Evaluation on the Stability of Pooled Sera for External Quality Assessment of Tumor Marker Assays

Park, Yongjung; Kim, Young Ran; Hwang, Mi Seon; Kim, Hyon Suk

Abstract

BACKGROUND
Specimens for the external quality assessment (EQA) need to be highly stable during the EQA process. Therefore, we evaluated the stability of pooled sera (PS) for tumor markers including alpha fetoprotein (AFP), carcinoembryonic antigen (CEA), carbohydrate antigen 125 (CA 125), and carbohydrate antigen 19-9 (CA 19-9).
METHODS
PS with 2 different levels (high and low) of each of the 4 tumor markers were collected and stored at -20degrees C, 4degrees C, and room temperature (RT). The concentration of each tumor marker was then measured after storage under these different conditions at baseline and on days 1, 3, 7, 14, 30, 90, and 181. Internal quality control (QC) results during the evaluation period were also analyzed.
RESULTS
Irrespective of storage conditions, coefficients of variation (CVs) of AFP and CA 125 levels in the PS during the evaluation period ranged from 3.3% to 7.5% in EQA assays and were similar to the CVs of QC assays. However, the levels of CEA detected in PS stored at -20degrees C, and 4degrees C, showed higher variability, with CVs ranging from 4.0% to 10.4%, and samples stored at RT showed especially high CVs, i.e., >8.3%. Samples for CA 19-9 testing stored at RT also showed lower stability than the QC samples as well as samples stored at -20degrees C, after 3 days.
CONCLUSIONS
CEA and CA 19-9 levels in PS showed higher variability than AFP and CA 125, especially when stored at RT. These results indicate that all EQA specimens for tumor marker assays need to be tested as soon as possible and not stored at RT for longer than 3 days during the EQA process.