Performance Analysis

The approach taken to evaluate performance for FXIII assays complies with specifications detailed in ISO 13528 Statistical Methods for use in proficiency testing by interlaboratory comparisons and is as follows:

The consensus median is taken as the central reference point or "target value".  

A z score is calculated using the following formula:

[result – mean] / SD

The mean is a “robust” mean value, with outliers excluded; the SD is a “robust” SD determined after exclusion of outliers.

A result identical to the mean will give a Z score of 0.  ISO13528 defines Z score above 3 or below -3 as an “action signal”.  A Z score above 2 and below 3 (or below -2 and above -3) is a “warning signal”.  It is suggested in this ISO guide that one action signal, or two successive warning signals require investigation. 

Assessment is against peer group (method-specific) medians, when n =>10. If the assay is performed by 10 but <20 centres, performance is assessed but centres should note that z-scores can be unreliable with small numbers of samples (n<20) as in this case. If <10 centres return results for a method, performance will not be assessed.