Pitfalls to Avoid in Growth Potential (GPT) Testing

Best Practice for Growth Potential Testing

It seems that there is a struggle to perform GPT in a co-ordinated and well documented approach. GPT testing is a regulatory expectation from a cGMP microbiology laboratory and there have been some issues with trepidations and data integrity observations in the microbiology labs around the subject of GPT testing. I have written what I believe is the best practice for GPT which you can find linked as a separate document. For advice on what pitfalls you can avoid I will run through some key issues which will hopefully help improve your process.

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Advice for improving your GPT testing procedure

Before starting the GPT test, big problems may arise if the media is used before release. Ensure that the media batch is at least labelled for quarantine and if possible segregated from batches released and in use. GPT is a time consuming process so save yourself time by physically checking the media against your criteria before testing. There is no point performing a GPT if the media isn’t suitable for use.

Avoid those awkward questions regarding your test organisms. They should be ATCC lines or the equivalent with evidence of suitability. Make sure your test strains are still fresh by using isolates no more than four generations from the original strain for consistency of results. Do remember that the more you sub-culture the more the characteristics of the micro-organism will change as it adapts growing in that environment. Another possibility is to include your regular environmental isolates to ensure the media can promote the growth of that microorganism.

Syringe_MOnto the testing itself, in my experience a frequent problem are issues regarding recovery your test organism. To give them the best chance of growth let the isolate or diluent reach room temperature before starting the test to prevent additional stress to the organism. Another issue is using the correct equipment and the correct technique. Without both you won’t get reliable results. Use pipette tips that are both low retaining and have a filter cap to avoid the possibility of cross contaminations. Pipette at an angle of no more than 20° (see the linked document for more detailed advice on technique). Avoid too many spreading and/or conjoined colonies by ensuring the surface of the agar plate is completely dry before testing. Following on from that make sure the agar has absorbed the cell culture before turning over for incubation or your colonies will not be entire and will be difficult to count.

By avoiding the pitfalls mentioned above you will have a more consistent and robust GPT procedure. As mentioned do read my advice on the whole procedure if you need more detailed advice.

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