Pushing proficiency testing results to their limits.

PT is a valuable component of laboratory quality control and management practice. PT results are useful to determine over time that a method is performing in a laboratory according to the manufacturer's design parameters, as exhibited by the performance characteristic of a large group of peer laboratories. PT peer group results give a good measure of the interlaboratory imprecision of a method, and summary reports document the state-of-the-art in user preference. PT results typically cannot be used to validate the accuracy of a method or its clinical utility or to compare the accuracy of various methods. These limitations are caused by the common and unpredictable matrix interferences with PT materials and most routine laboratory methods. In situations where a laboratory adjusts the calibration of one method to make patient results agree with those of another method, PT specimens results must be reported without calibration adjustments to allow those results to be compared to an appropriate peer group mean target value.