Effect of Repeated Freeze-Thaw Cycles on Maternal Serum Biological Markers for the Detection of Fetal Trisomies

Objectives: We studied the stability of maternal blood markers for screening for Down syndrome (alpha-fetoprotein, unconjugated estriol, intact human chorionic gonadotropin (hCG) and free β-human chorionic gonadotropin) upon repeated freeze-thaw cycles. Design and Methods: Forty-three samples collected from second trimester normal pregnancies were submitted to five freeze-thaw cycles. After each cycle, the markers were measured using kits and instruments from Wallac Canada (AutoDelfia®). Results were compared by repeated measures analysis of variance and by analysis of linear trend (after mathematical transformation of the results in order to decrease between-sample variation) as a function of the number of freeze-thaw cycles. Results: No significant differences were observed by ANOVA (p> 0.1) for any marker. Intact hCG showed a statistically significant linear downward trend (slope = −0.0063, p = 0.009) while free β-hCG increased (slope = 0.011, p = 0.004). After five freeze-thaw cycles, a mean decrease of 3.2% is predicted for intact hCG while free β-hCG would increase by 5.5% on average. Conclusion: We conclude that the studied markers do not show clinically significant changes under the evaluated conditions. The observed changes of intact hCG and free β-hCG would have a limited impact on the screening performance.