Effect of long-term storage on placental growth factor and fms-like tyrosine kinase 1 measurements in samples from pregnant women.

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 log(10)PlGF was positively correlated with parity (p=0.014) and gestational age (p=0.029), while log(10)sFlt-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.