In CLN’s April Ask the Expert column, Dr. Rex Astles offers advice on proficiency testing (PT), starting with a response to this question: I missed a PT challenge. What now?
Astles, a health scientist in the Centers for Disease Control and Prevention’s (CDC) Division of Laboratory Programs, Standards, and Services, described the steps forward as follows. “If the challenge involves one of the approximately 100 analytes explicitly required by CLIA regulations, then a miss causing an unsatisfactory PT event (<80% correct) is likely to have regulatory implications. CLIA requires that the root causes for any PT miss be investigated and fixed, and the outcomes documented.”
The consequences of a PT miss depend upon how many challenges were missed and the frequency of errors, explained Astles. PT events usually occur three times per year and most events involve five challenges. For most required analytes, missing only one of five challenges (80% score) is passing and the event is “satisfactory.” An “unsatisfactory” score will involve scrutiny by CLIA surveyors or accreditation organizations.
Fortunately, there is something laboratorians can do in this situation. By carefully reviewing PT results from every event, a laboratory can frequently detect and deal with analytical problems before they result in inaccurate patient testing.
What are the odds that a given PT result may be outside the CLIA acceptance limits (ALs)? Typically, PT results are judged against results from other laboratories that use the same test method; the target value is the mean of equivalent test methods in the peer group. Some ALs are based on percentages and/or concentrations around the peer group mean. In other cases, ALs are three standard deviations, in which case the miss rate per challenge will typically be about 1%—or greater due to non-Gaussian distribution—regardless of the peer group’s overall precision. The risk of a specific PT result failing a challenge depends upon not only the laboratory’s accuracy relative to its peer group, but also the stringency of the analyte’s AL.
Analytical performance goals should be tight enough to minimize the risk of missing a PT challenge, but perhaps even tighter to meet clinical needs based on the laboratory director’s judgment.
Pick up the April issue of CLN to read more from Astles on how laboratories should handle a PT miss.