Urinary placental growth factor stability as a critical factor in the reliability of preeclampsia diagnosis.

Introduction: Placental growth factor (PlGF) is a key biomarker for diagnosing and predicting preeclampsia (PE). While serum-based PlGF assays are well established, urine has emerged as a promising non-invasive alternative matrix. However, the absence of urinary PlGF stability data remains a major preanalytical limitation. This study aimed to assess urinary PlGF stability under common preanalytical conditions, including refrigerated storage and a double freeze-thaw cycle. Materials and methods: A prospective study was conducted using urine samples from ten pregnant women. Each sample was processed under standard laboratory conditions and aliquoted into five tubes. One aliquot was immediately frozen at - 80 °C (T0), three were stored at 2-8 °C for 48, 96, and 168 hours before freezing, and one underwent a double freeze-thaw cycle. Urinary PlGF concentrations were measured using the Elecsys PlGF immunoassay on Roche Cobas e801 analyzer. Percent degradation (PD%) was calculated relative to baseline. A linear regression model was applied to estimate the time to exceed a maximum permissible instability (MPI) of ± 10%. Results: Urinary PlGF remained stable at 2-8 °C for up to 48 hours, with a mean PD% of - 6% (95% confidence interval (CI): - 9.1 to - 2.8). The regression model (PD% = - 0.0834 x Time (h)) estimated the - 10% threshold at approximately 120 hours. After a double freeze-thaw cycle, the mean PD% was - 1.5% (95% CI: - 3.4 to 0.4%). Conclusions: Urinary PlGF shows acceptable stability for up to 48 hours under refrigeration and is stable over two freeze-thaw cycles. These findings provide essential preanalytical data supporting its potential use in clinical and research settings.