To be effective, PT schemes must be well-managed and provide samples that are representative of the ‘real’ situation in a laboratory. However, the proportion of positive samples is likely to be far higher with PT than with routine examinations. Samples must be sufficiently challenging and should be provided at regular intervals to ensure that the external assessment results allow an overview of performance over time.
The PT requirements for a testing laboratory should not be viewed in isolation; the laboratory needs to consider other aspects also such as:
- the weekly workload
- the frequency with which each examination is undertaken
- the number of positive samples normally seen for each routine test
- the degree of expertise of the staff
- the accuracy of previous PT results
- the frequency and acceptability of internal quality control procedures
Any of these factors may change over time so a laboratory’s PT requirements should be reviewed accordingly.