AFP Tetra Maternal serum screening can identify pregnant women who are at an increased risk for having a baby with certain birth defects. When a woman has reached a certain point in her pregnancy (between 15 and 21 weeks), a simple blood test can estimate her risk of having a baby with open neural tube defects, Down syndrome, or Trisomy 18, compared to other women her age. THIS TEST CAN ONLY BE ORDERED BY A LICENSED MEDICAL DOCTOR. Specimen Requirements: Minimum volume 3mL. Total volume 5mL. Gel-barrier tube, no thrombin additive. Collection: Collect in serum separator tube with gel barrier. Allow blood to clot, avoiding hemolysis. Separate serum from cells by centrifugation. Transport spun tube to testing laboratory. Pour off is not advised. Storage Instructions: Room temperature.