Objective. To assess the effect of storage time on the stability of placental growth factor (PlGF) and fms-like tyrosine kinase 1 (sFlt-1) levels in frozen serum samples from pregnant women.Methods. This is a matched case–control study using fresh and stored serum samples collected at 6, 12, 18, 24, 30 and 36 months prior to the collection of the fresh samples and frozen at –80°C. Forty-eight samples from each of the seven time-groups were matched for non-smoking Chinese, maternal weight, singleton-term pregnancy without major obstetric complications and extracted for PlGF and sFlt-1 assays. Multivariate analysis was performed to assess residual effects of the case-matching procedure. ANOVA was used to assess the effects of storage time.Results. Multivariate analysis of the 336 samples indicated that log10PlGF was positively correlated with parity (p = 0.014) and gestational age (p = 0.029), while log10sFlt-1 was inversely correlated with parity (p = 0.018). After correcting for the residual effect of gestation and parity, ANOVA showed no significant difference in PlGF and sFlt-1 levels between the fresh samples and all stored samples (p = 0.410 and p = 0.158, respectively).Conclusions. Serum PlGF and sFlt-1 levels are stable for at least 3 years when stored at –80°C. Parity is an independent factor of PlGF and sFlt-1 levels. PlGF levels are lower and sFlt-1 levels are higher in nulliparous women compared to multiparous.

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