fail without calculating Z scores.
IFM's policy on the treatment and reporting of non-detections:
where a count is expected, and the participant has reported “0” :
where a count is expected, and the participant has reported “less than” or "not detected per XX mL" :
where a count is not expected and the participant has entered “<detection limit” :
where a count is not expected and the participant has entered a count :
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