Evaluation of a new immunoassay for chromogranin A measurement on the Kryptor system

Abstract

Background: Chromogranin A (CgA) is a biomarker for neuroendocrine tumors (NETs). The aims of this study were to evaluate differences in measurement between the ThermoFisher Brahms CgA Kryptor assay and the CisBio assay and to investigate the influence of patient covariates. Temperature stability of CgA using both assays was determined.

Design and methods: 406 patients were analyzed for serum CgA using both assays. We performed a comparison study to determine whether several patient covariates (gender, use of protein pump inhibitors, impaired kidney function, referral department and tumor location) influenced the results. For the stability study, pooled serum samples were aliquoted and stored at different storage temperatures (room temperature, 4 °C and -20 °C) until assayed. In addition, 15 individual samples were evaluated after storage at 4 °C using the Kryptor assay.

Results: Differences in measured concentrations between the assays were statistically significant. Passing & Bablok fit showed ln Y(Kryptor)=1.05 ln X(CisBio) - 0.20 with a bias of 1.0% after logarithmic transformation. Patient covariates were not associated. Patients׳ sera showed variable stability for CgA in the Kryptor assay at room temperature and 4 °C, whereas the recovery in the CisBio assay was stable at both temperatures.

Conclusion: Differences in measured CgA concentration between the assays could not be explained by the investigated patient covariates. Serum should be stored at -20 °C prior to determination using the Kryptor assay.

Keywords: CgA, chromogranin A; Chromogranin A methods; ELISA, enzyme-linked immuno sorbent assay; ENETS, European Neuroendocrine Tumor Society; GEP-NET, gastroentropancreatic NET; H2RA, H2-receptor antagonist; LD, lactate dehydrogenase; MDRD, modification of diet in renal diseases; NET, neuroendocrine tumor; Neuroendocrine tumors; PPI, proton pump inhibitor; TRACE, time-resolved amplified cryptate emission; Temperature stability; Tumor markers.

Fig. 1

(A) Comparison between CisBio and Kryptor CgA assay (Passing & Bablok curve fit, all samples). y=1.05–0.20x. (B) Difference plot of Kryptor and CisBio CgA measurements (all samples). Bias of +1.0% (95% CI=0.0–1.9).

Fig. 2

Recoveries of CgA with the Kryptor assay at different temperature conditions in pooled samples. Percentages shown in the legend are recoveries on the last day of storage. Initial sample concentrations are also shown.

Fig. 3

Recoveries of CgA with the CisBio assay at different temperature conditions in pooled samples. Percentages shown in the legend are recoveries on the last day of storage. Initial sample concentrations are also shown.

Fig. 4

Recoveries of CgA with the Kryptor assay in 15 individual (unspooled) samples stored at 4 °C. Percentages shown are the highest and lowest recovery in these samples. The initial CgA concentrations for these samples are also shown.