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 beta-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 (Auto Delfia(R)). 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 beta-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 beta-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 beta-hCG would have a limited impact on the screening performance.