A-099 Validation of a copeptin immunoassay as a surrogate for vasopressin testing

Abstract Background Copeptin complements osmolality testing to aid in the evaluation of patients with polyuria-polydipsia syndrome and may also be used in conjunction with cardiac markers in the investigation of cardiovascular disorders. While derived from the same precursor as arginine vasopressin (AVP), copeptin has better ex vivo stability, is not affected by platelet binding as is AVP, and may allow for improved result turnaround time. The aim of this study was to evaluate the performance of the Thermo Scientific B·R·A·H·M·S Copeptin proAVP KRYPTOR immunofluorescent assay, based on TRACE Technology. Method: Validation studies included method comparison, linearity, imprecision, analytical sensitivity, specimen and freeze/thaw stability, sample carryover, interference, and reference interval verification. Method comparison was performed using de-identified residual patient samples and fresh-drawn EDTA plasma samples (n=56). A validated KRYPTOR analyzer at an external laboratory was used as the comparator method. Linearity was evaluated by diluting a high patient sample with assay diluent to create 5 levels of samples (0, 25, 50, 75, and 100% of the high pool), analyzed in duplicate. Imprecision was assessed by analyzing Kryptor control material and a patient pool over 5 runs, 4 replicates per level, per run. Limit of detection (LOD) was assessed by analyzing 10 replicates of the assay diluent and 3 replicates of a low patient pool. Instrument responses (RFU) were used for analysis in EP Evaluator. Specimen stability was studied by analyzing 2 pools of fresh-drawn EDTA plasma stored at room temperature, refrigerated, and frozen at various timepoints. Two cycles of freeze/thaw were evaluated. Sample carryover was assessed by alternating high and low patient pools. To evaluate hemolysis, icterus, and lipemia (HIL) interference, 3 patient pools were spiked with hemolysate (500 mg/dL), bilirubin (5, 10, and 20 mg/dL), and intralipid (500 mg/dL triglycerides) then compared to pools prepared to control for dilution. The reference interval was verified using fresh-drawn EDTA plasma from 70 volunteers (20-64 years old, 31 males and 39 females) who were without comorbidities or medications known to influence AVH and were not fasting or water deprived. All data were analyzed using EP Evaluator. Results Method comparison demonstrated favorable correlation with a slope of 1.02, bias of 0.14%, and correlation coefficient of 0.998 for results between 1.6-221.1 pmol/L. Acceptable linearity was observed across a measuring range of 0.0-494.2 pmol/L, with an observed slope of 0.997 and recoveries ranging from 80.7-100.0%. Imprecision studies observed total %CVs =5.4%. The manufacturer LOD claim of 0.88 pmol/L was confirmed. Copeptin was found to be stable for at least 7 days at room temperature, 7 days refrigerated, 30 days frozen, and up to 2 freeze/thaw cycles (average change of 0.3%, 8.5%, -8.0%, and 2.0%, respectively). Sample carryover was not observed. HIL interference resulted in an average change in copeptin concentration of ±8.6%. Copeptin adult reference interval of 1.0-13.0 pmol/L was verified. Conclusions: The Copeptin assay on the Thermo Kryptor analyzer has been evaluated and is found to be acceptable for clinical testing. Measurement of Copeptin provides a beneficial alternative to measuring arginine vasopressin hormone directly.