PAPPA and Down syndrome: van Heesch van Heesch PN PN de Rijke de Rijke YB YB Laudy Laudy JA JA Wildschut Wildschut HI HI Erroneous production of PAPP-A kits: the impact of a downward shift in PAPP-A concentration on the test performance of first-trimester combined screening for Down syndrome Erroneous production of PAPP-A kits: the impact of a downward shift in PAPP-A concentration on the test performance of first-trimester combined screening for Down syndrome Prenat Diagn Prenat Diagn 2011 2011 31 31 821 821 6 6 10.1002/pd.2775 10.1002/pd.2775 21692091 21692091