Preanalytical stability of maternal serum markers hCGβ and PAPP-A

Down syndrome maternal serum marker screening is based on a risk calculation including the free β - human chorionic gonadotropin (hCGβ) and pregnancy-associated placenta protein type A (PAPP-A). The aim of this study was to define the pre-analytical conditions of stability of these markers both in whole blood at 15-25 ̊C and, after centrifugation, in serum at 4-8 ̊C. 158 patients were included in the study. Two automated workstations were used for assays, Cobas 8000e602, Roche Diagnostics (58 patients tested) and DELFIAXpress, PerkinElmer (100 patients tested). The stability of markers was studied in whole blood (15-25 ̊C) 2, 4, 6 and 8 hours after sampling and in serum stored after centrifugation at 4-8 ̊C at 24, 72 and 120 hours. Variations were defined by (C_T - C₂)/C₂, C₂ being the marker concentration at 2 hours and C_T the concentration at time T. In whole blood kept for 8 hours at 15-25 ̊C, hCGβ increased by a mean 2.4%, whereas the mean increase of PAPP-A was < 1%. In the serum kept for 5 days at 4-8̊C, the mean increase of hCGβ was 4.2%, with no change in PAPP-A. The impact of these variations on risk calculation is low. In conclusion, maternal serum can be store 8 hours at 15-25̊C in whole blood and 5 days at 4-8̊C after centrifugation and serum separation for Down syndrome maternal serum screening.