When an assay is modified by the laboratory, or when an assay is developed by a laboratory (laboratory developed test), Clinical Laboratory Improvement Amendments (CLIA) requires that the laboratory establish performance specifications for that test. Measuring assay performance is achieved practically by measuring various metrics and comparing the results against a set of defined criteria or against a reference. We call these metrics performance characteristics. A good performance characteristic will be a metric or attribute that can successfully measure the efficiency and/or effectiveness of the assay. For example, an assay that demonstrates good accuracy is effective at producing true or correct results. When considering additional performance characteristics whose specifications should be established during a validation study, laboratories should therefore identify aspects of the testing process that could significantly impact assay performance and assess a corresponding metric.