Importance of Homogeneity and stability of PT samples
To run a successful proficiency testing (PT) program, the importance of homogeneity and stability of PT samples prepared for an inter-laboratory comparison study cannot be over emphasized, as these two factors can adversely affect the evaluation of performance.
The PT provider must ensure that the measurand (i.e. targeted analyte) in the batch of samples is evenly distributed and is stable enough before laboratory analysis at the participant’s premises. Therefore an assessment for homogeneity and stability for a bulk preparation of PT items must be done prior to the conduct of the program.
Checks for sample stability are best carried out prior to circulation of PT items. The uncertainty contributors to be considered include the effects of transport conditions and any variation occurred during the PT program period.
A common model for testing stability in PT is to test a small sample of PT items before and after a PT round, to assure that no change occurred through the time of the round. One may check for any effect of transport conditions by additionally exposing the PT samples retained for the study duration to conditions representing transport conditions.
A simple procedure for a homogeneity check
The homogeneity check aims to obtain a sufficiently small repeatability standard deviation (sr) after replicated analyses. The general procedure is as follows:
- Select a competent laboratory to carry out this exercise
- Take a number k of the PT samples from the final packaged bulk preparation through a random process
- Prepare at least m = 2 test portions randomly from each PT sample
- Take the k x m test portions in a random order and carry out single measurements for the targeted analyte concentrations
- Use 1-way ANOVA to analyze the data generated from (4)
- Homogeneity of the prepared bulk is achieved when the F test ratio of mean square between samples against the mean square within samples is smaller than the F critical value with the given degrees of freedom, i.e. (k-1) and k(m-1), respectively.