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:

• not detected per 10 mL, (or 1 mL, 100 mL etc), or

• less than 10/mL  (or another value expressing the detection limit)

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.

IFM has observed that the number of laboratories choosing to report “not detected” results as “0” (zero) is increasing.  Technically, reporting “not detected” or “less than (a detection limit)” is preferable to reporting zero.  IFM acknowledges that in the past, individual laboratory reports have shown the allocation of “0” as an over ride value for results entered as “<…” in order to permit statistical processing to occur. We also acknowledge that this may be influencing the manner in which participants report non-detection of target organisms.  

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.  

• For example, if a participant reports >1100 MPN per 100mL for a coliform count and the expected value is 3500 cfu per 100 mL, then clearly the participant is correct and a PASS can be allocated to that result without providing a Z score.  

• Similarly, if E. coli has not been inoculated into the sample and the laboratory reports <10 cfu /g, then this is also correct and the participant would be allocated with a PASS.  

• However, if E. coli was inoculated, the median result of participants is, say 2000 cfu/g but a participant reports less than 100/g, it could be concluded that the participant failed to detect the organisms and would be allocated with a FAIL.  

IFM's policy on the treatment and reporting of non-detections:

where a count is expected, and the participant has reported “0” :

• Exclude result from the data set used to determine statistical values

• FAIL Result and set Action required

• Do not calculate log value or Z-score

where a count is expected, and the participant has reported “less than” or "not detected per  XX mL" :

• Exclude result from the data set used to determine statistical values

• Do not calculate log value or Z-score

• If possible, based on information received, provide an assessment of the result in light of detection limit (note that unrealistic limits are not considered acceptable!)

where a count is not expected and the participant has entered “<detection limit” :

• PASS Result (however ensure that less than value is reasonable in terms of expected detection limits)

• The previous practice of entering a result override  value of “0” will no longer occur ; log value and z-score will not be calculated, nor shown in reports 

where a count is not expected and the participant has entered a count :

• FAIL Result and set Action required

• Do not calculate Z-score 

IFM Quality Services Pty Ltd

IFM, Working with you, for you

PO Box 877, Ingleburn 2565, AUSTRALIA

4/58 Stennett Road Ingleburn NSW 2565 Australia

Telephone:  +61 2 9618 3311

 Facsimile: +61 2 9618 3355

Email:  ContactIFM@ifmqs.com.au
                                                                            Date of page update: May 10, 2011