Clin Biochem Rev Vol 29 Suppl (i) August 2008

Summary
• For detection of errors where the standard deviation is wide compared to clinical requirements, consider use of the Capability Index (Cps).
• Cps is defined as ‘Allowable Limit of Error divided by the standard deviation of between-batch QC measurement’.
• ‘Capable’ assays have Cps >4 compared with ‘incapable’ ones with Cps <4. • Capability of an analyte can be used to optimise the amount of quality control (QC) required and still maintain appropriate error detection. • For EQA material, Cps can be used for continuously monitoring individual analytes, comparing laboratory performance with peers and comparison with industry-wide performance. • The capability approach can result in reducing the number of QCs run per day, reducing costs as well as significantly improving the performance of a number of assays.