The College of American Pathologists (CAP) is dedicated to ensuring the quality and accuracy of medical laboratory testing across the United States. Patient diagnosis and treatment decisions rely entirely on reliable test results. Proficiency testing (PT) is the primary mechanism utilized by CAP to verify the accuracy of a laboratory’s analytical performance. This external quality assessment process provides an objective measure of how well a laboratory’s methods and instruments perform in a real-world testing environment.
Defining Proficiency Testing and Its Purpose
Proficiency testing is a blind external quality assessment program where a laboratory analyzes unknown samples provided by a third-party provider like CAP. The laboratory staff must treat these PT samples exactly like patient specimens, using the same equipment, reagents, and procedural steps. The goal is not to pass a test, but to provide an honest, unbiased snapshot of the laboratory’s routine testing capabilities.
The fundamental purpose of PT is to ensure inter-laboratory comparability, confirming that a result for a given analyte would be consistent regardless of where the patient was tested. By comparing a laboratory’s results to a peer group using the same methodology, CAP can identify systematic errors or bias in a testing system. This process acts as an early warning system, detecting performance issues before they can negatively impact patient care. Participation in an approved proficiency testing program is a mandatory requirement for all regulated tests.
The CAP Proficiency Testing Process
The CAP proficiency testing process begins with the shipment of specialized PT kits to participating laboratories, typically three times per year. Each kit contains a specific number of individual samples, often five, which are known as “challenges” for each regulated analyte being tested. These challenges are designed to mimic real patient specimens as closely as possible, sometimes using human-based materials that have been processed to ensure stability and a range of concentrations.
Upon receipt, the laboratory must log the samples and integrate them into the routine workflow, ensuring that the same personnel who perform patient testing are the ones analyzing the PT challenges. It is strictly prohibited to communicate with other laboratories about the samples or send them out for testing before the submission deadline. Once the analysis is complete, the laboratory submits its results to CAP through an online portal, such as the e-LAB Solutions Suite.
CAP then compiles all the submitted data and groups the results by a “peer group,” which consists of other laboratories using the same or a very similar instrument and method. This peer group mean is used as the target value against which the laboratory’s performance is evaluated. The performance report is generated by calculating how far the laboratory’s result deviates from the peer group mean, often expressed as a Standard Deviation Index (SDI). The laboratory receives a graded report that allows them to benchmark their performance and identify areas where their methods may be showing imprecision or bias relative to their peers.
Regulatory Compliance and Consequence Management
Proficiency testing is a non-negotiable requirement enforced by the Clinical Laboratory Improvement Amendments (CLIA) regulations for laboratories performing non-waived testing. For most regulated analytes, a laboratory must achieve a score of at least 80% correct to be deemed satisfactory for a single testing event. This generally means getting at least four out of the five challenges correct for that analyte.
When a laboratory fails a PT event—meaning the score is less than the required passing threshold—immediate investigation and remediation are mandatory under CLIA. The laboratory must determine the root cause of the error, which may involve looking for transcription mistakes, systematic bias, or random imprecision, and then take corrective action. All steps of this investigation, including staff retraining and system checks, must be thoroughly documented.
More severe consequences occur with repeated failures, defined as an unsuccessful performance for the same analyte in two consecutive or two out of three testing events. In such a scenario, the laboratory is required to cease patient testing for that specific failed analyte or specialty. To resume testing, the laboratory must demonstrate to the regulatory authorities that the problem has been corrected and then successfully pass two consecutive proficiency testing events. Failure to follow these mandated remediation steps can lead to sanctions, including the suspension or limitation of the CLIA certificate and a potential loss of accreditation.