Stability of ten serum tumor markers after one year of storage at -18°C

OBJECTIVES

The storage of serum tumor markers (STM) at -18 °C for one year has been a legal requirement in France since 1999, but has been abolished in 2022. This raises the question of the relevance of maintaining these biobanks in terms of conditions of storage. These should only be implemented after validation; in order to maintain the integrity of the biological sample and must be controlled over time according to the laboratory's procedures. The aim of the study was to assess the suitability of storing 10 STMs by evaluating their stability after one year of storage at -18 °C.

METHODS

A new immuno-enzymatic assay (A+1) was conducted on samples that had been stored at -18 °C for one year after an initial assay (A) of one of the following STMs: carcino-embryonic antigen (CEA), alpha-fetoprotein (AFP), carbohydrate antigen 125 (CA125), carbohydrate antigen 15-3 (CA15-3), carbohydrate antigen 19-9 (CA19-9), total (TPSA), and free (FPSA) prostate-specific antigen, calcitonin (CT), thyroglobulin (TG), and neuro-specific enolase (NSE). The results were confronted to four different permissible error sources.

RESULTS

In total, 1148 A+1 assays were performed. A strong correlation between A+1 and A values was found for all analytes, but with a statistically significant reduction in the mean A+1 concentration compared to the mean A concentration in 7/10 STMs. The bias induced by conservation seems to be technically unsustainable if we rely on the repositories closest to the current analytical performances.

CONCLUSIONS

These results support the discontinuation of mandatory STM biobank storage at -18 °C, which requires considerable technical time and organizational effort.

Stability of total prostate-specific antigen and free prostate-specific antigen after 10 years’ storage

Introduction:

PSA is a serine protease composed of 240 amino acids in a single polypeptide chain and is a routine parameter in prostate cancer diagnostics. The aim of our study was to test the long-term stability of tPSA and fPSA after 10 years’ storage at −80°C.

Materials and methods:

We analyzed two aliquots from 55 serum samples. The first was assayed in routine testing at the time of establishing the diagnosis. The second was thawed for further testing after approximately 10 years’ storage at −80°C. The mean of storage time was 10.41 years (min–max: 9.35–11.40 years). We compared the results of tPSA and fPSA. We calculated the fPSA/tPSA ratio and compared the results of clinical evaluation. Serum tPSA and fPSA levels were assayed using chemiluminescent kits Access Hybritech PSA and free PSA. All measurements were performed using the instrument UniCel® DxI 800.

Results:

tPSA decreased 3.59% on average with a correlation r=0.9213, and fPSA increased at an average of 2.41% with a correlation r=0.9338. The fPSA/tPSA ratio increased 0.80% on average with a correlation r=0.9174. On clinical evaluation, five samples had fallen to a less malignant category and three samples had risen to a higher malignant category compared with the original results.

Conclusion:

The stability of tPSA and fPSA levels in serum is sufficient after 10 years’ storage at −80°C. Calculation of the fPSA/tPSA ratio is not recommended due to the change in the category of malignancy of 15% of the samples.