Adequacy of interlaboratory precision criteria in measuring intralaboratory performance

We compared the predictive value of the various criteria used for grading pH and blood-gas measurements in interlaboratory proficiency testing programs with performance as determined from actual intralaboratory quality-assurance data. The evaluation criteria were the two-standard-deviation interval (2 SDI) proposed by the College of American Pathologists (CAP); CAP's proposed fixed criteria; and the fixed criteria of the American Thoracic Society (ATS). These were compared with 95% confidence limits derived from the individual laboratories' actual intralaboratory data. We found that the CAP's most-stringent criterion (2 SDI) overestimated the number of outliers (unacceptable results) for PCO2 and PO2, whereas the proposed fixed limits underestimated them. For pH and PCO2 the ATS's limits, which are more stringent, more closely match the individual laboratory's actual performance as measured by conventional (mean +/- 2 SD) intralaboratory quality-assurance practices.