Insufficient mixing of thawed serum samples leading to erroneous results – experience from a field study and use of a correction procedure

Abstract

Incorrect analysis results that are close to expected might not be recognized in scientific studies or routine patient care. In two field studies we obtained unexpected results in a large number of samples. The present study aimed to identify the source of error in the samples from these studies and to validate a method to obtain correct results. Pre-analytical procedures were scrutinized, giving no indications of inappropriate pre-analytical sample handling in the field or during transport in a tropical climate. Using a new set of samples from volunteers in simulation experiments, we observed the known concentration gradient of analytes sampled in gel as well as plain tubes after freezer storage and thawing. Experiments demonstrated that mixing of samples by vortexing alone was not sufficient to disrupt the gradient formed by freezing and thawing, which appeared to cause the problem encountered when we in field studies analyzed and biobanked large sample sets by robot pipetting. A correction procedure was introduced, in which the obtained value of an analyte was multiplied by a correction factor calculated for each sample using the expected sodium level (140 mmol/L) divided by the measured sodium value. When it was validated on results from the simulation experiments, we repeatedly found that the correction lead to results very close to true values for analytes of different size and charge. Usefulness of the procedure was demonstrated when applied to a large set of field study results.

Keywords:

• Chemistry
• clinical
• clinical laboratory techniques
• diagnostic tests
• routine
• Pre-analytical phase
• analytic sample preparation methods
• specimen handling

Acknowledgement

We acknowledge the workers and field teams at the sugar mills, and the co-workers at the laboratories in El Salvador and Nicaragua. Anders Agrell and the staff at the Department of Occupational and Environmental Medicine, Sahlgrenska University Hospital in Gothenburg were essential for the chemical analyses and the experiments, as were the staff at the Department of Clinical Chemistry and the Department of Occupational and Environmental Medicine at Skåne University Hospital in Lund. Ying Li gave valuable comments for the statistical analysis and Professor emeritus Anders Grubb made an appreciated external evaluation of the findings.

Ethical approval

The field studies were approved by the National Ethics Committee for Clinical Research (Comité Nacional de Ética de Investigación de Salud) of El Salvador (OHRP IRB No. 0005660, FWA No. 00010986), and the Ethics Committee for Biomedical Research (Comité de Ética para Investigaciones Biomédicas [CEIB] “Dr. Uriel Guevara Guerrero”) of Universidad Nacional Autónoma de Nicaragua (FWA00004523/IRB00003342). The studies in Sweden were approved by the Regional Ethical Review Board of Lund (nos. 2015/7, 2016/233 and 2018/96).

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

ALF grants via Lund University and Region Skåne, and via University of Gothenburg and Västra Götalandsregionen.