Total observed error, total allowable error, and QC rules for canine serum and urine cortisol achievable with the Immulite 2000 Xpi cortisol immunoassay

Determining a simple quality control (QC) rule for daily performance monitoring depends on the desired total allowable error (TEa) for the measurand. When no consensus TEa exists, the classical approach of QC rule validation cannot be used. Using the results of previous canine serum and urine cortisol validation studies on the Immulite 2000 Xpi, we applied a reverse engineering approach to QC rule determination, arbitrarily imposing sigma = 5, and determining the resulting TEa for the QC material (QCM; TEa(QCM)) and the resulting probability of error detection (P(ed)) for each QC rule. For the simple QC rule 1(2.5S) with P(ed) = 0.96 and probability of false rejection (P(fr)) = 0.03, the associated TEa(QCM) were 20% and 35% for serum and 28% and 24% for urine QCM1 and QCM2. If these levels of TEa(QCM) are acceptable for interpretation of patient sample results, then users can internally validate the 1(2.5S) QC rule, provided that their QCM CVs and biases are similar to ours. Otherwise, more stringent QC rules can be validated by using a lower sigma to lower the TEa(QCM). With spiked samples (relevant cortisol concentrations in the veterinary patient matrix) at 38.6 and 552 nmol/L of cortisol, TEa(QCM) at sigma = 5 were much higher (54% and 40% for serum; 90.3% and 42.8% for urine). Spiked samples generate TEa that is probably too high to be suitable for daily QC monitoring; however, it is crucial to verify spiked sample observed total error (TEo; 26% and 18% for serum, 60% and 30% for urine) < TEa(QCM), and to use spiked sample TEo for patient result interpretation. In the absence of consensus TEa for cortisol in dogs, we suggest the use of a 1(2.5S) rule, provided that users accept the associated level of TEa(QCM) also as clinical TEa for results interpretation.