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LABORATORY ERROR

Microbiology has a well-deserved reputation for variability. This aspect of the data is both an outcome of the science itself and the manner in which we perform the work . It may be caused by how the samples are taken, the manner in which they are taken (with severe limitations in sample size contributing to the problem) and the innate variability of a process heavily dependent on human interaction using a biological test system. For example, let’s look at a relatively simple and basic operation such as plating. The scientists detailed a variety of errors (errors in the statistical sense of variability) involved in this operation:

The errors in this example might be divided into two main types - some that might be considered avoidable error (plating error, calculation error) and unavoidable error (sampling error, dilution error, distribution error). We cannot eliminate either type of error in the lab, but the general category of “unavoidable errors” are not amenable to correction by training or proper lab technique. In fact, some of the current practices of the lab, adopted for business purposes, may actually increase the effects of this type of variability. Among these unavoidable errors might be included:

Insufficient sample numbers - due to the expense involved, virtually all microbiological tests are performed with a sample size that is completely insufficient in terms of a statistical sample plan.

Insufficient number of replicate plates - While the research recommends a minimum of three replicates (the better to estimate the mean of the population and to recognize an outlier), industry practice is to use duplicate plating, and it is not uncommon to see labs only using single plates.

Difficulties in using living systems (which react to treatment and growth conditions).

This last point deserves some additional discussion. It is a great temptation to writers of regulations to view microorganisms as particles. This leads to the mistake of writing specifications that are unreasonably small, well below the limit of quantification for the test method. This unrealistic expectation, coupled with the lack of attention frequently paid to inoculum preparation, frequently leads to an unrealistic level of variability in the assay results. Simple “fixes” for variability (larger sample size, more replicate measures of each sample) are out of consideration for budgetary reasons.

c) Post-reading Stage.

- Answer the questions:

1) What reputation does microbiology have?

2) What may the microbiology error be caused by?

3) Who detailed the variety of errors?

4) What are two main types of errors division? Describe them.

5) What are unavoidable errors do you know?

6) Comment upon insufficient sample numbers.

7) Comment upon insufficient number of replicate plates.

8) What kind of errors are difficulties in using living system?

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