Standards interlaboratory trials results

The most critical part of the method selection is determining the relative merits of seemingly diverse HPLC separations. Unfortunately, there is no standard against which all HPLC methods for vitamins are presently compared. However, there are indicators of assay reliability for which the analyst should look. These include measures of precision, accuracy, and reproducibility recoveries from spiked food samples linearity of calibration limits of detection measures of peak purity comparisons with existing recognized methods and results of collaborative or interlaboratory trials (9,10). 

Some estimates of the precision of various typical modem procedures are given in Table 3. General clinical chemists may well be surprised at some of the values when compared with objective experience in other areas. Thus it is well known that the estimation of serum bilimbin has a coefficient of variation (C.V.) of around 15% in most laboratories, and that with creatinine a value of 8-10% is fairly normal (W3). In a recent blind (but not double-blind) interlaboratory trial of a standardized method for 17-ketogenic steroids and one for 17-ketosteroids, Gray et al. (GIO) obtained C.V. s varying between 4% and 14% for both methods. Six of the ten laboratories cooperating in the trial had special steroid experience and were asked to obtain duplicate estimates of any thirty routine specimens of urine. These results are for two well-established and relatively simple procedures. 

The presentation of statistical results of data from interlaboratory trials can be handled differently in CEN and ISO. CEN presents these data generally in the normative body of the standard, whereas ISO puts them in the informative annex of the standard. Besides the different philosophies in ISO and CEN on the l out of a standard, the organization and evaluation of an interlaboratory trial is obligatory for the standard developers in CEN and ISO, and the quality of the statistical data from the interlaboratory trial is the final categorical factor in the decision to publish the standard method, to postpone or to withdraw the project from the working list. The criteria for interlaboratory trial data to be met are given in ISO 5725, Part 2 (ISO, 2002). 

During development of the European standard, an interlaboratory trial was organized involving six laboratories within Europe to follow a defined protocol to assess aerosol output and size from two fundamentally different nebulizer systems. Results demonstrated that the methods were repeatable within +/- 10% of mean return for both aerosol output and aerosol size (unpublished data). Results of preliminary research investigating the correlation between in vivo response and in vitro estimate of aerosol output and size show a promising correlation (33), though further work is needed in this area. 

In a farsighted move in 1989, the European Union laboratory IRMM started a series of interlaboratory comparisons to provide objective evidence for the degree of equivalence and the quality of chemical measurements by comparing a participant s measurement results with external certified reference values (IRMM 2006). At the time most proficiency testing schemes used consensus results for the mean and standard deviation to derive z scores. With the IMEP-1 analysis of lithium in serum, the world was alerted to the problem of lack of accuracy in analytical measurements. The data of the first IMEP-1 trial are replotted in figure 5.6 notice that the apparent outlier was the only laboratory to come close to the assigned value. 

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